Friday, December 31, 2010

Olympus Mons

Olympus Mons is a large shield volcano mountain that is located on the planet Mars and it has a morphology similar to the large volcanoes that make up the Hawaiian Islands. Rising to a height of 22 kilometres above the surrounding plains or 21 kilometres above the standard topographic datum of Mars, Olympus Mons is the tallest mountain known in the Solar System. This makes Olympus Mons stand at just under three times the height of Mount Everest. The base of Olympus Mons measures over 600 kilometres across and the outer edge of the mountain is rimmed by an immense cliff which rises up to 8 kilometres above the surrounding terrain.

Due to the sheer size of Olympus Mons and from the fact that the average slope of the volcano’s flank is only 5 degrees, the entire vertical profile of Olympus Mons will not be visible to an observer who is standing at a great distance away on the surrounding plains as the curvature of the planet Mars would obscure the mountain’s summit. Similarly, an observer standing on the summit of Olympus Mons will be unable to view the surrounding plains as the slopes of the volcano would extend well beyond the horizon. However, the immense cliffs which surround almost the entire base of Olympus Mons will definitely make an impressive sight.

Olympus Mons is located on the northwestern edge of the Tharsis Bulge which also has some of the largest volcano mountains known in the Solar System. To the southeast of Olympus Mons are the mountains Arsia Mons, Pavonis Mons and Ascraeus Mons. Like Olympus Mons, these mountains are also immense shield volcanoes that rise to impressive heights, greatly dwarfing even the prominence of Mount Everest. The Tharsis Bulge, on which these colossal mountains are located, covers millions of square kilometres in area and the height of Everest’s summit is merely comparable to the surface elevation of the massive plateau above the standard topographic datum of Mars.

The extraordinary size of Olympus Mons is due to the fact that unlike the Earth, Mars does not have plate tectonics and this enables the crust of Mars to remain stationary over a hotspot. By doing so, magma coming out of the hotspot continuously builds the volcano in the same location and allows Olympus Mons to become so large. A unique observational aspect of Olympus Mons is that it is sufficiently high enough to penetrate above the frequent Martian dust storms that can occasionally be large enough to engulf the entire planet. This was the first observational hint of the incredible height of Olympus Mons, long before the first spacecraft arrived in orbit around Mars.

The atmospheric pressure on the top of Olympus Mons is about 70 Pascal and this is about 11 to 12 percent of the atmospheric pressure at the standard topographic datum of Mars which has a value of 610 Pascal. In comparison, the atmospheric pressure on the top of Mount Everest is about 31400 Pascal while the atmospheric pressure at sea level on the Earth is 101325 Pascal. To put this into an Earthly perspective, the atmospheric pressure on the top of Olympus Mons is like being at an altitude of 50.5 kilometres above sea level while the atmospheric pressure at the standard topographic datum of Mars is like being at an altitude of 34.5 kilometres above sea level.

Orographic clouds that are made up of particles of water ice have long been known to be associated with Olympus Mons and with the other great volcano mountains on Mars. These clouds form when air masses are forced from a lower elevation to a higher elevation as they move up the slopes of these great mountains. The air masses cool as they rise and the moisture content carried within them condenses into particles of water ice, forming orographic clouds.

Monday, December 27, 2010

Magnetar Flare

A neutron star is a type of compact star that is formed from the gravitational collapse of the core of a massive star during a supernova explosion. A typical neutron star has a diameter of around 20 kilometres and a mass that generally exceeds the mass of our Sun. In comparison, our Sun has a diameter of 1.392 million kilometres. This incredibly compact configuration for a neutron star means that just a single cubic centimetre of its material packs a mass of around a billion metric tons! The extreme compactness of a typical neutron star also gives it a surface gravity that is over 100 billion times the surface gravity on the Earth and an escape velocity of around one third the speed of light!

A magnetar is an exceedingly rare type of neutron star which possesses an extremely powerfully magnetic field. In fact, the magnetic fields of magnetars are the strongest known in the universe as these magnetic fields have intensities on the order of between a billion to a trillion teslas. For comparison, the strength of the Earth’s magnetic field is about 30 microteslas while the strongest permanent magnets can generate magnetic fields of up 5 teslas. Magnetars are so rare that less than 15 of them are known. These exotic stars give rise to occasional burst of X-rays and gamma-rays, thus manifesting themselves as either Soft Gamma-ray Repeaters (SGRs) or Anomalous X-ray Pulsars (AXPs). SGRs are generally more energetic than AXPs and the bursting/flaring events from magnetars can be roughly classified into 3 types – short bursts, intermediate flares and giant flares. Giant flares are far more energetic than the short bursts and intermediate flares, and only 3 giant flares have been recorded in the decades of monitoring high energy astrophysical events since the 1970s.

SGR 1806-20 is a magnetar that is located around 50 thousand light years away, on the other side of the of the Milky Way galaxy. At this distance, it takes light 50 thousand years to travel from SGR 1806-20 to the Earth. The stellar neighbourhood of SGR 1806-20 contains some highly unusual stars, including one of the most massive and luminous star known in the Milky Way galaxy. What makes SGR 1806-20 unique is that this magnetar has the strongest magnetic field ever discovered for any object in the universe and this magnetar is also the progenitor for one of the 3 giant flares recorded so far. The strength of the magnetic field of SGR 1806-20 is estimated to be on the order of a whopping one trillion teslas!

On Monday 27 December 2004, an extremely energetic giant flare was detected from SGR 1806-20. This giant flare was so energetic that it saturated all but the least sensitive particle detectors regardless of where the detectors were pointed and this event became the brightest blast of gamma-rays ever detected from an astrophysical source. The giant flare from SGR 1806-20 is estimated to have released more than 2000 trillion trillion trillion joules of energy in the form of X-rays and gamma-rays.  Almost all of the energy released from the giant flare was concentrated in an initial hard spike that lasted for around 0.2 seconds. This initial hard spike was then followed by a gradually decaying pulsating tail which shows about 50 cycles of high-amplitude pulsations over the duration of around 600 seconds. The high-amplitude pulsations show a period of 7.5 seconds and this period matches the rotational period of SGR 1806-20.

To place the amount of energy generated by the giant flare from SGR 1806-20 into perspective, the amount of power produced during the initial hard spike which lasted for around 0.2 seconds is on the order of a thousand times the combined luminosity of all the hundred of billions of stars in the Milky Way galaxy! In fact, the amount of energy produced during the 0.2 seconds of the initial hard spike is greater than the total amount of energy generated by our Sun over a period of 100 thousand years! Already, the amount of energy produced by our Sun in a single second is almost a million times the total worldwide energy consumption in 2009! The giant flare from SGR 1806-20 was so bright that even its echo off our Moon was detectable. Interestingly, if all the energy were converted into visible light, the giant flare would have been brighter than the full Moon during the 0.2 seconds duration of the initial hard spike! If the giant flare from SGR 1806-20 had occurred at a distance of 10 light years from the Earth, it will be similar to standing at a distance of 7.5 kilometres from a 15 kiloton nuclear explosion.

The giant flare detected on 27 December 2004 from SGR 1806-20 is hundreds of times more energetic than the two other known giant flare events. The release of such an immense amount of energy within such a short period of time managed to eject a significant amount of matter from the magnetar. The highly energetic ejecta formed an outflow which interacted with the external interstellar medium and produced a radio afterglow this is at least 500 times more luminous than the only other radio afterglow detected from a giant flare. Finally, it may be possible for ultra-high energy cosmic rays from the giant flare to be detected years after the event form the direction of SGR 1806-20, provided that the deflection of the ultra-high energy cosmic rays by galactic magnetic fields is not too large.

Friday, December 24, 2010

Dark Galaxy

VIRGOHI 21 is the name given to an intriguing object that is located approximately 50 million light years away in the Virgo Cluster. The Virgo Cluster is a cluster consisting of between one to two thousand member galaxies. VIRGOHI 21 was discovered through radio telescope observations of the 21 centimeter wavelength radio emissions from its neutral hydrogen content. The total mass of hydrogen in VIRGOHI 21 is estimated to be around 100 million times the mass of our Sun. Observations of the motion of hydrogen gas within VIRGOHI 21 shows that the hydrogen gas is moving far too rapidly to be explained by the gravity from just the mass of the detected hydrogen alone. In fact, the total mass of VIRGOHI 21 is inferred to be as large as 100 billion times the mass of our Sun!

Deep observations by the Hubble Space Telescope revealed no optical counterpart to VIRGOHI 21 and this makes VIRGOHI 21 an excellent candidate for a dark galaxy since it has a mass of a galaxy but is entirely devoid of stars. Almost all of the mass which makes up VIRGOHI 21 is expected to be in the form of dark matter and less than a fraction of a percent of its mass is ordinary matter. Dark matter is basically matter whose existence can only be inferred from its gravitational effects due to the fact that dark matter does not scatter nor emit electromagnetic radiation. Interestingly, a paper entitled “Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21” suggests that VIRGOHI 21 may not be a genuine dark galaxy and instead, it could be the result of a high-speed collision between two large galaxies.

Located half a million light years from VIRGOHI 21 is a large spiral galaxy called NGC 4254 and a filamentary structure of hydrogen gas connects VIRGOHI 21 with NGC 4254. This trail of hydrogen gas is almost devoid of stars and its velocity distribution is coherent with the outer disk of the spiral galaxy NGC 4254 to which it is morphologically connected. Furthermore, a tidal origin for this trail of hydrogen gas is unlikely since a counter trail is nonexistent in the opposite direction from the spiral galaxy NGC 4254. Instead, such a feature is consistent with a high speed collision between the spiral galaxy NGC 4254 and another galaxy since an event like this will cause little disturbance to the stars in the main disk of the spiral galaxy NGC 4254, resulting in the lack of stars in the trail of hydrogen gas that connects VIRGOHI 21 with NGC 4254. A high speed collision with another galaxy will also create a counter trail of hydrogen gas that is much fainter and shorter than the main trail. This counter trail will quickly fall back into the disk of the parent spiral galaxy and in a few hundred million years after the collision, a galaxy with just one trail of hydrogen gas will be observed.

The interloper galaxy which collided with the spiral galaxy NGC 4254 is probably a few million light years away by now since the collision is expected to occur at a velocity on the order of a thousand kilometers per second and it is estimated that a couple of billion years would have already elapsed since the collision. VIRGOHI 21 is located along the trail of hydrogen gas and the velocity distribution within VIRGOHI 21 differs remarkably from the rest of the trail. This can occur when denser parts of the trail contract and become self-gravitating. Eventually, a region like this can become an independent object with the mass of a dwarf galaxy, resulting in an object like VIRGOHI 21.

Observing the composition of the filamentary structure which connects VIRGOHI 21 to the spiral galaxy NGC 4254 can provide further evidence to proof if VIRGOHI 21 is a genuine dark galaxy or if it originated from a high speed collision between two galaxies. This is due to the assumption that genuine dark galaxies will be made up of pristine metal-poor gases as there will be no stars to fuse the hydrogen and helium into heavier elements. On the other hand, if VIRGOHI 21 formed out of matter spewed out from the spiral galaxy NGC 4254 after a high speed collision with another galaxy, VIRGOHI 21 will be observed to be enriched with elements heavier than hydrogen and helium from the many episodes of stellar fusion prevalent in the main stellar disk of NGC 4254. In conclusion, a high speed collision could provide an explanation for the origin of putative dark galaxies such as VIRGOHI 21.

Thursday, December 16, 2010

Consuming Planets

Red giant stars have diameters of around tens to hundreds of times larger than that of the Sun and they occur when stars like the Sun eventually exhaust the supply of hydrogen in their cores and switched to fusing hydrogen in a shell external to the core. The increased temperatures and reaction rates causes the star to expand into a red giant and as the star expands, it spins down due to the conservation of angular momentum. Therefore, red giant stars are expected to rotate much more slowly about their spin axis as compared to stars like the Sun.

An unusual class of red giant stars known as red giant rapid rotators are basically red giant stars that are known to spin much faster than what is predicted for them. Ordinary red giant stars have equatorial velocities of around 2 kilometres per second while red giant rapid rotators have equatorial velocities of around 10 kilometres per second or more. In a paper by Joleen Carlberg, et al. (2010) entitled “The Fate of Exoplanets and the Red Giant Rapid Rotator Connection”, it is suggested that as a red giant star expands; it can consume and accrete planets that happen to be orbiting in close vicinity. Planets accreted in this way can dump sufficient angular momentum into the red giant star and cause the star to spin up to become a red giant rapid rotator.

This mechanism of accreting planets only works for planets whose orbital periods are shorter than the rotational period of their host stars. In other words, the time it takes for the planet to orbit once around its star has to be shorter than the times it takes for the star to complete one rotation about its spin axis. In such a configuration, the tidal bulge raised on the star by the orbiting planet will always be trailing the planet and this allows angular momentum to be transferred from the orbiting planet to the rotation of the star. This causes the planet to lose orbital angular momentum, fall closer towards its host star and eventually getting accreted by the star.

The amount of angular momentum that is dumped into a red giant star by an accreted planet can be many times greater than the angular momentum of the star itself. In our solar system, the Sun holds less than 2 percent of the total angular momentum while the planet Jupiter holds 60 percent of the total angular momentum. However, the orbit of Jupiter is too distant for it to get consumed by the Sun when the Sun expands into a red giant star billions of years from now.

Most of the 500 or so extrasolar planets known to date are Jupiter-like planets which orbit very close to their parent stars, many of which have orbital periods in the range of a few days. These planets are termed hot-Jupiters and they form a large proportion of the currently known planets due to observational biases as these planets are the easiest to detect. Such a hot-Jupiter can dump a huge amount of angular momentum into its host star via accretion when the star expands into a red giant, turning the red giant into a rapid rotator. For example, a Jupiter-mass planet in a Mercury-like orbit around a star that is identical to our Sun will have about 10 times more angular momentum than the star itself.

The lithium abundance of a red giant rapid rotator can also provide further evidence to correlate it with accreted planets. Red giant stars are known to be depleted in lithium due to convective mixing and the accretion of a Jupiter-mass planet can significantly raise the lithium abundance of the red giant star. However, a better understanding of stellar evolution is still required to ensure that any observed lithium abundance or any other observed abundance anomalies are indeed anomalous for a given red giant rapid rotator such that it can be attributed to an accreted planet.

Friday, December 10, 2010

Historic Flight

SpaceX successfully launched its Dragon spacecraft into low-Earth orbit atop a Falcon 9 rocket on Wednesday, 8 December 2010 at 10:43 AM EST (4:43 PM UTC) from Launch Complex 40 at Cape Canaveral Air Force Station in Florida. The Falcon 9 rocket inserted the Dragon spacecraft into an orbit with a low point of 288 kilometers, a high point of 301 kilometers and an orbital inclination of 34.53 degrees. This orbit is remarkably close to the targeted orbit which called for an almost circular orbit 300 kilometers above the Earth’s surface with an orbital inclination of 34.5 degrees. Traveling at a velocity of nearly 28000 kilometers per hour, the Dragon spacecraft made almost two orbits around the Earth before reentering the Earth’s atmosphere and eventually landing on the surface of the Pacific Ocean at 3 hours and 19 minutes after liftoff.

This launch event marks the first time in history a commercial company has successfully recovered a spacecraft reentering from low-Earth orbit. Such a feat has been performed by only six nations or government agencies: the United States, Russia, China, Japan, India and the European Space Agency. Wednesday’s launch of the Dragon spacecraft marks a historic first for the future of space travel. No one was on onboard the Dragon spacecraft on its maiden flight even though the spacecraft has enough room for 7 astronauts. The entire mission from launch to splashdown in the Pacific Ocean was flawless and if there had been people in the Dragon spacecraft, they would have enjoyed the whole ride.

Timeline of Events:
T+00:00:00 – Liftoff
T+0:02:58 - 1st Stage Shut Down (Main Engine Cut Off)
T+0:03:02 - 1st Stage Separates
T+0:03:09 - 2nd Stage Engine Start
T+0:09:00 - 2nd Stage Engine Cutoff
T+0:09:35 - Dragon Spacecraft Separates from Falcon 9
T+0:13 - On-Orbit Operations
T+2:32 - Deorbit Burn Begins
T+2:38 - Deorbit Burn Ends
T+2:58 - Reentry Phase Begins (Entry Interface)
T+3:09 - Drogue Chute Deploys
T+3:10 - Main Chute Deploys
T+3:19 - Water Landing

As the Dragon spacecraft reenters the Earth’s atmosphere at a velocity of over 7 kilometers per second, the spacecraft experiences temperatures of around 2000 degrees Centigrade. To keep the interior of the spacecraft at room temperatures, against the ferocious heating during reentry, SpaceX worked with NASA to create a phenolic impregnated carbon ablator (PICA) heat shield called PICA-X. This heat shield is probably the most advanced heat shield ever to fly as it can be reused hundreds of times with little degradation, somewhat like an “on steroids” version of a Formula One racing car’s carbon brake pads.

Dragon is a reusable spacecraft that was developed by SpaceX under NASA’s Commercial Orbital Transportation Services (COTS) program and it was initially conceptualized by SpaceX in 2005. The Dragon spacecraft is made up of a pressurized capsule and an unpressurized trunk for the transportation of pressurized cargo, unpressurized cargo and/or crew members to low-Earth orbit. Basically, the Dragon spacecraft has 10 cubic meters of pressurized volume, 14 cubic meters of unpressurized volume and it can support up to 7 passengers in crew configuration. The crew and cargo versions of the Dragon spacecraft are designed to be nearly identical to facilitate a rapid transition between cargo and crew.

SpaceX’s Falcon 9 rocket was used to launch the Dragon spacecraft into space on this historic voyage. The Falcon 9 rocket is basically a two stage launch vehicle that is powered by liquid oxygen and rocket grade kerosene (RP-1). The first stage rocket booster is powered by nine Merlin 1C rocket engines which generate a combined thrust of 5 million Newton at liftoff, while the second stage rocket booster is powered by a single Merlin Vacuum rocket engine which generates a thrust of 411 thousand Newton in a vacuum. The Falcon 9 rocket can launch over 10 metric tons into low-Earth orbit and a yet to be launched heavy variant called the Falcon 9 Heavy can launch almost 30 tons into low-Earth orbit.

SpaceX is developing a family of launch vehicles and spacecraft that will increase reliability and performance of space transportation, while ultimately reducing costs by a factor of ten. Next year, the Falcon 9 rocket and the Dragon spacecraft will start delivering cargo, including live plants and animals to and from the International Space Station for NASA. Both the Falcon 9 rocket and the Dragon spacecraft were developed to one day carry astronauts.

SpaceX has also revealed plans for future rocket designs, namely the Falcon X, Falcon X Heavy and Falcon XX. All these launch vehicle designs are in the heavy-lift to super heavy-lift range. The Falcon X can deliver up to 38 metric tons to low-Earth orbit while the Falcon X Heavy can deliver up to 125 metric tons to low-Earth orbit. Finally, the Falcon XX is a behemoth which can deliver up to 140 metric tons to low-Earth orbit. If developed, SpaceX’s Falcon X Heavy and Falcon XX will be among the largest and most powerful rockets ever built, with the long retired legendary Saturn V rocket being the closest rival. For comparison, NASA’s Space Shuttle can deliver 24 metric tons to low-Earth orbit while the Saturn V rocket can deliver a massive 120 metric tons to low-Earth orbit.

Saturday, December 4, 2010

Dark Matter

The existence of dark matter in the universe can only be inferred from its gravitational effects on ordinary matter and on electromagnetic radiation. This is so because dark matter can neither emit nor scatter electromagnetic radiation. Dark matter constitutes 80 percent of the matter in the universe while ordinary matter makes up the remaining 20 percent. Ordinary matter is basically everything which makes up the Earth, the planets, the stars and the vast quantities of gas and dust across interstellar and intergalactic space.

Although ordinary matter can account for a tiny proportion of dark matter, the vast majority of dark matter is made up of something else entirely. While the properties of dark matter can be somewhat constrained, the particle constituents of dark matter continue to elude detection. Dark matter is not made up of atoms and it does not interact with ordinary matter via electromagnetic forces. Hence, the study of dark matter has so far been largely based on the observable gravitational effects that dark matter imposes on ordinary matter and on electromagnetic radiation.

There are a number of independent sources of evidence for the existence of dark matter. Stars are known to orbit around the centre of galaxies and their orbital speeds do not decrease with increasing distance from the galactic centre. This is rather unexpected because the galaxy must have much more mass than can be attributed to ordinary matter alone; otherwise the orbital speeds of stars should decrease with increasing distance from the galactic centre. Thus, dark matter is responsible for the additional mass that can’t be attributed to ordinary matter alone.

Gravitational lensing is another independent piece of evidence for the existence of dark matter and this phenomenon occurs when light from a background object gets deflected by the gravitational field of a foreground object. This can distort the image of the background object and also change its observed brightness. A more massive foreground object will create a more pronounced gravitational lensing effect. Observations of gravitational lensing by foreground galaxies have shown that the amount of mass required by the galaxy to generate the observed lensing far exceeds the combined mass of all its stars and ordinary matter.

A paper entitled “Planet-Bound Dark Matter and the Internal Heat of Uranus, Neptune, and Hot-Jupiter Exoplanets” by Stephen L. Adler from the Institute for Advanced Study at Princeton explores the possibility that the accretion of planet-bound dark matter by gas giant planets could significantly contribute to their internal heat.

The Milky Way galaxy is situated at the centre of a vast halo of dark matter. The dark matter in the vicinity of the solar system is believed to be distributed in a way such that a cubic volume of space measuring 10000 by 10000 by 10000 kilometres contains around 500 grams of dark matter. Although this may seem sparse, scaling up the volume of space to one cubic light year will give a mass of dark matter this is around 80 times the mass of the Earth. One light year is the distance light travels in a period of one year. The dark matter in the vicinity of the solar system orbits around the galactic centre of mass along with the solar system.

It is not known if there is dark matter that is gravitationally bound to the Sun or to the planets in the solar system. Gravitational conglomerations such as stars and planets can accrete the ambient galactic dark matter over time such that Sun-bound and planet-bound dark matter can have densities that are many orders of magnitude greater than the ambient density of dark matter.

Planet-bound dark matter can contribute to internal heating of the planet by depositing energy inside the planet as the dark matter particles lose orbital energy by interacting with the particles of ordinary matter that make up the planet. Planet-bound dark matter can also contribute to internal heating if the particles that comprise them are self-annihilating. When self-annihilating dark matter particles meet, they annihilate each other and convert their mass into energy which can be deposited within the planet as internal heating.

Interestingly, the contribution to internal heating of a planet by the accretion of dark matter can explain the anomalously low rate of internal heat production for Uranus as compared to Neptune. Uranus has an almost identical internal structure and composition as Neptune and it should be producing the same amount of internal heating as Neptune. However, one key difference between Uranus and Neptune is that Uranus is tiled 98 degrees with respect to the plane of the solar system, whereas Neptune is only tiled 28 degrees. For comparison, the Earth has an axial tilt of 23.4 degrees.

The large axial tilt of Uranus is believed to be caused by a massive impact event, whereby an object around the mass of the Earth slammed into Uranus. This impact event could have pushed Uranus out of its accreted planet-bound cloud of dark matter and leave it with a much lower rate of internal heat production than Neptune. Before the impact event, Uranus would have a similar rate of internal heat production as Neptune.

Sunday, November 21, 2010

Faraway Sedna

It is amazing to see how much our view of the solar system has changed over the past few years. Once upon a time, the solar system was known to be just a system of several planets in neat orbits around the Sun, together with a population of asteroids and comets. Back then, not much was known to exist beyond Pluto and the solar system seemed to be a simple place to be in.

Today however, the solar system is far from being a simple place as new objects are frequently being discovered as far out as Pluto and beyond. In fact, Pluto is far from being at the edge of the solar system as a huge number of newly discovered worlds are known to exist far beyond Pluto. Many of these newfound worlds rival Pluto in size and in 2005, a newly discovered object named Eris is found to be more massive and probably larger in size than Pluto.

This population of objects that orbit the Sun beyond Neptune are knows as Trans-Neptunian objects (TNOs) and they include objects such as Pluto and Eris. Several TNOs are known to be over 1000 kilometers in diameter and many more of such large TNOs are yet to be discovered. Since the discovery of Pluto in the 1930s, one of the most intriguing discoveries of a TNO was of an object named Sedna in 2003.

What makes the discovery of Sedna so interesting is its extremely elongated and far-flung orbit that is unlike any other TNOs. Sedna’s orbit brings it as close as 76 AU from the Sun out to as far as 960 AU from the Sun and it takes Sedna around 12 thousand years to complete one orbit around the Sun. An AU is a unit of measurement and one AU is basically the mean distance of the Earth from the Sun. When Sedna was discovered in 2003, it was located at a distance of 90 AU from the Sun and approaching perihelion. At its furthest distance of 960 AU, the Sun will appear as a point of light with less than half the brightness of the full moon.

Sedna is so distant that it never comes close enough to Neptune for it to be gravitationally scattered by Neptune into its current highly elongated orbit. In fact, the Earth comes closer to Neptune than Sedna ever does! Since its discovery in 2003, the answer as to how Sedna got kicked into its crazily elongated orbit is still not yet known, making it probably the only known object in the solar system whose orbit cannot be explained. Is something lurking in the outer parts of the solar system that could account for Sedna’s orbit?

Sedna could not have formed in it current orbit since the large relative velocities between planetesimals would have been disruptive rather than constructive. Hence, Sedna’s initial orbit must have been circular otherwise its formation by the accretion of planetesimals would not have been possible. A number of possibilities have been thrown in that might explain Sedna’s intriguing orbit.

The first possibility is that there is a large Earth-sized planet orbiting the Sun beyond Neptune that could have gravitationally scattered Sedna into its current orbit. This hypothesis might be a long shot because any Earth-sized planet located within 100 AU would have been easily detected, especially from its gravitational interactions with other TNOs. However, such a planet might once exist but may have been ejected from the solar system after the formation of the Inner Oort Cloud. The ejection of this planet would not substantially modify the orbits of the objects that have been scattered into Sedna-like orbits.

The second possibility that might explain Sedna’s odd orbit is a chance close encounter with a passing star. Such a star would have to come as close as 200 AU to 1000 AU from the Sun in order to excite TNOs into Sedna-like orbits. An encounter like this would have been “extremely close” give that the closest stars are already a few hundred thousand AU distant. In fact, the probability for such a close encounter in the past 4.5 billion years of the solar system’s history is around 1 percent. This is probably not good odds to base a theory on. For the second possibility, it can also be that Sedna once orbited a brown dwarf or a low mass star, and it was stripped from its parent star when it came too close to the Sun.

The third possibility, which is also the most likely one, assumes that the Sun was formed on a dense cluster of stars and perturbations from numerous neighboring stars gradually excited Sedna into its current elongated orbit. The view from inside one of these clusters would have been an incredibly awesome sight. After 4.5 billion years, the stars that once formed this cluster would have been long lost amongst the hundreds of billions of stars in the vast Milky Way galaxy. If this third possibility is true, then Sedna could serve as a “fossil record” of what happened during the Sun’s birth 4.5 billion years ago!

With two-thirds the diameter of Pluto, far-flung Sedna is already an interesting world in its own right. However remote the possibility may be, the thought that Sedna once orbited another star is rather fascinating because that will make Sedna the first known extra-solar dwarf planet in the solar system. What is Sedna trying to tell us? With just a single object, there will be no way of finding out and the next practical step will be to continue to search the skies for more objects like Sedna. All these explain why I personally think that Sedna is the most interesting TNO discovered so far since the discovery of Pluto.

Saturday, November 13, 2010

Weighing Up Extrasolar Moons

An extrasolar planet is a planet which orbits a star other than the Sun and from the Paris Observatory’s online Extrasolar Planets Encyclopedia, there are about 500 known extrasolar planets as of November 2010. This number is expected to increase dramatically in the next several months with follow-up observations of the hundreds of candidate transiting extrasolar planet released in the first data set by NASA’s Kepler space observatory - a ‘planet hunting’ space telescope. A transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit happens to bring it in front of the star.

As the number of known extrasolar planets continues to increase rapidly, it undoubtedly brings up the possibility of detecting moons orbiting around these extrasolar planets. Detecting moons around extrasolar planets will be very challenging since such objects are expected to be smaller and less massive than the Earth. However, NASA’s Kepler space observatory might have the sensitivity necessary to detect the largest of such moons around extrasolar planets. Moons around extrasolar giant planets that are close to the size of the Earth can be particularly interesting because a large number of extrasolar giant planets are know to orbit their parent stars at ‘comfortable’ distances where Earth-like surface conditions are possible on such moons!

Recently, I did some research on transit timing variations (TTV) and transit duration variations (TDV) caused by the presence of a planet’s moon perturbing the periodic transit of the planet in front of its parent star. I used the methods outlined in two papers published by David M. Kipping in 2008 and in 2009 respectively, and wrote a program which allows me to play around with the parameters. I used stars, planets and moons of different masses in various combinations and orbital configurations. Additionally, I also used various TTV and TDV inputs to determine the corresponding mass of the moon and the corresponding planet-moon orbital configuration that is responsible the various signals.

In one of my analysis, I have a star with the mass of our Sun and a planet with the mass of the Earth which orbits the star at a mean distance of 100 million kilometers. This planet has a moon that is one-twelfth its mass and the moon orbits the planet at a mean distance of 130000 kilometers. It is also assumed that the planet takes 40000 seconds to transit in front of its parent star. As the periodic transits of the planet in front of its parent star is measured, the moon will induce an observed TTV of around 20 seconds and a TDV of around 35 seconds.

In light of a paper by David M. Kipping (2010) entitled “How to Weigh a Star Using a Moon”, I wrote a separate program to study the methods outlined in this paper. Basically, if a star has a planet, and if that planet has a moon, and if both of them transit in front of their parent star, then the sizes and masses of the star, planet and moon can be precisely measured. Furthermore, knowing the size and mass of an object allows its bulk composition to be constrained. This particular method employs the TTV and TDV signals, and it requires a star to have both a planet and moon that transit it. Although no star is yet know to have both a planet and moon that transit it, NASA’s Kepler space observatory is expected to discover several of such systems.

This method of measuring the mass of a moon of an extrasolar planet is rather interesting because such a moon is likely to be less massive than the Earth and the mass of such an object will not be measurable with radial velocity measurements. Therefore, a method like this offers a means to accurately pin down the masses and sizes of the star, planet and moon respectively. The masses of moons measured in this way could well be the smallest masses that can be directly measured outside of our Solar System.

Thursday, October 21, 2010

Interstellar Traverse

The desire to reach for the sky runs deep in our human psyche.
- Cesar Pelli

Interstellar space travel refers to unmanned or manned travel to the stars and it is vastly more difficult that interplanetary space travel as the distances involved are many orders of magnitude greater, even for the nearest stars. The distances to the stars are so immense that a light-year is employed as the unit of measurement, where one light-year is the distance a beam of light travels in one year and it has a value of 9.46 trillion kilometers.

Alpha Centauri is one of the closest stars and it is already located at a distance of 4.37 light-years or 41.34 trillion kilometers away from us. To put this impressive distance into perspective, 41.34 trillion kilometers is over a billion times the circumference of the Earth, or over 100 million times the distance of the Moon from the Earth. Even traveling at a velocity of 100 kilometers per second, it will take over 13000 years to traverse that distance! Hence, in order to reach the nearest stars within a reasonable amount of time, a spacecraft will have to be accelerated to much larger velocities and this is where the immense difficulty of interstellar space travel arises.

If the total worldwide energy consumption in 2009 were used to accelerate a 10 ton spacecraft, it will only accelerate the spacecraft to a velocity of only 10 percent the speed of light and that spacecraft will still have to take over 4 decades to reach Alpha Centauri. Furthermore, upon reaching Alpha Centauri, the spacecraft will not be able to spend any meaningful amount of time at its destination since it will simply speed past Alpha Centauri at 10 percent the speed of light unless a similar amount of energy is employed to decelerate the spacecraft.

In this article, I will assume that the immense scientific and technological barriers of interstellar space travel have been crossed and the capability to accelerate to near the speed of light is possible. This possibility is enabled by having a propulsion system that can generate exhaust velocities at close to the speed of light and some hypothetical form of antimatter-based propulsion system can be a possible candidate. It should be noted that the speed of light in a vacuum is exactly 299792458 meters per second since one meter is officially defined as the distance traveled by light in a vacuum in 1/299792458 of a second.

To begin, I shall describe a set of equations that I developed not long ago which extends the classical rocket equations into the relativistic regime. In other words, the relativistic rocket equations that I have derived account for the effects of relativity as the rocket’s velocity approaches a significant fraction of the speed of light and such relativistic effects include time dilation and length contraction. Additionally, I have also written a program which employs the equations to compute the characteristic of various mission scenarios.

In almost all other literature that I have reviewed, a constant acceleration is assumed for the relativistic rocket equations. However, in the equations that I have derived, a constant proper thrust is assumed rather than a constant acceleration because in practice, it is more realistic for a rocket to maintain a constant thrust rather than having a varying thrust to maintain a constant acceleration. It should be noted that the thrust is constant from the perspective of an observer traveling together with the rocket. This observer will also experience a gradual increase in acceleration as the total proper mass of the rocket decreases due to the burning of propellant, while the thrust remains constant throughout.

Using the set of equations and the computer program that I have developed, I will start with an unmanned spacecraft that has a total initial mass of 1 million kilograms (one thousand metric tons). This spacecraft is in orbit around the Earth and it is poised for a one-way journey to the stars. Which destination should the spacecraft visit? Alpha Centauri? Tau Ceti? Sirius? In this mission, I shall choose the red dwarf star Gliese 581 as the interstellar destination for the spacecraft.

… to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no one has gone before.
- Gene Roddenberry

Why Gliese 581? The reason is that Gliese 581 has a total of six known planets in orbit around it and in my previous post, I wrote about one of the planets which is the most Earth-like one discovered so far. This planet is designated Gliese 581 g and it orbits Gliese 581 at a comfortable distance where the temperatures are estimated to be just right to support Earth-like conditions. The star Gliese 581 is located 20.3 light years or 192 trillion kilometers away from us and the spacecraft will need to accelerate to close to the speed of light to get there within a reasonable period of time. Upon reaching Gliese 581, the spacecraft will also have to decelerate itself from its incredibly huge velocity so that it will not merely zip pass Gliese 581.

As stated previously, the spacecraft has a total initial mass of 1 million kilograms and most of which is in the form of fuel. The spacecraft also has a propulsion system which can generate an exhaust velocity that is 80 percent the speed of light. Furthermore, the spacecraft’s propulsion system is able to generate a constant 24 million Newton of thrust and this force is equivalent to approximately 7 times the weight of a Boeing 747 airliner. It is important to note that the mass of the spacecraft and the generated thrust is measured from the perspective of an ‘observer’ traveling with the spacecraft since the effect of relativity will give a different measured reading for an observer at rest.

To generate a constant 24 million Newton of thrust, the spacecraft will have to burn its propellant at a rate of 0.1 kilograms per second and direct the high energy exhaust out at 80 percent the speed of light. From rest, the spacecraft will accelerate at a constant thrust for a total duration of 7.5 million seconds (86.8 days) as measured from onboard the spacecraft. However, due to the effect of relativistic time dilation, 8.6 million seconds (99.6 days) would have already elapsed on Earth during the entire acceleration phase!

Initially, the spacecraft will experience an acceleration of 2.40 g’s which gradually increases to 9.59 g’s at the end of the acceleration phase because the proper mass of the spacecraft decreases while the thrust remains constant. At the end of the acceleration phase, the spacecraft will attain a final velocity of 240949550 meters per second which is slightly over 80 percent the speed of light. During the acceleration phase, the spacecraft would have already traveled over a trillion kilometers, or approximately one-tenth of a light year.

The spacecraft then shuts off its engine and begins its high speed cruise across the vast expanses of interstellar space, towards the direction of Gliese 581. It should be noted that the total mass of the spacecraft is now 250 thousand kilograms. Cruising at an incredible velocity of 240949550 meters per second, the spacecraft still has to take 25 years to get to Gliese 581! Additionally, the effect of relativistic time dilation means that only 15 years would have elapsed for a hypothetical observer onboard the spacecraft.

Upon reaching Gliese 581, the spacecraft will turn on its engine and commence its deceleration phase with the same constant thrust of 24 million Newton. The spacecraft will take 1.875 million seconds (21.7 days) to decelerate from 80 percent the speed of light so that it will be slow enough to enter orbit around Gliese 581. However, due to relativistic time dilation, 2.151 million seconds (24.9 days) would have elapsed back on Earth during the entire deceleration phase. Initially, the spacecraft will experience a deceleration of 9.59 g’s which gradually increases to 38.4 g’s at the end of the deceleration phase because the proper mass of the spacecraft decreases while the thrust remains constant. The spacecraft would have traveled another quarter of a trillion kilometers during the deceleration phase.

Orbiting around Gliese 581, the spacecraft now has a total mass of just 62.5 thousand kilograms as 93.75 percent of its initial mass is basically the propellant required for the journey. It is up to you to imagine the various kinds of payloads that can makeup the 62.5 metric tons of the spacecraft’s final mass. Due to the finite speed of light, the Earth will only receive the first signals from the spacecraft 20.3 years after the spacecraft has reached Gliese 581…

Now when we think that each of these stars is probably the centre of a solar system grander than our own, we cannot seriously take ourselves to be the only minds in it all.
- Percival Lowell

Friday, October 8, 2010

Resembling Earth

For many planet hunters, though, the ultimate goal is still greater (or actually, smaller) prey: terrestrial planets, like Earth, circling a star like the Sun. Astronomers already know that three such planets orbit at least one pulsar. But planet hunters will not rest until they are in sight of a small blue world, warm and wet, in whose azure skies and upon whose wind-whipped oceans shines a bright yellow star like our own.
- Ken Croswell

Located 20 light-years or 190 trillion kilometers away is a humdrum red dwarf star called Gliese 581. As of October 2010, the star Gliese 581 has a total of six known planets in orbit around it and one of which is the most Earth-like planet discovered so far. This planet is designated Gliese 581 g and it is the fourth planet from its parent star. Gliese 581 g orbits its parents star at a distance of 22 million kilometers, taking 36.6 days to complete one orbit.

The orbit of the planet Gliese 581 g is located well within the habitable zone where the distance from its parent star is just right to support Earth-like surface temperatures. Thus, Gliese 581 g is located neither too close nor too far from its parent star. Since the star Gliese 581 is much less luminous that our Sun, the planet Gliese 581 g is able to support Earth-like surface temperatures even though it is located much closer to its parent star than our Earth is from the Sun.

In addition to orbiting its parent star within the “Goldilocks Zone”, the mass of Gliese 581 g is estimated to be between 3.1 to 4.3 times the mass of the Earth. If Gliese 581 g is a dense rocky planet like the Earth, its diameter will be somewhere between 1.3 to 1.5 times of the Earth’s diameter. The surface gravity of Gliese 581 g is also expected to be between 1.1 to 1.7 times the surface gravity of the Earth, making it not too different from the Earth. In fact, it will not be much of a problem for a human being to walk on the surface of Gliese 581 g.

With an Earth-like greenhouse effect, the average surface temperature of Gliese 581 g is estimated to be between 236 to 261 degrees Kelvin. In comparison, the average surface temperature of the Earth is 288 degrees Kelvin or 15 degrees Centigrade. However, because Gliese 581 g is more massive than the Earth, it is possible that the planet will have a more massive atmosphere which can create a larger greenhouse effect than Earth’s atmosphere. This can increase the average surface temperature of Gliese 581 g closer to the average surface temperature of the Earth.

One key difference between Gliese 581 g and the Earth is that Gliese 581 g is probably tidally locked whereby the same hemisphere of the planet perpetually faces its parent star. This is somewhat like the Earth-Moon system where the same side of the Moon always faces the Earth. In such a scenario, one side of Gliese 581 g will be in eternal daylight while the other side will experience eternal night. On such a world, temperatures can range from blazing hot at the sub stellar point on the day side to freezing cold on the night side. The sub stellar point on the surface of Gliese 581 g is where its parent star is forever directly overhead and it is the spot with maximum insolation. Between the two extremes, Earth-like temperatures can exist where a world that is not too different from ours is both easily conceivable and highly probable.

The mere fact that a potentially habitable planet has been discovered so soon around such a nearby star, suggests that habitable planets are far more common than previously believed. This means that potential of having many billions of Earth-like planets in our Milky Way galaxy alone is extremely probable. The paper detailing this discovery is by Steven S. Vogt, at al. (2010) and it is entitled “The Lick-Carnegie Exoplanet Survey: A 3.1 M Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581”.

We live in a changing universe, and few things are changing faster than our conception of it.
- Timothy Ferris

Of the 500 or so known extrasolar planets, Gliese 581 g is probably the most interesting one discovered so far. Apart from the remarkable discovery of Gliese 581 g, a paper by Daniel Kubas, at al. (2010) entitled “A frozen super-Earth orbiting a star at the bottom of the Main Sequence” describes the discovery of a super-Earth which orbits a faint red dwarf star whose mass is close to the lower limit for a star.

This planet is designated MOA-2007-BLG-192Lb and its mass is 3.2 times the mass of the Earth. The planet orbits its parent star at a distance of about 100 million kilometers and this is about two-thirds the distance of the Earth from the Sun. However, because the parent star of MOA-2007-BLG-192Lb is a mere 8.4 percent the mass of the Sun, the planet receives over a thousand times less insolation that the Earth gets from the Sun even though it is located closer to its parent star than the Earth is from the Sun.

MOA-2007-BLG-192Lb was discovered using the gravitational microlensing technique as it provides a unique opportunity for the detection of low mass planets that are currently beyond the reach of most other methods. Gravitational microlensing occurs when a foreground star passes in front of a background star and the gravity of the foreground star acts as a lens and magnifies the apparent brightness of the background star. If the foreground star has a planet orbiting it, the gravity of the planet can induce a perturbation to the microlensing light curve. The duration of the perturbation depends on the mass of the planet, where a more massive planet will induce a perturbation with a longer duration. The discovery of the MOA-2007-BLG-192Lb shows that planet formation can occur down to the very low mass end of the stellar population.

The surface temperature of MOA-2007-BLG-192Lb is estimated to be around 55 degrees Kelvin and this is just below the melting temperature of pure nitrogen. However, internal heat generated from the decay of radioisotopes can raise the temperature on the surface of MOA-2007-BLG-192Lb to beyond the melting temperature of pure nitrogen. This can enable seas and oceans of liquid nitrogen to exist on the planet’s surface as long as the atmospheric pressure on the planet’s surface exceeds 0.1 bars.

The flow of internal heat onto the surface of a terrestrial planet can be strongly heterogeneous, making it highly probably that the surface temperatures on specific locations on MOA-2007-BLG-192Lb can exceed not just the melting point of nitrogen, but also the melting point of methane and even water. Therefore, lakes of liquid hydrocarbons like those on Saturn’s moon Titan can exist on MOA-2007-BLG-192Lb and open bodies of liquid water may even exist in volcanically active locales.

Friday, September 17, 2010

Birth of a Quark Star

Quarks are elementary particles and they are a fundamental constituent of matter. For instance, a neutron is made up of one up-quark and two down-quarks, and a proton is made up of two up-quarks and one down-quark. Neutrons and protons then make up the nuclei of atoms. Quarks have never been studied individually because when two quarks move apart, the force between them increases until it becomes more energetically favorable at some point for a new quark-antiquark pair to appear out of the vacuum than for the two quarks to continue separating. The phenomenon whereby quarks cannot be individually isolated is called color confinement and the phenomenon whereby quark-antiquark particles can appear out of the vacuum is called hadronization.

A quark star is a type of exotic star that is made up of ultra-dense quark matter and they are even denser than neutron stars. Given sufficient pressure from a neutron star’s immense gravity, individual neutrons can break down into their constituent quarks and a neutron star can turn into an even more compact quark star. A typical quark star has roughly the mass of the Sun packed into a diameter of only around 10 kilometers and just a cubic centimeter of its ultra-dense material can have a mass of a few billion metric tons!

Gamma-ray bursts are the most energetic electromagnetic events known to occur in the universe and they emit titanic bursts of gamma-rays which last anywhere from milliseconds to several minutes. Gamma-ray bursts are believed to be narrow bipolar beams of incredibly intense radiation created during powerful supernova explosions and a typical gamma-ray burst produces as much energy in a few seconds as the Sun does over its entire 10 billion years lifespan!

There are two types of gamma-ray bursts – the long duration gamma-ray bursts and the less common short duration gamma-ray bursts. Long duration gamma-ray bursts last longer than 2 seconds and they are generally linked to the deaths of very massive stars. Additionally, long duration gamma-ray bursts are followed by bright and lingering afterglows. On the other hand, short duration gamma-ray bursts last less than 2 seconds and they produce very little afterglows as compared to long duration gamma-ray bursts. The true nature of short duration gamma-ray bursts still remains an enigma and the leading hypothesis is that these events originate from the coalescence of binary neutron stars.

A gamma-ray burst is generally characterized by an initial powerful blast of gamma-rays followed by an afterglow with a rapidly decaying intensity. In this article, I will only focus on the afterglows of long duration gamma-ray bursts and the observed plateau in the light curves of a number of these gamma-ray burst afterglows. Such a plateauing of the afterglow light curve of a gamma-ray burst can be attributed to the cooling behavior of a newly formed quark star.

Immediately after a gamma-ray burst, the newly formed quark star cools by emitting vast amounts of neutrinos and photons. This initial afterglow phase is characterized by a light curve with a gradually decaying intensity. The light curve of the afterglow then stops decaying and plateaus out with a constant intensity. This observed phenomenon can be explained by the solidification of the quark star as it undergoes a phase transition from liquid to solid. The latent heat released during the phase transition can provide a steady and constant supply of energy to power the afterglow of the gamma-ray burst. This is because the temperature of the central quark star will remain constant as it undergoes its phase transition.

After the phase transition, the light curve of the afterglow abruptly decays due to the extremely low heat capacity of the solid quark star. The entire phase transition of the quark star from liquid to solid occurs over a timescale of roughly 1000 seconds and the amount of energy generated from the phase transition alone is roughly equal to the total amount of energy the Sun gives off over a period of 10 billion years!

Therefore, the amount of energy produced during the phase transition of a quark star is sufficient and steady enough to produce the plateau in the light curve observed in the afterglow of a gamma-ray burst. Gamma-ray bursts are the most powerful explosions in the universe and when they do occur, they blaze with the glory of a billion billion Suns. Nevertheless, as magnificent as they are, their fleeting nature makes them elusive and challenging to study.

Friday, September 3, 2010

Around Sagittarius A*

Our Sun is one of the hundreds of billions of stars in the Milky Way Galaxy and it is located approximately 26000 light years from the center of the galaxy. One light year is the distance light travels in a year and its value is 9.46 trillion kilometers. A supermassive black hole named Sagittarius A* sits right in the center of the Milky Way Galaxy and this colossal black hole is estimated to have a mass of four million Suns.

Located within the close vicinity of Sagittarius A* is a very intriguing group of stars called the “S stars”. These stars are the closest known stars to the center of the Milky Way Galaxy and they orbit around Sagittarius A* at very high orbital velocities. Each of the “S stars” are several times more massive than our Sun and being much more massive than our Sun, these stars undergo a much more rapid rate of nuclear fusion which means that they have a lifespan of just several million years. In comparison, our Sun has a lifespan of over 10 billion years.

The “S stars” are intriguing because the environment around such a supermassive black hole is hostile to the formation of stars and the “S stars” would have to form somewhere much further out before migrating to their current extraordinarily close proximity to Sagittarius A*. However, the timescales involved in such a migration is longer than the short ages of the “S stars” and hence, these “S stars” constitute a “paradox of youth”.

S2 is the designation given to one of the “S stars” and what distinguishes S2 from the other stars is that S2 is by far the closest star in orbit around the supermassive black hole - Sagittarius A*. S2 orbits Sagittarius A* in a highly elliptical orbit and in such an extreme gravitational environment near a supermassive black hole, S2 takes just 15.5 years to complete one orbit around Sagittarius A* even though S2 is located at an average distance of about 140 billion kilometers from Sagittarius A*. In comparison, Pluto orbits the Sun once every 248 years at an average orbital distance of 6 billion kilometers from the Sun.

At its closest approach, S2 comes within just 17 light-hours from Sagittarius A* and this is roughly three times the distance of Pluto from the Sun. The highly elliptical orbit of S2 also brings it out as far as 10 light-days from Sagittarius A*. During closest approach, S2 zips around Sagittarius A* at a incredible velocity of over 5000 kilometers per second (about 2 percent the speed of light). The remarkable orbit of S2 around Sagittarius A* makes it uniquely valuable for testing various general relativistic and even extra-dimensional effects.

Friday, August 13, 2010

Planets Orbiting White Dwarfs

An extrasolar planet is basically a planet which orbits a star other than our Sun and a transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit brings it in front of the star’s luminous disk. Current missions such as NASA’s Kepler space telescope are sensitive enough to detect transiting extrasolar planets that are as small as the Earth around Sun-like stars.

Transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to our line of sight. Stars have randomly orientated planetary orbits and the probability that a planet is observed to transit its parent star is inversely proportional to the distance of the planet from its star. Therefore, a planet orbiting at a smaller distance from its star will have a higher probability of being observed to transit its star as compared to a planet orbiting at a larger distance.

For stars like the Sun, the fraction of light that a transiting planet blocks is small because the size of the star is much larger than the size of the planet. For example, if the Earth were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 0.01 percent while if Jupiter were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 1 percent.

White dwarfs are basically the end result of the evolution of main sequence stars such as the Sun and stars with less than 8 times the mass of the Sun eventually end up as white dwarfs. The Sun is 333 thousand times more massive than the Earth and a typical white dwarf can contain as much mass as the Sun gravitationally compactified into a dense sphere that is approximately the size of the Earth. In comparison, the Sun has a diameter that is 109 times larger than the Earth’s and a volume that can fit 1.3 million Earths.

As a result, when it comes to the relative fraction of starlight that can be blocked by a transiting planet, white dwarfs do offer a huge advantage over main sequence stars like the Sun. This is because the size of a white dwarf is much smaller than the size of a main sequence star and this allows a much greater fraction of a white dwarf’s light to be blocked by a transiting planet to generate a proportionally stronger transit signal. In fact, a Jupiter-size transiting planet can completely block out the light from a white dwarf since such a planet will be larger in size than the white dwarf.

The transit of an Earth-size planet across the luminous disk of a white dwarf will block out a significant fraction of the white dwarf’s light. In certain cases, it is even possible for the Earth-size planet to completely block out the white dwarf. Even the transit of an object as small as the Earth’s Moon in front of a white dwarf will occult a few percent of the white dwarf’s light. In comparison, the transit of an object as small as the Moon in front of a star like our Sun will only block out a minuscule 6 parts-per-million of the star’s light and such a weak signal will probably be hardly distinguishable from background noise. Therefore, white dwarfs offer an enormous advantage over main sequence stars as a means to detect small planetary objects.

Nonetheless, the detection of small transiting planetary objects around white dwarfs will require a higher observational cadence as compared to the detection of planets around main sequence stars like our Sun. This is because the small size of a white dwarf means that any transiting planet takes on the order of only a few minutes to transit across the entire luminous disk of the white dwarf. In comparison, the transit of a planet across the luminous disk of a main sequence star like our Sun takes on the order of a few hours which permits a much lower observational cadence.

White dwarfs are the final result of the evolution of main sequence stars like our Sun and there are ways in which planets can exist around white dwarfs. Before a star becomes a white dwarf, it will undergo a red giant phase where it will swell to many times its original size and engulf or vaporize planets that might be orbiting it in close vicinity. However, planets that orbit further out and planets that are sufficiently massive can survive the star’s evolution to a white dwarf and continue to orbit the white dwarf.

As a star evolves to a white dwarf, it can lose a significant fraction of its mass and this can destabilize any system of planets that is in orbit around the star. In such a scenario, planets can gravitationally interact with each other and can either be scattered into a tighter orbit around the white dwarf or get boosted into a more distant orbit around the white dwarf. It is also possible for planets to get completely ejected from the system. Planets that are scattered into a tighter orbit around the white dwarf will improve their probability of being observed to transit the white dwarf because the transit probability of a planet is inversely proportional to the distance of the planet from its star.

Interestingly, there is also an alternate mechanism that can allow planets to exist around white dwarfs. When a closely spaced pair of white dwarfs eventually merges due to the loss of orbital energy via the emission of gravitational radiation, a second generation of planets can form out from the disk of debris from the tidal disruption of the lower mass white dwarf. Therefore, the existence of an entire second generation system of planets, moons and asteroids in close orbit around a white dwarf is quite plausible.

The detection of transiting planetary objects around white dwarfs will help constrain the effectiveness of the mechanisms in which planets can exist around white dwarfs. To detect such transits, a large number of white dwarfs have to be continuously sampled at a high observational cadence. Currently, NASA’s Kepler space telescope is most suited for the detection of transiting planetary objects around white dwarfs. This is because Kepler has an extremely high observational cadence since its CCDs are read out every six seconds and Kepler is theoretically sensitive enough to detect objects as small as asteroids as they transit in front of white dwarfs.

Friday, August 6, 2010

Rogue Worlds

The formation of planets in protoplanetary disks around stars is a messy process and a significant number of protoplanets with masses similar to planets such as the Earth or Mars may get ejected from their solar systems by gravitational interactions with massive gas giant planets. It is even possible that more protoplanets are ejected than retained in protoplanetary disks around stars and a considerable number of such ejected worlds might be wandering in the immense and dark expanses of interstellar space. Interstellar space is basically the vast and enormous spaces between stars.

An ejected planet with the mass of the Earth can retain an atmosphere of hydrogen in the frigid temperatures of interstellar space since the atmosphere of hydrogen will be cold enough to be bounded by the gravity of the planet. In comparison, at the distance where the Earth is from the Sun, a planet has to be over 10 times more massive than the Earth for its own gravity to be sufficiently strong enough to keep an atmosphere of hydrogen from escaping into space. In this article, I shall denote such ejected worlds as interstellar planets. From a distance, an interstellar planet will appear as a dark silhouette against a field of distant background stars.

The effective temperature of an interstellar planet is expected to be just several degrees Kelvin above absolute zero and any form of water at this temperature will be frozen solid. However, if an interstellar planet has a sufficiently thick atmosphere of hydrogen where the pressure at the bottom of such an atmosphere ranges from a hundred to a few thousand bars, the pressure-induced infrared opacity of molecular hydrogen will greatly insulate the planet from dissipating its internal radiogenic heat into space. At high pressures, molecular hydrogen is very effective at trapping heat and this significantly reduces the amount of heat lost into space.

With such an overlying atmosphere of hydrogen, the surface temperature of an interstellar planet can potentially exceed the meting point of water! In an environment like this, it will be possible for oceans of liquid water to exist on the planet’s surface and the ocean can be kept warm by the persistent flux of heat generated by the decay of radioactive isotopes in the planet’s interior.

On the surface of an interstellar planet, the sky will appear totally back and it is highly unlikely that stars will be visible from the planet’s surface due to the thick hydrogen atmosphere. However, areas of the planet’s surface can still be illuminated by occasional flashes of lightning. In the lower and warmer reaches of the atmosphere, it is possible for clouds of water droplets to form and drive a hydrological cycle. The atmospheric temperature decreases higher up in the atmosphere, possibly enabling other gases such as ammonia and nitrogen that have lower condensation temperatures to condense into clouds at these higher altitudes. Therefore, an interstellar planet can have several cloud layers of different condensates in its atmosphere.

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.
- Metrodorus of Chios, 4th century BC

An interstellar planet can provide a stable environment for life for billions of year until the surface temperature declines below the meting point of water as the heat generating radioisotopes in the interior of the planet slowly gets depleted. Interstellar planets will be extremely difficult to detect as the amount of radiation they emit is exceedingly miniscule compared to the large amount of solar radiation the Earth reflects back into space as the Earth basks in the warm vicinity of the Sun. Finally, if interstellar planets do exist in significant numbers in the vast and uncharted expanses between the stars, these dark worlds might serve as common sites for life-supporting environments in the universe.

Friday, July 30, 2010

Triton's Shooting Stars

Triton is by far the largest moon in orbit around the planet Neptune and with a diameter of 2700 kilometers; Triton is the seventh largest moon in the Solar System. Interestingly, Triton is the only large moon in the Solar System with a retrograde orbit, whereby it orbits Neptune in the opposite direction to Neptune’s rotation. Triton has a relatively high rock/ice ratio of approximately 70/30 and this composition is remarkably similar to large Kuiper Belt objects such as Pluto and Eris. Because of Triton’s retrograde orbit and its similar composition to objects such as Pluto and Eris, Triton is believed to be a Kuiper Belt object that was captured into orbit around Neptune.

Triton orbits Neptune at a distance of 354800 kilometers and it takes 5 days and 21 hours for Triton to orbit once around Neptune. Like the other large moons in the Solar System, Triton is tidally locked to Neptune where it keeps the same hemisphere oriented towards Neptune at all times. Triton was discovered by British astronomer William Lassell in 1846, only several days after Neptune itself was discovered. The first and only closed-up images of this elusive and far-flung moon of Neptune came on August 1989, after a close fly-by of Triton by NASA’s Voyager 2 spacecraft.

Apart from Saturn’s moon Titan, Neptune’s moon Triton is the only other moon in the Solar System with an appreciable atmosphere. Similar to both the Earth and Titan, nitrogen is the main constituent of Triton’s atmosphere. The atmospheric pressure on the surface of Triton is over 50000 times less than the atmospheric pressure at sea-level on the Earth. This is actually equivalent to the atmospheric pressure up in the Earth’s mesosphere at well over 50 kilometers above the Earth’s surface. Triton’s extremely frigid and cold environment allows the nitrogen in its atmosphere to be deposited onto the surface as frozen nitrogen. On the surface of Triton, the Sun will appear almost a thousand times dimmer than from the Earth’s surface.

The atmosphere of Triton contains thin clouds of nitrogen ice particles that are located a few kilometers above the surface. Above the clouds of nitrogen ice particles is a haze layer which extends up to 30 kilometers above Triton’s surface. This haze layer is made up of hydrocarbons and nitriles created when ultraviolet light from the Sun breaks down methane in Triton’s atmosphere. The surface of Titan also contains numerous erupting geysers of nitrogen gas. By itself, nitrogen gas is invisible and it is the entrained dust within the nitrogen gas which allows the geysers to be seen as plumes rising from the surface up to a height of 8 kilometers above Triton’s surface. The entrained dust within a plume can be deposited to over a hundred kilometers downwind of the plume and these plumes are responsible for creating the long and dark streaks on the surface of Triton.

Although Triton’s atmosphere is rather tenuous compared to the Earth’s atmosphere, it is still dense enough to ablate micrometeoroids when they pass through. The combination of a micrometeoroid’s orbital velocity around the Sun, the orbital velocity of Triton around Neptune, the orbital velocity of Neptune around the Sun and the gravitational accelerations of both Triton and Neptune, can allow the micrometeoroid to achieve a fast enough impact velocity that will enable the micrometeoroid to be sufficiently heated to visibility by friction with the air molecules in Triton’s atmosphere.

As Triton orbits Neptune, most micrometeoroids will impact the leading hemisphere of Triton and this is where most micrometeoroids can be observed. When a micrometeoroid penetrates an atmosphere, it heats up due to friction with the air molecules in the atmosphere. This causes material to sputter off the surface of the micrometeoroid particle in a process called ablation. Sufficiently strong heating and ablation can enable a micrometeoroid to become a visible meteor.

Due to the thin atmosphere of Triton and the much lower speeds at which micrometeoroids will impact the atmosphere of Triton as compared to the Earth, micrometeoroids can penetrate all the way down to the surface of Triton. This is unlike the Earth where micrometeoroids vaporize entirely high in the atmosphere. On Triton, visible meteor trails can extend all the way down to the surface. Icy micrometeoroids are expected to produce brighter meteor trails than stony micrometeoroids because icy micrometeoroids have a greater rate of ablation and the brightness of a meteor trail is directly related to the rate of ablation of the micrometeoroid.

Additionally, large variation in the brightness of meteors is expected to occur along different phases of Triton’s orbit around Neptune. The brightness of a meteor trail is expected to be the greatest when Triton’s orbital velocity around Neptune adds up most positively with Neptune’s orbital velocity around the Sun. The Neptune-Triton system is unique, because unlike other planet-satellite systems, it features a remarkably large variation in the meteoroid impact velocity onto Triton as a function of Triton’s orbital phase position around Neptune.

In addition to producing visible meteor tails, the ablation of incoming micrometeoroids can deposit metallic atoms and molecules that were once part of the micrometeoroid into the atmosphere of Triton. These metallic atoms and molecules can condense into dust particles in the atmosphere of Triton and these dust particles could serve as nucleation centers for the condensation and formation of cloud and haze particles observed in Triton’s atmosphere. Most of these metallic atoms and molecules will eventually get deposited together with the condensates onto the surface of Triton.

Saturday, July 24, 2010

Stellar Behemoth

The Tarantula Nebula is an extremely luminous nebula that is located approximately 165000 light years away in the Large Magellanic Cloud and it is the largest and most active star formation region known in the Local Group of galaxies. The Large Magellanic Cloud is basically a nearby irregular galaxy which is roughly one-tenth as massive as the Milky Way Galaxy.

At the heart of the Tarantula Nebula lies an exceptionally dense cluster of stars called R136 which generates most of the light that illuminates the Tarantula Nebula. 4 exceedingly massive and luminous stars sit in the core of the R136 star cluster and they are designated R136a1, R136a2, R136a3 and R136c respectively. Each star is well over 100 times more massive than the Sun and each star is millions of times more luminous than the Sun.

The most massive and luminous of the 4 central stars in the R136 star cluster is the star called R136a1. This behemoth is currently the most massive and luminous star discovered so far. R136a1 has a current mass that is 265 times the mass of the Sun and an initial mass that is estimated to be 320 times the mass of the Sun! Since its birth, R136a1 has shed over 50 times the mass of the Sun in extremely powerful stellar winds.

R136a1 also shines with an “off-the-charts” luminosity that is approximately 10 million times greater than the Sun’s luminosity! To put this extreme luminosity into perspective, R136a1 emits as much energy in 3 seconds as the Sun emits in an entire Earth year! With an age of just over a million years, R136a1 is already a middle-aged star. In comparison, our Sun is already 5000 million years old and the Sun is only halfway through its lifespan.

Although the R136 star cluster contains approximately 100000 stars, its 4 brightest stars (R136a1, R136a2, R136a3 and R136c) account for approximately half of the wind and radiation power of the entire cluster of stars! Because very massive stars are so exceedingly rare, it is unlikely that there is any other star in the Tarantula Nebula or possibly even in the entire Local Group of galaxies that will be comparable to the brightest components of the R136 star cluster. An ultra-massive star like R136a1 is considered to be a very extreme case for a star and R136a1 might well be one in a trillion.

Source: Paul A Crowther, et al. (2010), “The R136 Star Cluster Hosts Several Stars Whose Individual Masses Greatly Exceed the Accepted 150 M_Sun Stellar Mass Limit”, arXiv:1007.3284v1