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.

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.

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

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.

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.

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.

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.

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.

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.

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