Extrasolar Carbon Planets

In the inner solar system, the terrestrial planets - Venus, Earth and Mars are silicate planets as the bulk of their mass is primarily composed of silicon-oxygen compounds. These planets were formed from the coalescencing of planetesimals which condensed out of a protoplanetary disk of material orbiting the young Sun at around five billion years ago. In the case for the inner region of our solar system, the condensation of silicon-oxygen compounds to form silicate planets is the domineering process because the carbon to oxygen ratio of the protoplanetary disk in this region is only around 0.5, making oxygen the dominant component. In our region of the solar system, iron-peak elements condensed at the highest temperatures, followed by silicates at slight lower temperatures, water at 180 degrees Kelvin and eventually other volatiles such as ammonia and methane at lower temperatures. Hence, the Earth is comprised of an iron-nickel core within a large silicate mantle and topped on the exterior surface by water and other volatiles.

The condensation sequence of the material in a protoplanetary disk can be dramatically different if the carbon to oxygen ratio is above 0.98 whereby instead of silicates, the high temperature condensates will be carbon-rich compounds such as graphite and carbides, resulting in an entirely different class of planets. These planets are termed carbon planets where carbon is the most abundant component. A carbon planet will have an iron-nickel core just like our Earth. However, the layers of material surrounding the iron core will be very different as the mantle of a carbon planet will be comprised of silicon carbide and titanium carbide. Above the planet’s mantle, a layer of graphite will extend up to the surface of the planet, making up the crust of the planet. The deeper parts of this graphite crust will be subjected to high pressures and it will result in the formation of a global shell of crystalline diamond covering the entire planet.

The atmosphere of a carbon planet will be primarily composed of carbon monoxide or methane and the surface may be covered by precipitations of tar-like substances and other carbon-rich compounds. Such an atmosphere will be reducing instead of oxidizing. A carbon planet that orbits at a very close distance from its host star can loose its atmosphere due to atmospheric escape from the extreme heating, thereby directly exposing its solid surface to the vacuum of space. Such a carbon planet will remain exceptionally stable against the extreme heat as it will be protected by layers of heat resistant shells of graphite, silicon carbide or even diamond. In comparison, a silicate planet will have less protection due to the much lower melting and vaporizing temperatures of silicate compounds. The heat resistance of carbon compounds is exemplified in silicon carbide which is a ceramic used for lining the cylinders of automotive engines and in diamond which remains solid up to a temperature of around 4000 degrees Kelvin.

For a terrestrial planet like our Earth, the atmosphere is characterized by the presence of oxygen-rich gases such as carbon dioxide, oxygen and ozone. However, the atmosphere of a carbon planet will have an absence of these oxygen-rich gases and instead, the atmosphere will be dominated by carbon monoxide or by methane for a cold carbon planet. Cold and low mass carbon planets are conducive for the survival of long chains of photochemically synthesized carbon compounds. On such a planet, the temperatures can even be low enough for methane and ethane to condense and rain out of the atmosphere to form lakes and seas of hydrocarbons, similar to those found on Titan. Carbon planets are probably more common in regions closer towards the galactic centre because the stars there tend to contain a larger proportion of carbon as compared to stars like our Sun which is located further away from the galactic centre.

Worlds Like Titan

A reddish colour dominated everything, although swathes of darker, older material streaked the landscape. Towards the horizon, beyond the slushy plain below, there were rolling hills with peaks stained red and yellow, with slashes of ochre on their flanks. But they were mountains of ice, not rock. An ethane lake had eroded the base of the hills, and there were visible scars in the hills' profiles.

- Stephen Baxter, Titan

In human terms, Titan is a cold and frigid world with an average surface temperature of minus 180 degrees Centigrade and a surface atmospheric pressure that is 1.45 times the atmospheric pressure at sea-level on Earth. These conditions allow for the existence of liquid methane on Titan’s surface in the form of lakes and seas. A large number of these lakes and seas can be found in Titan’s north polar region and the largest of them is named Kraken Mare - a large sea of liquid methane and ethane that is estimated to be similar in size to the Caspian Sea on Earth. Titan is also characterised by a thick atmosphere which extends hundreds of kilometres above its surface and a global atmospheric haze layer that is transparent to infrared wavelengths but opaque to ultraviolet and visible wavelengths. In this article, I will be considering how Titan will be like if it were to orbit a red dwarf star instead of the Sun and also if it were a rogue planet wandering in the dark depths of interstellar space.

The global atmospheric haze layer of Titan blocks incoming ultraviolet and visible light but allows infrared radiation from the surface to freely escape into space, thereby creating an anti-greenhouse effect. In comparison, a greenhouse effect allows visible light in but blocks outgoing infrared radiation. The clouds in the atmosphere of Titan rain liquid methane and ethane, completing a ‘methanological cycle’ that is akin to the hydrological cycle on Earth. Benner et al. (2004) were the first to suggest that liquid methane on Titan could potentially be the basis for life there, playing the same role as water does for life on Earth. Methane-based life on Titan could consume organic molecules similar to Earthly life, but they would probably inhale hydrogen instead of oxygen and exhale methane instead of carbon dioxide. The discovery of any methane-based life on Titan will have incredibly interesting implications. In this article, it will be assumed that methane-based life on cryogenic Titan-like worlds is a possibility. Hence, the term liquid methane habitable zone (LMHZ) will correspond to Titan-like worlds while the term liquid water habitable zone (LWHZ) will correspond to habitable Earth-like worlds.

Suddenly I was aware of something new. The air in front of me had lost its crystal clearness… I was aware of a faint taste of oil upon my lips, and there was a greasy scum upon the woodwork of the machine. There was no life there. It was inchoate and diffuse; extending for many square acres and then fringing off into void. No, it was not life. But might it not be the remains of life? Above all, might it not be the food of life, a monstrous life, even as the humble grease of the ocean is the food for the mighty whale?

- Arthur Conan Doyle, The Horror of the Heights

Red dwarf stars have much lower masses than our Sun and they comprise the vast majority of stars. Being much more numerous that Sun-like stars, red dwarf stars are particularly interesting in the search for potentially habitable worlds; both in the LMHZ for Titan-like worlds and in the LWHZ for Earth-like worlds. The much lower luminosities of red dwarf stars mean that a planet orbiting a red dwarf star will have to be located much closer in just to receive the same amount of radiation as if it were located around the Sun. For a habitable Earth-like planet around a red dwarf star, the LWHZ will be situated very close to the star, causing the planet to be in a tidally locked state whereby one hemisphere of the planet perpetually faces its host star. However, the LMHZ for a Titan-like planet around a red dwarf star is located much further out from the star and this gives the planet a much better chance of not being in a tidally locked state, thereby creating a less stringent condition for life to exist.

The light from a red dwarf star contains a higher proportion of infrared radiation as compared to the light from the Sun. If Titan were orbiting around a red dwarf star instead of the Sun, a greater proportion of the light from the red dwarf star will reach the surface of Titan as the atmospheric haze of Titan is transparent to infrared wavelengths. If Titan is placed at an appropriate distance from the red dwarf star such that it receives the same amount of radiation as it currently receives from the Sun, the increased infrared fraction of the incoming radiation that makes it to Titan’s surface will warm the surface by an additional 10 degrees Centigrade or so. This warming effect is based on the assumption that a Titan-like world orbiting around a red dwarf star has a haze layer that is as thick as Titan’s. However, because red dwarf stars produce a lower proportion of ultraviolet light as compared to the Sun and because red dwarf stars can also produce a greater deal of high energy radiation that is associated with flares as compared to the Sun, the haze production rate for a Titan-like world in orbit around a red dwarf star can range from being much lower to much higher than that for Titan.

A habitable Titan-like world orbiting within the LMHZ of an M4-type red dwarf star will now be investigated. The M4-type red dwarf star is assumed to have a surface temperature of 3130 degrees Kelvin and a luminosity that is 2500 times less than the Sun’s. For a Titan-like world with a haze layer thickness that is reduced by a factor of 100 in comparison to Titan’s haze layer, it will have to orbit its parent M4-type red dwarf star at a distance of 0.23 AU in order to maintain a surface temperature of minus 180 degrees Centigrade. However, if the haze layer thickness of the Titan-like world is increased by a factor of 100 in comparison to Titan’s haze layer, the planet will need to orbit its parent M4-type red dwarf star at a much closer distance of 0.084 AU in order to maintain the same surface temperature. The temperature of minus 180 degrees Centigrade is the current surface temperature of Titan and it allows for the existence of liquid methane. Therefore, within a range factor of 10000 for the haze layer’s thickness, the liquid methane habitable zone (LMHZ) for a Titan-like world around an M4-type red dwarf star varies from 0.084 AU to 0.23 AU.

Instead of orbiting around the planet Saturn in the solar system, now imagine Titan as a lone planet drifting in interstellar space, with no parent star to provide any form of light and warmth. This is the case of Titan as a rouge planet and how it might still support a surface temperature of minus 180 degrees Centigrade as it drifts in the much colder depths of interstellar space. In order to maintain such a surface temperature, Titan with its current haze layer thickness will require an average geothermal heat flux of 1.4 watts for each square meter of its surface. Nevertheless, this value is around 20 times more than the average geothermal heat flux for the Earth and although this value might be consistent with a planet that is somewhat larger than the Earth, it is not realistic for a world the size of Titan. However, if Titan’s atmosphere is 20 times thicker than its current thickness, a more plausible average geothermal heat flux of 0.1 watts for each square meter of its surface will be sufficient to maintain a surface temperature of minus 180 degrees Centigrade.

Thus, for billions of years, Titan waited… An object looking a little like a comet streaked across the sky of Titan, battering atmospheric gases to a plasma twice as hot as the surface of the Sun itself. Cooling, it fell towards the surface slush. A parachute blossomed above it.

- Stephen Baxter, Titan

If any methane-based life is discovered on Titan, it should be widespread on Titan’s surface because liquid methane is also widespread on the surface. Direct evidence from the Huygens Probe has shown that the surface of Titan at the probe’s landing site is soaked with methane and radar imagery from Cassini has revealed numerous lakes on both the northern and southern polar regions of Titan. Life on a cryogenic world which runs on a methanological cycle will be extremely interesting. This is because the discovery of methane-based life on Titan or on any other Titan-like worlds will greatly improve our understanding of the range of worlds and chemical models that might support liquid-based life.

Sources:
1. Ashley E. Gilliam and Christopher P. McKay “Titan under a Red Dwarf Star and as a Rogue Planet: Requirements for Liquid Methane” (2011), Planetary and Space Science, doi:10.1016/j.pss.2011.03.012.
2. Steven A Benner et al. “Is there a common chemical model for life in the universe?” (2004), Chemical Biology, doi:10.1016/j.cbpa.2004.10.003.

Dark Matter and Alien Planets

In the dark and immense vastness of interstellar space, there can be lone planets that do not orbit around any parent star. Such planets do not receive warmth from stars and any surface inhabitant will experience perpetual night. It appears very unlikely that these dark and seemingly frigid worlds may support life and sustain alien ecologies. However, a combination of mechanisms such as radiogenic heating, tidal heating or having a thick hydrogen atmosphere that is very effective at trapping heat, can sufficiently raise the surface temperature of such a planet to a point where liquid water can exist on the planet’s surface. In this article, I will consider another possible source of heating which can contribute to raising the surface temperature of a ‘sunless’ planet and that source of heating comes from the annihilation of dark matter particles.

All of the dark matter in the known universe contains a total amount of energy that is on the order of 10 thousand times greater than all of the energy that could be released through the fusion of all the hydrogen in the universe into helium. Unlike normal matter, dark matter has a scattered nature and does not interact at sufficient rates to meaningfully contribute to heating a planet. An exception is when dark matter particles are gravitational captured by a planet, whereby interactions with the matter making up the bulk of the planet can cause the dark matter particles to lose momentum and become gravitationally bound to the planet. This causes dark matter to accumulate in the planet’s interior and the annihilation of dark matter particles produces high energy secondary particles which are then absorbed and deposited as heat into the surrounding bulk of the planet, thereby providing a source of internal heat.

For the Earth, the capture and annihilation of dark matter particles in the planet’s interior does not produce any significant amounts of energy and even in the most optimistic scenarios, the energy contribution from the annihilation of dark matter particles is billions of times less than the energy the Earth receives from the Sun. However, the density of dark matter is expected to be hundreds to thousands of times greater in the central regions of the Milky Way galaxy and in the dense cores of dwarf spheroidal galaxies than it is in our solar system. This means that the energy contribution from the annihilation of dark matter particles for planets located in these regions can be very different.

Furthermore, dark matter residing in this unique regions have extremely low relative velocities and this greatly increases the capture rate of dark matter particles by a planet that is located in such a region. This is due to the fact that the low relative velocities of the dark matter particles make them more efficient in being gravitationally focused toward the planet or becoming gravitationally bound to the planet following collisions in which the particles lose just a small amount of momentum. This enables dark matter particles to accumulate in much greater quantities in planets located in these regions, such that the annihilation of dark matter particles can become the dominant source of energy to the extent of providing sufficient warmth for liquid water to exist on the surfaces of these planets even in the absence of warmth from a parent star.

The energy released from the annihilation of dark matter particles can enable rouge planets that do not orbit around any parent star to become potentially habitable and sustain an alien ecology. Around the center of the Milky Way galaxy, Earth mass planets with very low atmospheric emissivity can efficiently trap the energy released from the annihilation of dark matter particles to maintain surface temperatures that are possible for liquid water to exist. For atmospheres with higher and more Earth-like emissivities, super-Earths with a few times the mass of the Earth will then be required to trap sufficient annihilation energy to maintain surface temperatures that are capable of sustaining liquid water. This is due to the fact that although high emissivity atmospheres are less efficient in trapping heat as compared to low emissivity atmospheres, super-Earths can accumulate more dark matter than Earth-mass planets due to their more massive bulk.

The timescale over which a rouge planet can maintain sufficient warmth to have liquid water on its surface solely by the energy released from the annihilation of dark matter particles is on the order of trillions of years. This surpasses even the exceedingly long lifespans of low mass red dwarf stars. Due to the rarity of very high density dark matter environments, planets that are heated by the annihilation of dark matter particles are expected to be very rare. Nevertheless, such planets can provide the energy required to sustain an alien ecology over trillions of years, even in the absence of warmth from any parent star! Given their exceedingly long lifetimes, these rare alien worlds may prove to be the ultimate cradles of life in the universe.

Black Hole Propelled Starship

A black hole is essentially an object that is so dense and compact that within a sufficiently close distance from it, its immense gravitational pull does not let even light to escape. This critical distance is the event horizon of the black hole and anything which crosses the event horizon, including light, can never escape. If the entire Earth is crushed to form a black hole, its event horizon will have a diameter of only 18 millimeters! In this article, I am going to describe the possibility of using micro black holes as a means of propulsion for interstellar space travel and also compare it with other forms of propulsion. So far, all black holes known range from monstrous supermassive black holes in the cores of galaxies to stellar mass black holes, spanning in mass from billions of times the mass of the Sun to a few times the mass of the Sun respectively. In this article, the black holes described are micro black holes that are on the order of only a hundred thousand metric tons or so.

In the 1970s, the physicist Stephen Hawking theorized that black holes can emit radiation due to quantum effects and this phenomenon became known as Hawking radiation. In the absence of any mass accretion, an isolated black hole will gradually lose mass via the emission of Hawking radiation until the entire black hole eventually disappears. The power emitted by a black hole in the form of Hawking radiation increases as the mass of the black hole decreases. Therefore, as a black hole shrinks in mass, it will emit Hawking radiation at an ever increasing rate until it eventually disappears in an incredible burst of energy. A black hole with a mass of a billion metric tons will take almost 3 trillion years to complete decay via the emission of Hawking radiation even though it is slightly smaller than the size of the nucleus of an oxygen atom.

The Alpha Centauri star system is located 4.37 light years away and it is among the nearest stars. Traveling at a velocity of say 100 kilometers per second, which is already much faster than the fastest speed attained by any spacecraft to date, it will take over 13000 years to reach Alpha Centauri. Therefore, to get to the stars within a reasonable amount of time, a spacecraft will have to be accelerated up to a significant fraction of the speed of light and an entirely new means of propulsion will be required for such a feat.

As a black hole decays through the emission of Hawking radiation, almost all of the mass of the black hole is directly converted into energy and the only other known process with such a good mass to energy conversion efficiency is the annihilation of matter with antimatter. A 100 percent efficient conversion of mass to energy produces about 90 thousand trillion joules of energy for every kilogram of mass. Today’s best chemical propulsion methods can only get up to a few million joules per kilogram of fuel. Even nuclear fission and nuclear fusion pale in comparison as less than one percent of the mass of the fissile or fusion material is converted into energy. Hence, the almost perfect mass to energy conversion efficiency from the emission of Hawking radiation by decaying micro black holes can make them a viable means of propulsion for interstellar space travel.

A micro black hole with a mass of 100 thousand metric tons or so is able to produce thousands of times more power in the form of Hawking radiation than the average total power consumption by the entire human world in 2008. Such a micro black hole can be use to accelerate a spaceship to the incredibly huge velocities required for interstellar space travel by directing the high energy radiation from the decaying black hole to generate thrust. Because a black hole of this mass has a lifespan of only a few months, matter is continuously required to feed the black hole to sustain it. In fact, any form of matter including the extremely tenuous gases making up the interstellar medium between the stars can be use to feed and sustain the black hole.

To put the numbers into perspective, a micro black hole with a mass of 404 thousand metric tons will have a net power output of 370 petawatts from its emission of Hawking radiation and this is about 25000 times more power than the average total power consumption by the entire human world in 2008! If the total power output of this black hole is sustained for one year to accelerate a 1 million metric ton spaceship which also includes the mass of the black hole itself, the spaceship will acquire a final velocity of almost half the speed of light or 150 thousand kilometers per second. This will get the spaceship to Alpha Centauri in about a decade or so. However, such a scenario assumes that 100 percent of the energy emitted by the black hole is used for the acceleration of the spaceship. The black hole must also be constantly fed with mass such as interstellar gas collected along the way to sustain it as it journeys to the stars.

Forming an initial black hole will first require crushing a large amount of mass into an extremely tiny volume of space. The technology required to accomplish such a feat is probably far beyond today’s capabilities. However, once an initial black hole is created, additional mass can be fed into the black hole to allow it to grow to the required mass for it to be used as a means to accelerate a spaceship for interstellar space travel. Compared to the annihilation of matter with antimatter, the use of micro black holes as a means of propulsion is probably much more energy efficient because the production of antimatter requires vastly more energy as an input that what can be obtained by the annihilation process. In addition, once a micro black hole is created, it can be made to provide power via the emission of Hawking radiation for an indefinite period of time as long as the black hole is fed with mass to sustain it.

Drifting Crust

Titan is the largest moon of Saturn, the only moon that is known to have a dense atmosphere and the only known object in the Solar System other than Earth with stable bodies of surface liquid. With a diameter of 5150 kilometres, Titan is the second-largest moon in the Solar System as it is slightly smaller than Jupiter’s moon Ganymede. However, when placed together with Ganymede, Titan will actually appear larger because Titan’s dense and opaque atmosphere extends many kilometres above its surface and increases its apparent diameter. NASA’s Cassini spacecraft is currently in orbit around Saturn and it frequently makes flybys of Titan.

On Titan, the average surface temperature is roughly minus 180 degrees Centigrade and the surface atmospheric pressure is 1.45 times the atmospheric pressure at sea-level on Earth. For every square meter of Titan’s surface area, the overlying atmosphere 7.3 times more massive in comparison to the Earth’s. The surface gravity of Titan is one-seventh the surface gravity of the Earth such that when combined with the dense atmosphere, Titan’s gravity is sufficiently low to allow humans to consider flying through the atmosphere on their own strength by flapping artificial wings strapped to their arms!

Beneath an icy crust that has a thickness of perhaps a hundred kilometres or so, Titan is believe to have a global subsurface ocean of liquid water. The presence of a subsurface ocean dynamically decouples the crust of Titan from its much more massive interior bulk, thereby lowering the effective moment of inertia of the moon’s crust. This allows the global circulation of air within Titan’s thick atmosphere to drag and torque the entire crust around such that the crust does not rotate at exactly the same rate as the rest of Titan.

Like the other large moons of Saturn, Titan’s rotation is synchronous, which implies that Titan rotates once with each orbit around Saturn. However, because Titan’s crust is decoupled from its interior by the subsurface ocean, it allows the crust to be freely dragged around by the movement of air in Titan’s thick atmosphere. Surface features imaged by Cassini during one flyby are observed to be offset by as much as a few tens of kilometres when imaged in subsequent flybys. The entire surface of Titan shifts by one-third of a degree each year as the winds in Titan’s thick atmosphere freely torques the entire crust. Therefore, surface features on Titan will be noticeably offset in images of the same locations that were taken on different dates.

Having an atmosphere which pushes around the entire surface of a moon is not something that is new. In fact, the same thing happens on the Earth where the length-of-day changes by about one millisecond over the duration of a year because of winds speeding up and slowing down in the atmosphere. However, that is a tiny amount when compared to Titan because the Earth is much more rigid and more massive than Titan, and the Earth’s atmosphere is less dense than Titan’s atmosphere. On Titan, it seems that the entire world has to be considered, from its thick atmosphere to its icy crust to its interior ocean, just to explain the length of its day and the locations of its surface features. This makes Titan a world that is probably no less complex and dynamic as the Earth.

White Sun

The search for habitable Earth-size planets has primarily been focused on stars similar to our Sun. In recent years, the search has also gone on to focus on low mass red dwarf stars as these stars are by far the most common and an Earth-size planet around such a star will be much easier to detect due to the lower mass and luminosity of a red dwarf star. In this article, I will be exploring the possibility of detecting Earth-size planets located in the habitable zone of cool white dwarf stars. White dwarf stars are the final evolutionary state of all stars that are not massive enough to explode as supernovae and this includes stars such as our Sun. Typically, a white dwarf star has a mass that is comparable to our Sun and all its mass is contained within a tiny volume that is comparable to the size of the Earth. Hence, a white dwarf star is a very dense object as each cubic centimetre of its material can weight over a metric ton.

White dwarf stars are as common as Sun-like stars and as they slowly cool, they can provide energy to planets in orbit around them for billions of years. A paper entitled “Transit Surveys for Earths in the Habitable Zones of White Dwarfs” describes the prospect of detecting habitable Earth-size planets around white dwarf stars by searching for transits of such planets in front of white dwarf stars. Compared to a typical Sun-like star, the habitable zone around a white dwarf star will be located much closer in due to the much lower luminosity of a white dwarf star. The most common surface temperature for white dwarf stars is around 5000 degrees Kelvin and white dwarf stars with surface temperatures of over 10000 degrees Kelvin are rare because white dwarf stars spend little time at high temperatures as they cool very rapidly at such high temperatures. Furthermore, the high ultraviolet flux from a hot white dwarf star that has a surface temperature of over 10000 degrees Kelvin will affect the retention of an atmosphere around an Earth-size planet. Therefore, only cool white dwarf stars will surface temperatures that are considerably less than 10000 degrees Kelvin are considered for the detection of habitable Earth-size planets.

A white dwarf star does not have an internal source of energy like a typical star and this means that it will gradually radiate away its energy and cool down over a period of billions to trillions of years. Hence, the term “continuously habitable zone” is defined as the range of orbital distances from a white dwarf star where an Earth-size planet can stay habitable for a specified minimum duration. For an Earth-size planet to remain habitable for at least 3 billion years, the continuously habitable zone will extend from a distance of 0.005 AU to 0.02 AU for white dwarf stars with masses ranging from 0.4 to 0.9 times the mass of our Sun, whereby 1.0 AU is basically the mean distance of the Earth from our Sun.

The orbital period of any planet in the continuously habitable zone of white dwarf stars will range from around 4 to 32 hours and the planets are expected to be tidally-locked whereby the star-facing hemisphere of the planet will experience permanent day, while the other hemisphere will experience permanent night. The night side of such a planet can be warmed by the global circulation of heat from the day side of the planet which can prevent the formation of a cold-trap on the night side. Since the orbital period and spin period of a tidally-locked planet are both the same, an Earth-size planet in the continuously habitable zone of a white dwarf star will experience Coriolis and thermal forces that are similar to those on the Earth.

Earth-size planets in or near the continuously habitable zone of white dwarf stars can be detected via the transit method where the individual photometric output of a large number of white dwarf stars can be continuously monitored to look for any dimming that can be associated with the transit of an Earth-size planet in front of a white dwarf star. Due to the small size of a white dwarf star, the transit of an Earth-size planet will block out a significant fraction of the white dwarf star’s total photometric output or even completely block out the entire star if the star is sufficiently small. The small size of a white dwarf star also favours the detection of transiting objects that are smaller than the size of the Earth. The transit durations of Earth-size planets in the continuously habitable zone of white dwarf stars are estimated to last for a couple of minutes or so, thereby requiring high cadence observations to record the proper light curves that are indicative of such transit events.

Measurements of the distance and spectrum of a white dwarf star will allow its mass, luminosity, atmospheric composition and radius to be determined. Therefore, with the size of the white dwarf star known, the measured transit depth of a transiting planet enables the size of the transiting planet to be directly determined. On the contrary, the mass of the transiting planet cannot be determined from Doppler measurements as the spectra of cool white dwarf stars are generally featureless. However, if the white dwarf star has multiple transiting planets, gravitational interactions among the planets can cause measurable transit timing variations which can be use to estimate the mass for each of the planets.

Arriving At Mercury

NASA’s MESSENGER spacecraft was launched into space onboard a Delta II 7925 rocket on 3 August 2004 at 06:15:56 UTC from Space Launch Complex 17B at the Cape Canaveral Air Force Station in Florida. After travelling through space for 6 years, 7 months and 16 days and covering an impressive distance of 7.9 billion kilometres, MESSENGER finally entered orbit around the planet Mercury on 18 March 2011 at 01:00 a.m. UTC after a 15 minutes Mercury orbit insertion (MOI) engine burn. MESSENGER is the second mission to Mercury after a final flyby performed by Mariner 10 in 1975 and it is the first spacecraft to enter orbit around the planet. The primary mission of MESSENGER will be to study the chemical composition, geology and magnetic field of Mercury.

Getting to Mercury from the Earth requires a large velocity change because the closeness of Mercury to the Sun places the planet deep within the Sun’s gravitational potential well. Furthermore, Mercury’s extremely tenuous atmosphere makes it impossible for an aerobraking manoeuvre to be employed to sufficiently slow an incoming spacecraft for capture into orbit around Mercury. To solve this issue, MESSENGER extensively used gravity assist manoeuvres by making flybys of the inner planets to gradually decelerate the spacecraft such that the amount of propellant required to slow the spacecraft into orbit around Mercury is greatly reduced. However, this comes at the cost of prolonging the trip to Mercury by a few years. The trajectory that MESSENGER took through the inner solar system to get to Mercury included one flyby of Earth, two flybys of Venus and three flybys of Mercury itself.

On 18 March 2011 at 12:45 a.m. UTC, the orbital insertion manoeuvre brought MESSENGER into a highly elliptical orbit around Mercury whose lowest point is 200 kilometres above the planet’s surface while the highest point is over 15000 kilometres above the planet’s surface. The three previous flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that has changed our understanding of the enigmatic innermost planet of the solar system. However, these flybys are merely a sneak preview of the discoveries that are expected to come as MESSENGER is now the first spacecraft ever to orbit Mercury for long-term observations. Visit http://messenger.jhuapl.edu/index.php to learn more about MESSENGER and its mission around Mercury.

Galactic Blast

The Fermi Gamma-ray Space Telescope (FGST) is a space observatory which observes the universe in gamma-rays from its vantage point in low Earth orbit. One interesting discovery by Fermi are two enormous gamma-ray-emitting bubbles that extend about 30 thousand light years above and below the centre of the Milky Way galaxy. The existence of the two gamma-ray-emitting bubbles was first hinted by previous detections of a localized excess of radio signals. In this article, the two gamma-ray-emitting bubbles will be referred to as the Fermi Bubble. I recently read a paper entitled “Origin of the Fermi Bubble” and this paper suggests that the periodic capture of stars by the supermassive black hole at the centre of the Milky Way galaxy can inject the required amounts of high energy plasma into the galactic halo to form the Fermi Bubble.

A supermassive black hole with a mass of approximately 4 million Suns sits in the heart of the Milky Way galaxy. Stars which happen to come too close to the supermassive black hole can be destroyed by tidal disruption. When a star gets tidally disrupted by the supermassive black hole, about half of its mass becomes tightly bound to the black hole while the other half gets violently ejected. The amount of energy carried by the ejected mass can significantly exceed the amount of energy released by a normal supernova explosion. Approximations have shown that the supermassive black hole at the galactic centre destroys a star by tidal disruption at a rate of roughly one star every ten thousand years or so. This means that tens of stars are expected to get tidally disrupted every one million years.

The ejecta from each tidally disrupted star expand as a spherically symmetric wind of high energy plasma and ‘snowploughs’ its way out of the galactic centre to form a pair of bipolar outflows which contribute to the existence of the Fermi Bubble. The high energy outflows from each tidal disruption event expand hydrodynamically out of the galactic centre and into the galactic halo, forming shock fronts which accelerate electrons to near the speed of light. Interaction of the high energy electrons with background photons via synchrotron radiation and inverse Compton scattering produces the observed radio and gamma-ray emissions respectively. Since the mean interval between each tidal disruption event is smaller than the timescale for energy loss, the gamma-ray emissions produced from each individual shock front can be approximated to be uniformly distributed over the entire Fermi Bubble.

Finally, the existence of the Fermi Bubble cannot be explained by a previous episode of starburst activity in the galactic centre because there is no evidence of an excessive amount of supernova explosions in the past 10 million years or so in the galactic centre. Furthermore, supernova remnants can be traced by the radioactive aluminium-26 they produce and the sparse concentration of aluminium-26 in the galactic centre does not support a previous episode of starburst activity.

Cool Dwarf

Brown dwarfs are objects that are too low in mass to sustain hydrogen fusion in their cores and they occupy the mass range between gas giant planets and the lowest mass stars. The upper limit for the mass of a brown dwarf is around 80 times the mass of Jupiter while the lower limit for the mass of a brown dwarf is undefined as it overlaps with the masses of gas giant planets. Methane-bearing spectral class T brown dwarfs are the coolest known class of brown dwarfs. Although a large number of brown dwarfs are know, there remains a large gap between the temperature of the coolest known brown dwarfs and the gas giant planets in our solar system. The coolest known brown dwarfs have temperatures of around 500 degrees Kelvin while the gas giant planets in our solar system have temperatures of around 150 degrees Kelvin. Theoretical studies have shown that brown dwarfs in this temperature range exhibit spectroscopic characteristics that are distinct from the spectral class T brown dwarfs, such as ammonia absorption lines and scattering from clouds of water ice. Any brown dwarfs in this temperature range can be categorized into a new and cooler spectral class known as spectral class Y.

A paper by Kevin Luhman, et al. (2011) entitled “Discovery of a Candidate for the Coolest Known Brown Dwarf” describes the discovery of what might be the coolest known brown dwarf and a likely prototype for the spectral class Y. With an estimated temperature of 300 degrees Kelvin, WD 0806-661 B is a candidate for the coolest known brown dwarf and also cool enough for its atmosphere to contain clouds of water ice. WD 0806-661 B is in a wide orbit around a white dwarf star and if a similar age to its host star is assumed, WD 0806-661 B will be around 1.5 billion years old. Furthermore, based on evolutionary models of cooling brown dwarfs, WD 0806-661 B is estimated to have a mass of around 7 times the mass of Jupiter and this falls well within the range of masses for the more massive extrasolar planets. There are two mechanisms in which an object like WD 0806-661 B could have formed. Firstly, WD 0806-661 B could have formed from the coalescence of a fragmented cloud of gas at its current large distance from its host star. Secondly, WD 0806-661 B could be a gas giant planet that had been dynamically scattered into a much more distant orbit around its host star. If subsequent observations confirm WD 0806-661 B to be the coolest known brown dwarf, it will become a valuable target for studying atmospheres in an entirely new temperature regime that will consequently aid searches for the coldest brown dwarfs with facilities such as the Wide-field Infrared Survey Explorer (WISE) and the James Webb Space Telescope (JWST).

Interstellar Drifters

The formation of planets around a young star occurs within a disk of gas and dust encircling the young star called a protoplanetary disk. This results in a close alignment between the rotation axis of the star and the orbital motion of the planets after they have formed. However, measurements of the relative spin-orbit alignment of transiting extrasolar planets via the Rossiter-McLaughlin effect have shown that a number of these planets have orbits that are significantly misaligned with the rotation axes of their host stars. Gravitational interactions with other planets or the Kozai mechanism are two proposed means in which planets in initially aligned orbits can get perturb into misaligned orbits. A recently published paper entitled “Misaligned and Alien Planets from Explosive Death of Stars” proposes an alternative mechanism to explain the planets that are observed to be in misaligned orbits around their host stars.

In this paper, planets whose orbits are misaligned with the rotation axes of their host stars are suggested to have formed from high-speed blobs of gas that are produced in supernova remnants and planetary nebulae. Supernova remnants are formed following the death of massive stars in supernova explosions while planetary nebulae are formed from the death throes of Sun-like stars as they shed off their outer layers into space. These blobs of gas have been observed in great numbers around supernova remnants and planetary nebulae. High resolution images of young supernova remnants and planetary nebulae have shown that each of them is surrounded by thousands of blobs of gas that have cometary-like appearance possibly shaped by overtaking winds. As these blobs of gas travel through interstellar space, they sweep up ambient matter along the way, causing them to increase in mass and decelerate. Over time, these blobs of gas gradually cool by emitting radiation. Once the blobs of gas become sufficiently massive and cool, self-gravity takes over and cause the blobs of gas to collapse gravitationally to form gas giant planets.

Regardless of whether these blobs of gas eventually contract gravitationally to form gas giant planets, they can explain the considerable number of planets that are found to be in misaligned orbits around their host stars through a number of different ways. For a blob of gas that has already collapsed gravitationally to form a gas giant planet, it can be captured into a misaligned orbit around a star or perturb the original planets that have previously formed around a star into misaligned orbits. On the other hand, an uncollapsed blob of gas can be captured into a misaligned orbit around a star to form a misaligned disk of gas and dust from which the in situ formation of planets with misaligned orbits can occur. Furthermore, uncollapsed free-floating blobs of gas can also be captured by stars and strongly perturb their planetary systems.

Around young supernova remnants and planetary nebulae, a typical blob of gas has a similar mass as the Earth and a ‘head’ which spans tens of billions of kilometers across. Shaped into cometary-like morphologies by faster overtaking winds, a typical blob of gas travels with an average projected velocity on the order of a few hundred kilometers per second outwards from its parent supernova remnant or planetary nebula. Blobs of gas in young supernova remnants can be classified into two populations distinguished by their velocities where the low velocity population formed in the ejecta of the progenitor star before its supernova explosion while the high velocity population formed following the supernova explosion.

As the blobs of gas travel outwards, they cool by radiation and grow in mass by accreting gas and dust in the interstellar medium. The accretion process causes the blobs of gas to decelerate, making the blobs of gas or the eventual gas giant planet slow enough to be gravitationally captured during sufficiently close encounters with stars. Finally, very high velocity blobs of gas from supernova explosions that travel through very low density regions of space will escape into the low-density intergalactic space where they will expand before they ever reach the required mass and compactness necessary for them to collapse gravitationally into gas giant planets. These very high velocity blobs of gas will enrich the intergalactic medium with heavy elements produced from supernova explosions.