The Quest:
For as long as humans have walked the earth we have looked up at the stars and wondered whether we are alone in the universe. Until very recently, we did these two things independently, unaware that one day we might wonder whether or not we are actually gazing at other worlds in the night sky. With the invention of the telescope, with Galileo, Copernicus, Kepler and the rest, we redefined our understanding of the universe, and it wasn’t long before we began to question our uniqueness in the cosmic expanse. Beginning with Huygens in the 1600s and continuing up to the present day, a new field of astronomical inquiry was born, dedicated for the search for extrasolar planets, or planets orbiting stars other than our Sun.
This quest for other worlds hold’s significance for us in several areas: not only does it aid in our understanding of the structure of our galaxy, it also offers the possibility of discovering extrasolar life, the holy grail of many branches of astronomy. While we have discovered almost 1,000 exoplanets (and thousands more candidate planets), we have yet to refine our technology to the point of being able to detect life on any of them. The discovery of life beyond Earth would revolutionize our understanding of the cosmos and have gigantic implications for humanity in general, from religion to politics to even economics. Consequently, various nations have devoted large amounts of resources and technological expertise to the cause.
The methods used to detect exoplanets have evolved over the years as our technology has developed, but we still have only a handful of ways we can catch a glimpse of our neighboring worlds. The simplest is to observe a star over a long period of time and monitor it for slight changes in position that could indicate the gravitational tug of an orbiting body. This technique, known as astrometrics, was used to discover the first exoplanets in the 1900s. Since then we have created more precise ways of measuring the orbital wobble of planet-supporting stars, such as the Doppler Shift technique, which detects shifts in the wavelength of light emitted by a star (an indication of changing velocity), and pulsar timing, in which researchers calculate the change in timing of pulses emitted from neutron stars to determine whether they exhibit the orbital wobble. Other methods such as transit detection (observation of a star “dimming” when a planet passes in front of it), gravitational microlensing (observation of a star’s light being magnified by the gravitational field of a planet), and even direct imaging make up the rest of discoveries. However, most of these methods push the boundaries of our current technology, and leave plenty of room for improvement.
(Light curve of WASP-12, used to detect the transit of WASP-12b)
One particular planet, discovered with the transit method, has provided astronomers with an intriguing sight. Discovered in April of 2008 by the SuperWASP (Wide Angle Search for Planets) international survey team, WASP-12b is the only planet ever to be discovered in the process of being consumed by its parent star. At 1.39 times the mass of Jupiter, orbiting a G0-Type star (just slightly hotter than the sun, and 1.57 times the size), 12b seems to be a fairly typical exoplanet at first sight. What makes it unusual is its orbital distance. This massive gas giant zips around its parent star at 0.0229 AU, or 1/44 of the Earth’s distance from the sun, completing a full orbit every 1.09 Earth days. This extreme proximity to its planet star has caused 12b to warp into slightly egg-shaped form, and is stripping the planet of its atmosphere by 189 quadrillion tons per year. The planet is essentially being eaten by its star.
Is It Habitable?
With a deteriorating atmosphere and the distorting warp of its cannibalistic parent star, WASP-12b appears to be the perfect stage for science-fiction apocalypse story. It’s tempting to imagine an advanced race of aliens desperately trying to escape the surface of their dying planet, building arks or sending out emissaries to carry on their legacy, much like the classic Superman-Krypton story. However, as far as the habitability of 12b goes, the truth is far less exciting. Based on our understanding of life on Earth and our knowledge of the conditions on 12b, there is no possible way life could exist on its surface today.
For instance, let’s examine its orbit. At 0.0229 AU (3,425,791 kilometers) away from its parent star, the amount of solar radiation absorbed by 12b is enormous. Given its distance and the luminosity of WASP-12, we can use the equation for equilibrium temperature…
… where Ab is the planet’s albedo, D is its average orbital distance in AU, Lstar is the luminosity of WASP-12 (in solar luminosities) and Lsun is the luminosity of our sun, to calculate the planet’s average surface temperature. With an albedo of about 0.1, an orbital distance of 0.0229 AU and an approximate luminosity of 1.26 Lsun, the calculations produce a surface temperature of 2560 Kelvin.
(Artist’s impression of WASP-12b and parent star)
A surface temperature of 2560K is much too high to accommodate liquid water, let alone liquid methane or ethane. This fact alone means that 12b fails the so-called “litmus test” of habitability. Without a liquid medium for chemical reactions to occur, the occurrence of life is impossible. In order to maintain temperatures suitable for life, 12b would have to orbit within the “habitable zone” of WASP-12b, or the orbital area within which planets should be able to maintain liquid medium on their surface. The limits of this zone can be calculated with relative ease. Given the luminosity of the star, we can use the equations…
… where Dinner and Douter are the distances from the star to the inner and outer boundaries of the habitable zone, respectively. Plugging in the luminosity of WASP-12 (about 1.26 solar luminosities), we calculate the inner boundary to be about 1.07 AU away from the star, and the outer to be 1.57 AU. 12b, at only .0229 AU, is a far cry from the sweet spot. In fact, unless the planet has migrated in towards the star from a more distant orbit throughout its lifetime (which scientists think may be possible explanation for the position of many large exoplanets), it is unlikely that 12b has ever existed within the habitable zone of its star, making life on its surface an impossibility. Additionally, as its low mass parent star evolves, becoming a red giant when it depletes its fuel supply of hydrogen, it will eventually envelop 12b before dying and expelling its outer layers in a planetary nebula, making the case for potential habitability even more obsolete.
However, despite this seemingly condemning piece of evidence, let’s continue our examination of other characteristics of the planet that might affect its habitability. The composition of 12b is still being debated today, but a recent spectroscopy taken by the Hubble Space Telescope has given scientists some data on which to base their hypotheses. The Hubble imaging indicates that the planet has a higher carbon-to-oxygen ratio than does the Sun, which seems to indicate that it is a carbon-rich gas giant. The carbon should be contained in its atmosphere, which is swollen to three times the radius of Jupiter, in the form of carbon monoxide and methane, a mixture that would be toxic to most life on Earth. Another point against the planet’s habitability (at least for life as we know it).
Science Fiction:
However, if we stretch our imaginations slightly it is possible to visualize what life might be like if it could survive on WASP-12b. It is hypothesized that, much like Jupiter, 12b’s outer layers are made up of differentiated levels of gas clouds. Assuming that it has at least some water content, it is possible that at a certain altitude, the temperature and pressure ranges would create a small layer suitable for water vapor. It is here that life might be able to exist, utilizing small droplets of water in the air as a liquid solvent to facilitate chemical reactions. Given that the temperature would still be somewhere much higher than is suitable for most life on Earth, we might only find 12b’s version of extremophile bacteria floating in this habitable layer. However, because of the strong vertical winds of 12b’s atmosphere, any free-floating bacteria would inevitably be swept to higher or lower altitudes, into the uninhabitable zones of the atmosphere. More complex life might be able to develop techniques to counteract this turbulence, perhaps by developing large, gas-filled sacs capable of creating a buoyancy to maintain a stable atmosphere. However, in order to reach the right balance, these creatures would have to be enormous. Not to mention capable of breathing carbon dioxide and methane. You can almost imagine huge, floating, jellyfish-like aliens, bobbing around through the giant’s atmosphere, snagging floating bacteria or the occasional, unwitting space ship for their afternoon snack.
What Can it Tell Us?
While it hasn’t aided us in our search for habitable planets beyond our solar system, WASP-12b has given us valuable insight into the nature of solar system formation and the orbital patterns of gas giants. While it may be incapable of supporting life, 12b has at the very least given us an extreme view of the conditions existing in the universe. At approximately the same size as Jupiter, orbiting a star approximately the same size as our sun but at only a fraction of Earth’s orbital distance, 12b sports surface temperatures of 2560 K and a toxic atmosphere of carbon dioxide and methane. If nothing else, this blazing hot ball of gas, this planet being devoured by its own sun has probably given science fiction writers something to chew on.
Bibliography
“WASP-12b.” Wikidpedia. Wikipedia , 04 Sep 2013. Web. 20 Oct 2013. <http://en.wikipedia.org/wiki/WASP-12b>.
“WASP-12.” Wikidpedia. Wikipedia, 04 Sep 2013. Web. 20 Oct 2013. <http://en.wikipedia.org/wiki/WASP-12>.
“Planet WASP-12 b.” Exoplanet.eu. Exoplanet.eu. Web. 20 Oct 2013. <http://exoplanet.eu/catalog/wasp-12_b/>.
David, Wilson. “SuperWASP Planets.” SuperWasp.org. N.p.. Web. 23 Oct 2013. <http://www.superwasp.org/wasp_planets.htm>.
