GJ 1214b Water World
The Kepler space telescope, which started operation on March 6th, 2009, is one of a number of new projects aimed at discovering extra-solar planets. This goal, which was once just a dream of astronomers, has been realized in recent years due to a number of new telescopes, spearheaded by the Kepler space telescope, a 37 inch telescope dedicated to finding new planets (Britannica). Although it was previously believed that planets might be scarce, and that our system rather special, the new data revealed by Kepler and other telescopes have revealed hundreds of planets orbiting foreign stars. Though initially the only planets we were able to detect were large, massive Jovian planets, more recently we have been able to detect many terrestrial planets. Most are a few times more massive than Earth, and have been dubbed, “super earths” for their size and terrestrial nature.
The methods for finding planets are varied and numerous. The six methods used today are pulsar timing, astrometry, Doppler shift, direct imaging, transits and micro-lensing. Of these six, by far the most commonly used are transits, astrometry and Doppler shift. The transit method, used by the Kepler telescope, looks at the changes in luminosity of stars as planets transit their disc (Britannica). This method can be very accurate, but is not infallible; multi star or planet systems need complex models to be understood, and this method is prone to finding very large planets, not the small terrestrial ones that could support life. Another weakness is that this method requires the plane of the system to be in line with ours, further limiting its usefulness. Doppler shift and astrometry are very similar. Both look at the movement of stars as they orbit the center of mass of their system to find planets. By breaking down the star’s orbit and fitting it to a model, we can predict the mass and placement of the surrounding planets. The difference in these methods lies in how they collect data. The Doppler shift method, as the name suggests, uses the red or blue shift of the star to model the orbit while astrometry uses imaging. Both methods are prone to detecting Jovian planets much more frequently than terrestrial ones, but both have been used to great effect.
One planet detected by the transit method is Gliese 1214b or GJ 1214b. Detected as part of the MEarth Project, GJ 1214b has a radius roughly 2.6 times larger than that of Earth, and mass around 7 times greater than Earths. Orbiting a small, dim, M class dwarf star called GJ 1214, GJ 1214b is only 47 light years from Earth, just a step away compared to most other super-Earths. Though little is directly known of the planet’s composition due to its thick atmosphere, its low density tells us that it is likely to be a water world, with much less rock than Earth (Wiki, GJ 1214b). The current proposed model, pictured above, would make this planet a prime candidate for life, with a hot atmosphere and a huge, warm sea. The planet’s heat is due to its proximity to its host star. With a semi major axis of just .014 AU, GJ 1214b orbits closer to its star than any body in our solar system. This proximity is not problematic for life due to the star’s (GJ 1214’s) low temperature and luminosity (Wiki, GJ 1214b). The atmosphere of GJ 1214b is very thick, low Raleigh scattering provides evidence of a water rich atmosphere. Raleigh scattering is a technique that looks at how starlight is scattered as it goes through the atmosphere of a planet to determine its composition. Because some low levels of scattering are observed, but very little light goes all the way through the atmosphere, we can conclude that a cloudy, water rich atmosphere is likely (Daily Mail). One more factor that makes life possible on GJ 1214b has to do with the star it orbits. Dwarf stars are very stable and very long-lived, giving life lots of time to evolve, unlike high mass stars which burn through their fuel faster, and so live shorter lives.
Due to these factors GJ 1214b is a prime candidate for further study, and one of the most likely places that life could exist in a system close to ours. The conservative boundaries of the habitable zone around a star are given as such:
Dinner = .95AU√(Lstar/Lsun) and
Douter = 1.4AU√(Lstar/Lsun)
Given GJ 1214’s luminosity we can calculate the habitable zone
Dinner = .95AU√(.00328☉)
Douter = 1.4AU√(.00328☉)
So the habitable zone around GJ 1214b is between .054 AU and .081 AU.
Though the orbit of GJ 1214b is outside these limits, it does not mean that the planet cannot support life. These equations do not take into account any atmospheric effects, nor does it use albedo to determine temperatures. To find a more accurate indication of temperature we can use an equation for equilibrium temperature. This too does not include atmospheric effects, but will still give a better idea of the temperatures on GJ 1214b.
4πσ(Teq)4 = (1-A)(L★/4d2)
Where σ is the Stefan-Boltzmann constant (5.670373 × 10−8 W m−2 K−4), A is the albedo, L★ is the luminosity of the star in watts, d is the semi-major axis of the planet’s orbit in meters and Teq is the equilibrium temperature in degrees kelvin. Though we cannot directly measure the albedo of GJ 1214b, our best models put it at around .4 (astro.ex.ac.uk).
4π(5.670373 × 10−8 W m−2 K−4 )(Teq)4 = (1-.4)(1.259192 x 1024 W/4((2.139 × 109m)2)
(7.12560086 x 10-7 W m−2 K−4)(Teq)4 = 41240 kg / s3
(Teq)4 = (41240 kg / s3)/ (7.12560086 x 10-7 W m−2 K−4)
Teq = 490.4 K
This temperature, while hot, could still support liquid water at different depths (and pressures). Using different models and different albedos scientists have put GJ 1214b’s temperature at somewhere between 393 and 555 degrees kelvin, or around 120 – 282oC.
Though these temperatures are to hot for most terrestrial life to survive in, Methanopyrus kandleri and Geogemma barossii are both microorganisms that can survive and even reproduce in temperatures of greater than 120oC (NSF). While that is still on the lower end of the predicted range, evolution could make more species suitable for these conditions.
As previously mentioned, the star GJ 1214 is an M-class red dwarf. This is important in determining the possibility of life on GJ 1214b because dwarf stars have very long lives, giving plenty of time for life to evolve. A red dwarf’s life cycle is different than that of a massive star. A high mass star, such as VY Canis Majoris, which has a radius of approximately 6.6 AU, and is around 20 times more massive than the sun, has a lifetime of around two million years, far too short for life to evolve. Unlike these massive stars, red dwarfs are expected to live up to 10 trillion years, giving life plenty of time to evolve.
GJ 1214b shows promise as a life-bearing planet due to a number of characteristics. Its composition, primarily water, is perhaps the most exciting of them all, water is a well known necessity for terrestrial life, and its use as a solvent and catalyst is crucial for life. While other liquids could be used for the same purpose, water is the best we have found so far, with a wide variety of temperatures at which it can remain liquid. The planet’s temperature, while extreme by terrestrial standards are still within the limits of life on Earth, and could be considered pedestrian by extraterrestrial life. Other characteristics such as its water rich atmosphere and long-lived star provide additional support to the idea that life may exist on GJ 1214b. Its proximity and the aforementioned characteristics make GJ 1214b a prime candidate for further investigation and study.
Works Cited:
“GJ 1214” Wikipedia: The Free Encyclopedia. Wikimedia Foundation. Web. Oct. 22, 2013. http://en.wikipedia.org/wiki/GJ_1214_b
“Kepler.” Encyclopædia Britannica. Encyclopædia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2013. Web. 22 Oct. 2013. <http://www.britannica.com/EBchecked/topic/1474027/Kepler>.
“Kepler Telescope” Wikipedia: The Free Encyclopedia. Wikimedia Foundation. Web. Oct. 22, 2013.http://en.wikipedia.org/wiki/Kepler_telescope
Lovley, Derek. “Microbe from Depths Takes Life to Hottest Known Limit” NSF. NSF.gov. Web. Oct 22, 2013. http://www.nsf.gov/od/lpa/news/03/pr0384.html
Marcy, Geoffrey. “Extra Solar Planets: Water World Larger Than Earth” Nature. Nature.com. Web. Oct. 22, 2013. http://www.nature.com/nature/journal/v462/n7275/full/462853a.html
Zolfagharifard, Ellie. “Super-Earth 40 light years away ‘is rich in water with a thick, steamy atmosphere’, confirm Japanese astronomers” The Daily Mail. Daily Mail. Web. Oct. 22, 2013. http://www.dailymail.co.uk/sciencetech/article-2412151/Super-Earth-GJ-1214b-40-light-years-away-rich-water-steamy-atmosphere.html
