What is it that drives us to search for planets outside our solar system? Possibly because an ingrained curiosity to further our understanding of the world around us. Or maybe we owe it to future generations to find a new home, as Wernher von Braun suggested, before Earth is swallowed by our sun, destroyed by an asteroid, or ruined due to our own negligence. Almost one thousand exoplanents have been discovered to date (999 as of this past Saturday according to Exoplanet.eu), but only a tiny fraction of these could potentially harbor life. Astronomers have been searching for more than twenty years and still have yet to find a plant that resembles Earth. The practical goal now is not to find another Earth, but instead a planet that could sustain life.
Hope
A promising lead in the right direction came almost two years ago when researchers at ESO or the European Southern Observatory, confirmed the existence of a planet called Gliese 667 Cc. The researchers discovered the planet using the High Accuracy Radial velocity Planet Seracher, more commonly known as HARPS, located at La Silla Observatory in Chile. This method works by measuring a star’s speed towards and away from us, which is slightly influence by the gravity of the planets orbiting it. Through these slight changes in the stars velocity we are able to confirm the presence of planets. Gliese 667 Cc is located in the triple-star system Gliese 667, which resides in the Scorpius constellation twenty two light years away. Two stars, Gliese 667 A and B, lie in the center of the constellation, orbiting each other at a separation from 20AU to 5AU. Gliese 667 C on the other hand orbits the previously mentioned stars at an average of 230AU, which is nearly five times the distance from our sun to Pluto. The latter is particularly interesting because orbiting it are at least five rocky planets, three of which are within its habitable zone. Gliese 667 C is a M1.5V type red dwarf star that has a luminosity only a fraction of our Sun’s and is relatively cool at 3700K, yet it still emits a fair amount of energy in the infra-red spectrum.
Cold Hard Facts
Gliese 667 Cc is a prime candidate for a habitable exoplanet and here’s why. The planet is located inside of the habitable zone around the star it orbits and could hold liquid water. It is thought to be composed of rocky materials like those on Earth. It is big enough to retain a molten core; necessary for generation of a magnetic field to protect the planet. Given the proper atmosphere it is likely that Gliese 667 Cc could sustain life.
A visual of the habitable zone around Gliese 667 C, (credit: Planetary Habitability Laboratory @UPR Arecibo)
A key selling point to scientists of Gliese 677 Cc, as mentioned, is that it is comfortably within its parent star’s habitable zone or the area that is able to support liquid water around the star. To calculate the inner and outer boundaries of the habitable zone Dinner and Douter, respectively, we use the following two equations for the inner and outer edges, where L is the luminosity of a star in terms of Lsun.
By using a value of 0.0137 for the luminosity we get a habitable zone stretching from 0.11AU to 0.16, which works out well for Gliese 667 Cc because it orbits at an average of 0.125AU way from its parent star. This ability to support water is due to an ideal temperature range on the planet which we can estimate using the following equation, where A is the planet’s albedo or reflectivity, D is the distance in AU from the planet to the star it orbits, and L is the parent star’s luminosity in terms of Lsun.
Since this planet was detected using the Dopler method it has not been directly observed and therefore scientists have yet to calculate a value for its albedo. To estimate a range of possible temperatures, assuming the planet has relatively similar atmosphere to Earth, we use albedo values of 0.01, 0.99, 0.125AU for D, and 0.0137 for L to get a temperature range from 85.1K-268.3K. However, according to more precise blackbody calculations from scientists the temperature is probably closer to 277.4K. Two other factors here play a huge role in the planet’s temperature. Since the planet is so close to the star it is orbiting, it is likely tidally locked and therefore only one side of the planet ever receives light from the star, which would make that side much hotter. The next thing to take into account is the planet’s atmosphere, if thick enough it could distribute this uneven heating to the dark side of the planet. Finally we must take into account the size of Gliese 667 Cc. Current calculations place it at around 4.5 times more massive than Earth, while its exact size is unknown it is certainly large enough to have current molten core. This is extremely important in generating a magnetic field, which in turn shields the planet’s atmosphere from being swept away by solar radiation.
A comparison of light on Earth (left) to predicted light on Gliese 667 Cc (right), (Credit: Sven Wedemeyer-Böhm)
A Bit of Speculation
If life were to exist on 677 Cc it would certainly be different to life here on Earth. One major difference is the sunlight. The planet receives about 90% of the light that we do, but almost all of it is in the infra-red spectrum. This means that life there would have genetically adapted to see more in the IR spectrum and plants would be able to photosynthesize using more IR light than visible. The surface of the planet is likely covered in rocky land masses and oceans on which the planets inhabitants live on and in. Since the planet is tidally locked an intelligent species living there would have built much of its infra-structure that requires solar energy, such as agriculture on the sunny side of the planet. Another aspect of life to consider is the force of gravity. Since the planet is more massive than Earth we would experience up to 1.6 times heavier than we are. Also due to its larger mass the planet would have a much heavier atmosphere which could add much more pressure than we are currently use to. So any life that evolved on Gliese 667 Cc would be much more suited to living at these higher pressures.
Back to Reality
There are still many more exoplanets out there to be discovered and vast amounts of information to be studied about ones already found, especially in the case of the Gliese 667 system. Whether life exists on Gliese 667 Cc is yet to be determined, but if it does it would certainly be different form life here on Earth. Hopefully through further analysis we will learn more about the planet’s atmosphere and surface composition, so we can answer the question of habitability once and for all. If we discover that Gliese 667 Cc is inhabitable, there is no need to worry because two other planets, Gliese 667 Cf and Ce, within the star’s habitable zone.
Sources:
http://science.kqed.org/quest/2013/03/22/gliese-667-cc-musing-the-possibilities-of- another-earth
http://www.mn.uio.no/astro/english/research/news-and-events/news/astronews-2012-02-17.html
http://arxiv.org/abs/1202.0446
http://arxiv.org/abs/1212.4058v2
http://www.eso.org/public/archives/releases/sciencepapers/eso1328/eso1328a.pdf
