An international team of astronomers has announced the discovery of a potentially-habitable Super-Earth around the nearby red dwarf star Gliese 832. Located 16 light-years from Earth, it's considered one of the closest and best habitable-world candidates so far.
Gliese 832-c features a brief orbital period of 36 days and a mass at least five times that of Earth's. This planet may be close to its red dwarf parent star, but it receives about the same average energy as Earth does from the Sun. According to lead astronomer Robert Wittenmyer from UNSW Australia, the planet might have Earth-like temperatures, though with large seasonal shifts (assuming it has an atmosphere similar to ours). More likely, however, is that it's a Super-Venus — an oversized terrestrial planet with a hot, dense atmosphere that's hostile to life.
"Gliese 832-c seems to have the right temperature for life but not necessarily the right size," Abel Méndez tells io9. "It could be a gas or water world instead of a rocky world but we simply don't know now. The orbital eccentricity of Gliese 832 c might cause large seasonal temperatures changes but not enough to preclude life. A dense CO2 atmosphere could make this planet too hot for life anyway."
Méndez, a planetary astronomer at the Planetary Habitability Laboratory at the University of Puerto Rico at Aricebo, was not involved in the study.
But Méndez still says it's a planet worth investigating. He tells io9:
The Earth Similarity Index (ESI) of Gliese 832 c (ESI = 0.81) is comparable to Gliese 667C c (ESI = 0.84) and Kepler-62 e (ESI = 0.83). This makes Gliese 832 c one of the top three most Earth-like planets according to the ESI (i.e. with respect to Earth's stellar flux and mass) and the closest one to Earth of all three, a prime object for follow-up observations. However, other unknowns such as the bulk composition and atmosphere of the planet could make this world quite different to Earth and non-habitable.
But in fact, there are other candidates that might turn out to be better bets. Earlier this month, astronomers discovered a potentially habitable planet around Kapteyn's star that's only 11 light-years away. There's also Gliese 581-d to consider, which is 20.2 light-years away.
Gliese 832 hosts another planet, a cold Jupiter-like world that was discovered in 2009. It's farther out than 832-c (it has a 9.4 year orbit), so the solar system is a kind of miniature version of our own. So this newly discovered giant planet might play a similar role to the one played by Jupiter in our own system.
Gliese 832-c orbits near the inner edge of the conservative habitable zone. Average temperature is similar to Earth's, but with large shifts on account of its high eccentricity.
Addendum: I just received some more information from Wittenmyer:
The new planet is a "super-Earth" with at least 5.4 Earth mass. It can be considered "potentially habitable" because it receives roughly the same amount of energy from its star as the Earth does from the Sun. But (and this is a big "but"), the habitability critically depends on the properties of the atmosphere. Consider Earth and Venus — planet-hunters looking at our own solar system would see two practically identical, "habitable" planets based on the masses and orbital distances. But Venus has 90 times the atmosphere, so the extreme greenhouse effect makes it completely inhospitable.
Unfortunately, we know nothing about the atmosphere (if any!). The only way we can know that is if the planet transits across its star, but as far as we know, GJ 832c does not. So, we must speculate on the habitability based on what we do know about other super-Earths and their properties. Again, we are disadvantaged since our solar system does not have any Super-Earths — they are a relatively mysterious class of planet. GJ 832c is at the inner edge of the habitable zone, so it may be too hot. In general, larger planets are expected to have thicker atmospheres, so that works against us here. Despite what we still don't know, the GJ 832 system is interesting because it is so close to us (16 light years, just around the corner), and because there is a Jupiter-like planet in the system as well. This makes it a miniature version of our own solar system, and that architecture is extremely rare among the more than 1500 planetary systems known.
Read the entire study at the arXiv pre-print archive: "GJ 832c: A super-earth in the habitable zone". Supplementary information via PHL.
Image: PHL@UPR Aricebo, NASA Hubble, Stellarium
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