Wednesday 15 February 2012

Gliese 667; possibly habitable planets in a tertiary star system.

Gliese 667 (or GJ 667) is a tertiary star system slightly under 23 light years from the Earth in the constellation of Scorpius. It comprises three stars, Gliese 667A, B &C; Gliese 667A & B orbit their mutual centre of gravity every 42.15 years, in eccentric orbits with an average separation of 12.6 AU (12.6 times the distance between the Earth and the sun) but varying between 5 AU and 20 AU; the equivalent of moving between the orbits of Jupiter and Neptune in our Solar System. Gliese 667A & B are K-type orange dwarf stars with 73% and 69% of the mass of the sun, respectively. Gliese 667C is an M-type red dwarf star with 37% of the Sun's mass, which orbits the Gliese 667AB pair at a distance of about 230 AU; nearly eight times the distance between Neptune and the Sun.

In a paper published on the online arXiv database at Cornell University Library on 2 February 2012, a team of scientists lead by Guillem Anglada-Escudé of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington and the Institut für Astrophysik at Universität Göttingen, describe a search for planets orbiting Gliese 667C, using data from the European Southern Observatory 3.6-m telescope at La Silla Observatory in the Atacama Desert of Chile.

Anglada-Escudé et al. concluded that there are at least two planets orbiting Gliese 667C, dubbed Gliese 667Cb & Gliese 667Cc, and there was some evidence for a third, Gliese 667Cd.

Gliese 667Cb orbits its parent star at a distance of 0.049 AU (4.9% of the distance at which the Earth orbits the sun), every 173 hours. It has a mass 5.68 times that of the Earth, and is likely to be a hothouse world with a dense atmosphere.

Gliese 667Cc orbits Gliese 667C at 0.123 AU, less than a third of the distance at which Mercury orbits the Sun, but in the habitable zone (the zone in which water could potentially exist as a liquid on a planet) of Gliese 667C, since this star is so much smaller and cooler than our sun, with an effective temperature of 3700 K, compared to 5778 K for our Sun. Gliese 667Cc has a year of just over 28 days, and is 4.54 times as massive as the Earth. This large mass has implications for the chances of life on Gliese 667Cc, as it makes it more likely the planet has a thick atmosphere that may cause a runaway greenhouse effect, but our understanding of these processes is still in its infancy, so this does not rule out liquid water or life. Gliese 667 A and B are too distant to have a notable impact on the planets temperature.

The team were less certain about Gliese 667Cd; they have produced probable statistics for it based upon their research, but cannot be certain if the planet exists at all, so if it does it is quite likely that some of these preliminary statistics will turn out to be wrong. As currently envisaged Gliese 667Cd orbits the star at a distance of 0.235 AU, every 75 days. It has a mass of 5.65 times that of the Earth. Gliese 667 Cd would be at the outer limits of the habitable zone of Gliese 667C, but in this case its large size would count in its favor as a possible habitable world, since at this distance from the star a strong greenhouse effect might help to keep the planet from freezing over, though, as noted above, our limited understanding makes this highly speculative.

Comparative diagrams of the Gliese 667C system of our own, showing estimated habitable zones and the inner three planets. Not to scale. Mars falls within the habitable zone of our system tough it has no water due to its small size and thin atmosphere; were it as large as the Earth it would be able to maintain liquid water in its current position, though it would still be much colder. From Anglada-Escudé et al. (2012).