Tuesday, December 18, 2012

Potentially Habitable Planets around Gliese 667C

Gliese 667C is a low mass red dwarf star located at a distance of 22 light years away. It is one-third as massive as the Sun and has 1.4 percent the Sun's luminosity. Observations using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph mounted on the European Southern Observatory's (ESO) La Silla telescope have revealed the presence of multiple planets around Gliese 667C. HARPS detect planets by measuring the Doppler shifts in a star's spectrum caused by small gravitational tugs on the star by the possible presence of one or more planets orbiting the star.

The 6 Keplerian signals detected by HARPS for Gliese 667C is consistent with a system of up to 6 planets with orbital periods of 7.2, 28.1, 30.8, 38.8, 53.2 and 91.3 days. The 7.2 and 28.1 days signals correspond to the orbital periods of two previously known planets around the star. It should be noted that the signal with a period of 53.2 days may not be from a planet since this period also corresponds to the 2nd harmonic of the star's rotation. The five planets detected by HARPS with orbital period, distance and mass in parenthesis are:
Gliese 667Cb (7.2 days, 0.049 AU, 5.4 MEarth),
Gliese 667Cc (28.1 days, 0.123 AU, 4.8 MEarth),
Gliese 667Cd (30.8 days, 0.130 AU, 3.1 MEarth),
Gliese 667Ce (38.8 days, 0.152 AU, 2.4 MEarth), and
Gliese 667Cf (91.3 days, 0.268 AU, 5.4 MEarth).

With only 1.4 percent the Sun's luminosity, the habitable zone around Gliese 667C is expected to be located much closer to the star. Here, the habitable zone is defined as the region around the star where temperatures are suitable for liquid water to exist on the surface of a rocky planet. As a result, the three planets with orbital periods of 28.1 days (Gliese 667Cc), 30.8 days (Gliese 667Cd) and 38.8 days (Gliese 667Ce) all happen to reside in the centre section of the habitable zone of Gliese 667C. Although the outermost planet Gliese 667Cf is just within the outer edge of the habitable zone, its eccentric orbit means that it actually spends most of its time beyond the habitable zone, possibly making it too cold to be considered potentially habitable. With 3 potentially habitable planets, Gliese 667C makes a particularly interesting target for follow-up observations that can determine the habitability of these telluric worlds.

Philip C. Gregory (2012), “Evidence for Multiple Planets in the Habitable Zone of Gliese 667C: A Bayesian Re-analysis of the HARPS Data”, arXiv:1212.4058