Thursday, July 3, 2014

Super-Earth in the Habitable Zone of a Cool Star

Hundreds of extrasolar planetary systems have been discovered over the past 2 decades. The discoveries show a surprising diversity of planetary systems, with many of these systems far stranger than imagined. Among these discoveries are numerous Earth-size and super-Earth-size planets circling in the habitable zone (HZ) of their host stars where temperatures are believed to be just right for liquid water to exist. Using data gathered with the 6.5 m Magellan Telescope located at Las Campanas Observatory in Chile, R.A. Wittenmyer et al. (2014) report on the discovery of yet another super-Earth-size planet in the HZ.

Figure 1: Artist’s impression of GJ 832c as compared with Earth. Image credit: PHL @ UPR Arecibo.

The planet, identified as GJ 832c, lies just 16 light-years away. It orbits an M dwarf star that is much smaller and cooler than the Sun. The suffix “c” identifies GJ 832c as the second planet detected around the star. A Jupiter-like planet identified as GJ 832b was the first planet detected around the star and it orbits in a nearly-circular 9.4-year orbit. Much closer to the host star is GJ 832c, super-Earth-size planet situated within the HZ, albeit towards the warmer edge of the HZ. On average, GJ 832c receives as much insolation from its host star as Earth does from the Sun. The planetary system comprising GJ 832b and GJ 832c resembles a miniature version of the Solar System, with giant planets orbiting further out and small rocky worlds closer in.

GJ 832c circles its host star with a period of 35.68 ± 0.03 days and it has an estimated minimum mass of 5.4 ± 1.0 times the Earth’s mass. Although GJ 832c is sufficiently far from its host star for it to be cool enough for liquid water to exist on its surface, it is not necessarily habitable because there are a large number of other contributing factors that determine habitability. The close proximity of GJ 832c from its host star means it is likely trapped in a spin-orbit resonance, causing the planet to have a slow rotation rate. Work by Yang et al. (2014) show that a slow rotation rate might boost the habitability of GJ 832c. They have employed 3D global climate models to suggest that planets with slower rotation rates would be able to remain habitable at higher stellar flux levels than planets with more rapid rotation rates.

Figure 2: Radial velocity curve for GJ 832c using data from the Planet Finder Spectrograph (PFS) (blue) on the 6.5m Magellan Telescope and the High Accuracy Radial Velocity Planet Searcher (HARPS) (red). GJ 832c was detected from the tiny but measurable gravitational tug it exerts on its host star, causing its host star to “wobble” and produce a radial velocity signature. R.A. Wittenmyer et al. (2014).

Figure 3: Orbit of GJ 832c around its host star. Notice that GJ 832c orbits near the inner edge of the HZ. Image credit: PHL @ UPR Arecibo.

On the contrary, being a few times more massive than Earth, the surface of GJ 832c might be shrouded in a dense atmosphere, creating an enhanced greenhouse effect which might make the planet too hot to be habitable. If GJ 832c has a thick atmosphere and given that the planet is close to the warmer edge of the HZ, it is reasonable to suggest that GJ 832c might not be a habitable planet and might instead be a super-Venus. One can say that GJ 832c is in the optimistic habitable zone but outside the conservative habitable zone.

- R.A. Wittenmyer et al. (2014), “GJ 832c: A super-earth in the habitable zone”, arXiv:1406.5587 [astro-ph.EP]
- Yang et al. (2014), “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, arXiv:1404.4992 [astro-ph.EP]