Despite having lost two of its reaction wheels, the Kepler mission has proven itself still capable of making discoveries. Now in a mission extension called K2, in which radiation pressure from the Sun stabilizes the spacecraft, Kepler has continued to detect planets in distant solar systems. And one of its latest discoveries is an especially intriguing pair of Earth-sized planets transiting a small, cool star only ~200 light-years away!
Earth-sized planets that orbit close to their host stars are thought to be remarkably common. They’re predicted to exist around more than a quarter of Sun-like stars, and to be nearly ubiquitous around the smaller, cooler M dwarfs. Unfortunately, systems with M-dwarf hosts are hard to find, since they’re often very faint; a large survey is needed to spot the few M dwarfs near enough to be easily detectable. Luckily, Kepler has risen to the occasion!In a recent paper, a team of scientists led by Erik Petigura (Hubble Fellow at the California Institute of Technology) reports the discovery of two new transiting, Earth-sized planets around nearby M dwarf K2-21. The team followed up with spectroscopy of the host star, which allowed them to estimate that the two planets, K2-21b and K2-21c, have radii roughly 1.6 and 1.9 times the radius of Earth. These sizes mean that they straddle the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes.
One unanswered question about close-in, small planets common around dwarfs is whether they form in situ, or form far from their host and migrate inward. K2-21b and c have orbital periods of approximately 9.3 and 15.5 days, which means they are very nearly in a 5:3 resonance. This may be evidence that they formed further out and migrated inward, as planets evolving according to this model often get trapped in resonance during their migration.
Another interesting feature of the K2-21 system is that the planets receive fairly low levels of radiation from their host star. This is unusual: more than 80% of the planets we’ve found with radii of R<2 Earth radii receive more than 100 times the stellar flux we get on Earth — which irradiates their atmospheres and drives mass loss. This system’s levels of incident radiation are much closer to those of Earth, and it’s nearby enough that we can follow up with studies that look at transit timing, radial velocity, and even atmospheric transmission once James Webb is operational!
Erik A. Petigura et al 2015 ApJ 811 102. doi:10.1088/0004-637X/811/2/102