Thursday, August 22, 2013

Star-Hugging Planets

A preliminary search for very short period transiting planets in the publicly available Kepler dataset has revealed 13 planet candidates with orbital periods ranging from 3.3 to 10 hours. Confirming whether these planet candidates are indeed planets will require additional follow-up observations and data analysis. If revealed to be true planets, they can only be small rocky planets since gas giant planets cannot survive at such small distances to their host stars.

Even with just a few Earth masses, these planet candidates can induce stellar radial velocity signatures (~ 10 m/s) that are large enough to be detectable by current ground-based instruments. The reason is because these planet candidates orbit incredibly close to their host stars and exert much larger gravitational tuggings on their host stars as compared to similar planets orbiting further out. For example, a 10 Earth mass planet in a 5 hour orbit around a solar mass star induces a radial velocity of 10 m/s.

The upcoming Transiting Exoplanet Survey Satellite (TESS) mission has a short observational cadence length of one minute. This makes TESS ideal for finding transiting planets with very short orbital periods that may be missed by Kepler’s 30 minutes observational cadence. If very short period planets are common, they should represent a significant fraction of the planets TESS finds.

Very short period planets are unlikely to form where they are currently observed to be. This is because at such close distances from their host stars, the temperatures are simply too high for rock material to condense. One possible way to get a very short period planet is by the inward migration of a gas giant planet. In this scenario, a planet that is located closer to its host star gets captured into resonance by an inward migrating gas giant planet and the end result is a very short period planet with a gas giant planet further out. This process can be tested by looking for outer, more massive planets that accompany very short period planets.

Another possible origin for very short period planets is by the tidal disruption of gas giant planets that come too close to their host stars. In this process, what is left of the tidally disrupted gas giant planet is its small rocky core in a very close-in orbit around its host star. The detection of these 13 very short period planet candidates adds yet another fascinating and unanticipated species to the growing menagerie of planetary systems.

Jackson et al. (2013), “A Survey for Very Short-Period Planets in the Kepler Data”, arXiv:1308.1379 [astro-ph.EP]