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