Kepler-51 is a fairly young star with an estimated age of ~300 million years and it is also slightly more luminous than the Sun. Observations of Kepler-51 by NASA’s Kepler space telescope found that it hosts three transiting planet candidates - Kepler-51 b, Kepler-51 c and KOI-620.02. The three planets have orbital periods of 45.2 days (Kepler-51 b), 85.3 days (Kepler-51 c) and 130.2 days (KOI-620.02), placing them close to a 1:2:3 resonance. By measuring the amount of light each planet blocks as it transits its host star, the size of each planet is found to be 7.1 (Kepler-51 b), 9.0 (Kepler-51 c) and 9.7 (KOI-620.02) times the Earth’s diameter.
Figure 1: Artist’s impression of a gaseous planet.
Figure 2: Phase-folded transit light curves of Kepler-51 b (top), Kepler-51 c (middle) and KOI-620.02 (bottom). Black dots are the observed fluxes and coloured solid lines show the best-fit models.
As the three planets circle their host star, they gravitationally perturb one another. This leads to transit timing variations (TTVs) where each planet transits the host star at slightly earlier or later timings, deviating somewhat from strictly periodic transit intervals. By studying the TTVs, Masuda (2014) derived the mass for each of the three planets to be 2.1 (Kepler-51 b), 4.0 (Kepler-51 c) and 7.6 (KOI-620.02) times the Earth’s mass. With the size and mass of each planet known, all three planets were found to have remarkably low densities of less than 5 percent the density of water, possibly the lowest densities yet determined for exoplanets. In comparison, the Earth has a mean bulk density of 5.52 times the density of water. With this finding, the Kepler-51 system serves as yet another example of a very low-density compact multi-transiting planetary system.
The planets around Kepler-51 have mean densities that are much lower than any of the planets in the solar system. To explain their “puffiness”, each planet probably possesses an extended outer hydrogen-helium envelop surrounding a denser core. Assuming the planetary system has an age of ~300 million years; calculations show that the observed radii of the Kepler-51 planets can be explained if they have about 10 percent (Kepler-51b), 30 percent (Kepler-51c) and 40 percent (KOI-620.02) of their masses in their hydrogen-helium envelopes. All three planets are unlikely to be habitable, at least for the type of life found on Earth, given that the planets have thick gaseous envelopes and equilibrium temperatures that exceed 100°C.
Masuda (2014), “Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event”, arXiv:1401.2885 [astro-ph.EP]