Using data collected by NASA’s Kepler space telescope, four transiting planetary candidates were found around the star KOI-152. The four planet candidates, identified as KOI-152 b, c, d and e, range in size from 3.5 to 7 times the size of Earth. They circle KOI-152 with orbital periods of 13.5, 27.4, 52.1 and 81.1 days - near a 1:2:4:6 chain of commensurability. All four planet candidates orbit their parent star in a region that is tighter than Mercury’s orbit around the Sun. The planet candidates gravitationally perturb one another and cause the transit timing of each planet candidate to exhibit variations. By analysing the transit timing variations, the masses of all four planet candidates were found to be rather small for their sizes. This means that all four planet candidates around KOI-152 have low densities (Figure 3).
Figure 1: Artist’s impression of a low density planet with a substantial gaseous envelope.
Figure 2: Artist’s impression of a low density planet with a substantial gaseous envelope.
Figure 3: The planet candidates of KOI-152, with masses and radii in Earth units. The final column shows the amount of stellar flux each planet candidate receives, where a value of 1 represents the amount of flux Earth receives from the Sun.
The planet candidates around KOI-152 have masses ranging from 4.1 to 10.9 times the mass of Earth. Planets with such masses have no known analogues in the Solar System because the Solar System has no object intermediate in mass between Earth and Uranus/Neptune, both of which are more than 14 times the mass of Earth. Of the four planet candidates, KOI-152 b has the highest bulk density even it is similar in size to KOI-152 c and e. This suggests that KOI-152 b is comprised of either a substantial mass fraction of water and/or a relatively thin hydrogen-helium envelope, with a denser rocky interior. The less dense KOI-152 c and e are likely to have more voluminous envelopes of water and/or hydrogen-helium.
Finally, the largest of the four planet candidates, KOI-152 d, has a remarkably low density of just 9 percent the density of liquid water. KOI-152 d is expected to have a very voluminous hydrogen-helium envelope comprising more than 10 percent, but less than 50 percent of its mass. Compared to dense rocky planets such as Earth with 5.5 times the density of liquid water and Kepler-10b with 8.8 times the density of liquid water, the low density planet candidates around KOI-152, especially the extremely low density KOI-152 d, show that there is a tremendous compositional variation amongst planets with ~1 to 10 times Earth’s mass.
Daniel Jontof-Hutter, Jack J. Lissauer, Jason F. Rowe1 and Daniel C. Fabrycky (2013), “KOI-152’s Low Density Planets”, arXiv:1310.2642 [astro-ph.EP]