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.
Reference:
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]