The main objective of NASA’s Kepler space telescope is to
determine the frequency of Earth-like planets around Sun-like stars. Analysis
of the data collected by Kepler has already revealed a wide diversity of Earth
mass and super-Earth mass planets. These planets have masses ranging from ~1 to
~10 times Earth’s mass. Using data collected by Kepler together with follow-up
observations by ground-based telescopes, Aviv Ofir et al. (2013) report the
discovery of a gas giant and a very low density super-Earth around the star
Kepler-87. The gas giant is identified as Kepler-87b while the low density
super-Earth is identified as Kepler-87c.
Kepler detects planets by looking for the tell-tale signature
when a planet passes in front of its parent star and blocks some of the star’s
light. It allows the planet’s size to be directly measured since a larger
planet will block more of the star’s light than a smaller planet. Because
Kepler-87b is a gas giant, it has a deeper transit depth (i.e. blocks more of
the light from its star) than Kepler-87c. Based on the transit depths,
Kepler-87b measures 13.49 ± 0.55 Earth radii and Kepler-87c measures 6.14 ±
0.29 Earth radii.
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Figure 1: Artist’s impression of a super-Earth with a
substantial outer envelop of hydrogen and helium. Credit: Paul A. Kempton.
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Figure 3: Mass-radius relation for all known planets with
masses below 30 times the mass of Earth. It is obvious that Kepler-87c occupies
a unique position on this parameter space as the lowest density planet for its
super-Earth mass range. (Aviv Ofir et al., 2013)
Kepler-87, the parent star of Kepler-87b and Kepler-87c, is
somewhat more luminous than the Sun. This is because, although Kepler-87 has
about the same surface temperature as the Sun, it spans 1.82 times the Sun’s
diameter. Kepler-87 also has an estimated age of 7 to 8 billion years. The time intervals between successive transits
show orbital periods of 114 days for Kepler-87b and 191 days for Kepler-87c. As
such, the distances of both planets from Kepler-87 can be determined and the
estimated equilibrium temperatures of Kepler-87b and Kepler-87c are 205°C and
130°C, respectively. In addition, the data from Kepler also shows the presence
of two more short-period super-Earth sized planet candidates - KOI 1574.03 and KOI
1574.04. Unlike Kepler-87c, both planet candidates are each less than 2 Earth
radii in size. Follow-up observations will be required to confirm their
planetary status.
Using the transit timing variations (TTVs) method, the
masses of Kepler-87b and Kepler-87c are determined to be 324.2 ± 8.8 Earth
masses and 6.4 ± 0.8 Earth masses, respectively. The TTVs method involves
measuring the timing variations between consecutive transits when Kepler-87b
and Kepler-87c transit their parent star. These timing variations are caused by
gravitational perturbations on the outer planet (Kepler-87c) by the inner
planet (Kepler-87b) and vice versa. With their sizes and masses known, the bulk
densities of Kepler-87b and Kepler-87c are determined to be 73 percent and 15
percent the density of liquid water, respectively. As a result, Kepler-87b is a
rather typical Jupiter mass planet with a Saturn-like density while Kepler-87c
is an extremely low density planet in the super-Earth mass regime.
Kepler-87c is the lowest density planet currently known in
the super-Earth mass regime and its density is even similar to the least dense hot-Jupiters
such as TrES-4b and WASP-17b. However, unlike these hot-Jupiters, Kepler-87c is
not strongly irradiated by its parent star. Kepler-87c’s equilibrium
temperature of 130°C pales in comparison with the temperatures of hot-Jupiters
that can get up to ~2000°C. The extremely low density of Kepler-87c is consistent
with ~20 percent or more of its mass in the form of a low density gaseous outer
envelope comprised of hydrogen and helium. For comparison, the super-Earth
Kepler-10b has an estimated bulk density of 8.8 times the density of liquid
water. Such a high density implies that Kepler-10b must be predominantly made
of rock and iron. The almost two orders of magnitude density difference between
Kepler-87c and Kepler-10b demonstrates the enormous compositional variety for
planets with masses between ~1 to ~10 times Earth’s mass.
Reference:
Aviv Ofir et al. (2013), “An Independent Planet Search In
The Kepler Dataset. II. An extremely low-density super-Earth mass planet around
Kepler-87”, arXiv:1310.2064 [astro-ph.EP]