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.
Figure 1: Artist’s impression of a super-Earth with a substantial outer envelop of hydrogen and helium. Credit: Paul A. Kempton.
Figure 2: Transit light curves of the planets and planet candidates in the Kepler-87 system. From top to bottom: transiting exoplanets Kepler-87b and Kepler-87c, and planetary candidates KOI 1574.03 and KOI 1574.04. Above each light curve are the model residuals. (Aviv Ofir et al., 2013)
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.
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]