Tuesday, May 24, 2016

Discovery of the First Transiting Jupiter Analog

Kipping et al. (2016) present the discovery of the first transiting Jupiter analog. This planet is identified as Kepler-167e and it is part of a planetary system which contains three known transiting super-Earths that are in relatively close-in orbits around their host star. Kepler-167 is the host star of this planetary system. It is a K3V star with ~0.77 times the Sun's mass, ~0.73 times the Sun's radius and an effective temperature estimated to be ~4890 K.

Figure 1: Artist's impression of a gas giant planet.

The three super-Earths in this planetary system are Kepler-167b, c and d. Kepler-167b has ~1.615 times the radius of Earth, a 4.393 day orbital period, an estimated equilibrium temperature of about 914 K and it receives 116 times the amount of flux Earth gets from the Sun. Kepler-167c has ~1.548 times the radius of Earth, a 7.406 day orbital period, an estimated equilibrium temperature of about 758 K and it receives 57.7 times the amount of flux Earth gets from the Sun. Kepler-167d has ~1.194 times the radius of Earth, a 21.804 day orbital period, an estimated equilibrium temperature of about 536 K and it receives 13.7 times the amount of flux Earth gets from the Sun.

Kepler-167e orbits much further out compared to the three inner super-Earths. Kepler-167e measures ~10.15 times the size of Earth, which is roughly 90 percent the radius of Jupiter. It orbits its host star every 1071 days, its estimated equilibrium temperature is about 130 K and it receives only ~7 percent the amount of flux Earth gets from the Sun. Kepler-167e is about twice as far from its host star as Earth is from the Sun and its orbit around its host star is close to being perfectly circular. With these properties, Kepler-167e bears a great deal of resemblance to Jupiter and hence, it is termed a Jupiter analog.

The compact trio of super-Earths around Kepler-167 raises the possibility that many of the currently known compact multi-planetary systems may host Jupiter analogs on distant orbits. Kepler-167e is a rare find because it transits its host star once every 2.9 years. Since one would need 2 transits to confirm its planetary nature and given that the primary Kepler mission ran for 4.3 years, the detection of Kepler-167e was indeed a fortunate one. Because Kepler-167e is a transiting planet, it also offers a unique opportunity for the characterisation of the atmosphere of a Jupiter analog.

Figure 2: Folded transit light curves of Kepler-167b, Kepler-167c, Kepler-167d and Kepler-167e. For the upper three, data (gray points) are binned to a 10 minute cadence. Light curve of Kepler-167e uses 30 minute binning and uses circles to denote the first transit (Q4) and squares to denote the second transit (Q16). Note that all of the transits were fitted using the original unbinned data. Kipping et al. (2016)

Figure 3: Catalogue of known transiting exoplanets with colour depicting the peak wavelength colour of the parent star. Solar system planets are shown with black symbols, and the Kepler-167 planets with squares. The blue box depicts Jovian-sized planets beyond the snow-line, with Kepler-167e being the first transiting planet to be in this space. Kipping et al. (2016)

Figure 4: Schematic illustrating the scale of the Kepler-167 system. Planet sizes are scaled relative to the key, rather than the orbital distances in order to make them visible. The four known planets display remarkable coplanarity and near-circular orbits with the habitable-zone notably devoid of transiting planets. Kipping et al. (2016)

Reference:
Kipping et al. (2016), "A Transiting Jupiter Analog", arXiv:1603.00042 [astro-ph.EP]