A System of Five Super-Earths and a Saturn-Mass Planet

Vogt et al. (2015) present the discovery of a six-planet system orbiting the nearby K3V star HD 219134. The planetary system consists of at least 5 super-Earth mass planets in a compact configuration and a Saturn-mass planet orbiting much further out. The six planets have orbital periods of 3.1, 6.8, 22.8, 46.7, 94.2 and 2247 days, spanning 3.8, 3.5, 8.9, 21.3, 10.8 and 108 times the Earth’s mass, respectively. All 5 inner planets orbit HD 219134 closer than Mercury’s average distance from the Sun.

The six planets around HD 219134 were detected by precisely measuring the tiny wobbles the planets induce on their host star. The 5 inner planets are orbiting too close, and hence, too hot to be habitable. Nevertheless, HD 219134 is 0.31 times as luminous as the Sun and a planet orbiting it at 0.56 AU, corresponding to an orbital period of 167 days, would receive the same intensity of stellar radiation Earth receives from the Sun. If this hypothetical planet has a mass equal to that of Earth, the amount of wobbling it induces on its host star would be challenging but not impossible to detect.

Reference:
Vogt et al. (2015), “A Six-Planet System Orbiting HD 219134”, arXiv:1509.07912 [astro-ph.EP]

Giant Planet Transiting a Rapidly Rotating A-Type Star

HAT-P-57b is a giant planet in a close-in orbit around a rapidly rotating A8V star. A-type stars are more massive and much more luminous than F, G and K-type stars. To date, only a handful of planets have been found around A-type stars. HAT-P-57b was detectable because it transits its host star every 2.465 days, which is also the orbital period of the planet around its host star. Such a short orbital period implies HAT-P-57b is in an extremely close-in orbit, only about 5 stellar radii from the surface of its host star. HAT-P-57b is estimated to have an equilibrium temperature of roughly 2200 K.

The host star of is estimated to have 1.47 ± 0.12 times the mass, 1.500 ± 0.050 times the radius and 6.4 ± 1.1 times the luminosity of the Sun. With an equatorial rotation velocity of at least 102 km/s, the host star of HAT-P-57b has the highest rotation velocity of any star currently known to host a transiting planet. The next most rapidly rotating stars to host transiting planets are KOI-89 with a rotation velocity of at least 90 km/s and WASP-33 with a rotation velocity of at least 86 km/s. Finally, the mass of HAT-P-57b is estimated to be no more than 1.85 times the mass of Jupiter.

Transit light curve indicating the presence of HAT-P-57b. Hartman et al. (2015)

Reference:
Hartman et al. (2015), “HAT-P-57b: A Short-Period Giant Planet Transiting A Bright Rapidly Rotating A8V Star Confirmed Via Doppler Tomography”, arXiv:1510.08839 [astro-ph.EP]