A Hot Jupiter with Friends

WASP-47b is a hot-Jupiter in a close-in 4.16 day orbit around a Sun-like star. It has 337 times the mass and 12.7 times the radius of Earth, making it similar in mass and size to Jupiter. Using data from the K2 mission, Becker et al. (2015) present the discovery of two additional planets in the system. The two planets are identified as WASP-47c and WASP-47d. WASP-47c is a super-Earth with an orbital period of 0.79 days, 1.82 times the radius of Earth and is estimated to have less than 8.9 times the mass of Earth. WASP-47d is a Neptune-sized planet with an orbital period of 9.03 days, 3.60 times the Earth’s radius and is estimated to have 8.5 times the Earth’s mass.

With a super-Earth interior to it and a Neptune-sized planet exterior to it, WASP-47d is the first known hot-Jupiter with close-in planetary companions. The orbits of the two planetary companions are coplanar with WASP-47b, and all three planets have almost perfectly circular orbits. WASP-47b and its two close-in companion planets form an extremely compact planetary system. The orbits of all three planets fit well within Mercury’s orbit around the Sun. Nevertheless, calculations indicate that the planetary system is dynamically stable in the long run.

Phase-folded K2 transit light curves of WASP-47b, c and d overlaid with the best-fit transit models (red curves), and binned points (purple circles). Becker et al. (2015)

Hot-Jupiters are basically Jupiter-sized planets that formed further out and subsequently migrated closer in to their host stars. There appears to be a lack of hot-Jupiters with close-in planetary companions. This supports the idea that hot-Jupiters form via the high eccentricity migration (HEM) process. Such a process would destabilise the orbits of other planets in the system, resulting in the lack of close-in planetary companions to hot-Jupiters. However, studies have also shown that not all hot-Jupiters form via the HEM process and these hot-Jupiters can have close-in planetary companions. WASP-47b and its close-in planetary companions demonstrate that Jupiter-sized planets can migrate toward their host stars in a dynamically stable manner.

Reference:
Becker et al. (2015), “WASP-47: A Hot Jupiter System with Two Additional Planets Discovered by K2”, arXiv:1508.02411 [astro-ph.EP]

Massive Brown Dwarf in Resonance with its Host Star

CoRoT is a space observatory which hunts for exoplanets by searching for dips in the brightness of stars when a planet happens to pass in front of its host star. Using data collected by CoRoT during its mission, Csizmadia et al. (2015) present the discovery of a massive brown dwarf that orbits a Sun-like star in a close-in orbit with a period of 5.82 days. The brown dwarf is identified as CoRoT-33b. Together with radial velocity measurements, CoRoT-33b is estimated to have 1.1 times the radius and 59 times the mass of Jupiter. The bulk density of CoRoT-33b is 55 g/cm³, several times the density of iron.

Although giant planets and brown dwarfs are similar in size to Jupiter, their masses can range from roughly the same mass as Jupiter up to ~80 times the mass of Jupiter, the maximum mass for brown dwarfs. CoRoT-33b is a relatively rare object and it is in the so called brown dwarf desert. Brown dwarfs in close-in orbits around solar-type stars (i.e. FGK main sequence stars) appear to be ~10 times less common than giant planets in similar close-in orbits around solar-type stars. CoRoT-33b may be in a 2:3 resonance with its host star. The host star of CoRoT-33b rotates twice for ever 3 orbits of CoRoT-33b.

Reference:

Csizmadia et al. (2015), “Transiting exoplanets from the CoRoT space mission XXVIII. CoRoT-33b, an object in the brown dwarf desert with 2:3 commensurability with its host star”, arXiv:1508.05763 [astro-ph.EP]