Wednesday, October 23, 2013

A Trio of Inflated Hot Jupiters

Figure 1: Artist’s impression of a hot Jupiter.

As part of the SuperWASP (Wide Angle Search for Planets) program, an international team of researches have reported the discovery of three highly irradiated and bloated hot Jupiters. Like Jupiter, these planets are gas giants and are comprised almost entirely of hydrogen and helium. However, unlike Jupiter, all three planets circle in very tight orbits around stars that are hotter and larger than the Sun - F5 to F7-type stars with effective surface temperatures of 6,250 K to 6,500 K. In comparison, the Sun is a cooler G2-type star with an effective surface temperature of 5,780 K. The three planets, identified as WASP-76 b, WASP-82 b and WASP-90 b, take just 1.81, 2.71 and 3.92 days respectively to circle their parent stars.

All three planets are bloated, with diameters of 1.6 to 1.8 times the diameter of Jupiter, and their intensely irradiated day-sides are scorched to temperatures of ~2,000 K. WASP-76 b, WASP-82 b and WASP-90 b belong to a class of inflated hot Jupiters. Planets of this nature tend to orbit in very close proximity to stars that are somewhat hotter than the Sun. In fact, planets with as much as ~2 times the diameter of Jupiter have been found. An example is the hyper-inflated WASP-17 b. It appears that stellar irradiation plays a key role in determining where a hot Jupiter is inflated, because all known inflated hot Jupiters receive more than 150 times the amount of stellar irradiation Earth gets from the Sun (or 4,000 times the stellar irradiation Jupiter gets from the Sun). There is an extensive literature regarding the mechanisms for inflating hot Jupiters. Such mechanisms include tidal heating and Ohmic dissipation.

Figure 2: Parameters for WASP-76 b, WASP-82 b and WASP-90 b.

 Figure 3: A number of planets with measured mass, radius and stellar incident flux. The black points are planets with less than 150 times the mass of Earth and the red points are planets with more than 150 times the mass of Earth (i.e. giant planets). Giant planets with inflated radii are shown at the top-right corner of the plot. (Weiss et al., 2013)

- West et al. (2013), “Three irradiated and bloated hot Jupiters: WASP-76b, WASP-82b & WASP-90b”, arXiv:1310.5607 [astro-ph.EP]
- Weiss et al., “The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux”, 2013 ApJ 768: 14