Highly-Irradiated Compact Hot Jupiters

The HATSouth survey is a network of 6 telescopes designed to detect transiting exoplanets around relatively bright stars visible from the Southern Hemisphere. As part of the survey, Brahm et al. (2015) report the discovery of two highly-irradiated hot Jupiters that appear relatively compact in comparison with other hot Jupiters with similar irradiation levels.

The two hot Jupiters are HATS-9b and HATS-10b. HATS-9b is in a 1.9 day orbit around an old, slightly evolved star with similar mass as the Sun. The planet has 0.837 times the mass and 1.065 times the radius of Jupiter. The other hot Jupiter, HATS-10b, is in a 3.3 day orbit around a Sun-like star. HATS-10b has 0.53 times the mass and 0.97 times the radius of Jupiter.

Figure 1: Artist’s impression of a giant planet.

Despite the intense irradiation due to their closeness to their host stars, HATS-9b and HATS-10b do not appear inflated. When plotted on a temperature versus radius diagram, both planets deviate from the known correlation between a planet’s radius and the intensity of irradiation it receives. HATS-9b and HATS-10b should have 1.36 and 1.22 times the radius of Jupiter, respectively. However, their observed sizes are 3σ and 5σ below these values. Both HATS-9b and HATS-10b can be classed as non-inflated hot Jupiters.

Hot Jupiters are basically Jupiter-like planets composed primarily of hydrogen and helium. However, hot Jupiters orbit very close to their host stars and their orbital periods are measured in days rather than years as in the case for Jupiter. One possible explanation for the compactness of HATS-9b and HATS-10b is both planets contain significant amounts of heavy elements in massive cores ~60 times Earth’s mass.

HATS-9b will be observable by NASA’s Kepler space telescope as part of the K2 mission. Kepler’s exquisite precision should allow the reflected light signature of HATS-9b to be measured so that the planet’s albedo (i.e. reflectivity) can be estimated to give a better measurement of the planet’s temperature.

Figure 2: Mass-radius diagram of hot Jupiters receiving similar insolation levels as HATS-9b (marked with a triangle). Filled symbols are coloured according to the metallicity of the host star (i.e. the proportion of heavy elements in the host star). HATS-9b deviates from the known correlation. Brahm et al. (2015).

Reference:
Brahm et al. (2015), “HATS-9b and HATS-10b: Two Compact Hot Jupiters in Field 7 of the K2 Mission”, arXiv:1503.00062 [astro-ph.EP]