Figure 1: Artist’s impression of a gas giant planet.
Hartman et al. (2015) present the detection of HATS-6b, a warm gas giant planet in orbit around a low-mass M-dwarf star. HATS-6b is about the same size as Jupiter. It also has ~0.32 times the mass of Jupiter, about the same mass as Saturn. Circling its host star at a distance of 0.036 AU, HATS-6b takes 3.3 days to complete an orbit. The host star of HATS-6b has 0.57 times the Sun’s mass and only ~1/18th the Sun’s luminosity. HATS-6b and its host star are ~500 light-years away.
HATS-6b receives considerably less stellar irradiation than hot Jupiters, which is why it is termed a “warm” gas giant planet. Based on the luminosity of its host star and its distance from it, HATS-6b is estimated to have a temperature of about 700 K. Its atmospheric chemistry is expected to be quite different compared to hotter planets. HATS-6b appears to be a promising target for atmospheric characterization with the upcoming James Webb Space Telescope (JWST).
The host star of HATS-6b is one of only four stars with less than ~0.6 times the Sun’s mass known to host a gas giant planet in a relatively close-in orbit - the other three are WASP-80, WASP-43 and Kepler-45. Furthermore, the combination of a large planet with a relatively small star means that the transit depth (i.e. the fraction of starlight that is blocked by the planet when the planet transits) of HATS-6b is one of the deepest known.
Figure 2: Transit light curve of HATS-6b. The solid line shows the best-fit transit model. Hartman et al. (2015).
Figure 3: Comparison of HATS-6b with other known transiting gas giant planets in a plot showing host star mass versus planet mass. HATS-6b is indicated by the filled red triangle. Hartman et al. (2015).
Hartman et al. (2015), “HATS-6b: A Warm Saturn Transiting an Early M Dwarf Star, and a Set of Empirical Relations for Characterizing K and M Dwarf Planet Hosts”, Astronomical Journal 149, 166.