Figure 1: Artist’s impression of a gas giant planet.
HATS-17b is a warm Jupiter-mass planet in a circular orbit around a Sun-like star. Every 16.255 days, HATS-17b transits its host star. The host star of HATS-17b has ~1.13 times the mass and ~1.09 times the radius of the Sun. Additionally, the host star’s surface temperature is estimated to be 5846 ± 78 K and the luminosity of the host star is ~25 percent grater than the Sun’s. By measuring how much light the plant blocks when it transits its host star, the radius of HATS-17b is estimated to be 0.777 ± 0.056 times the radius of Jupiter.
Subsequent radial velocity measurements indicate that HATS-17b has 1.338 ± 0.065 times the mass of Jupiter. With the size and mass known, the density of HATS-17b is found to be ~3.50 g/cm³, roughly 30 percent denser than aluminium. The density of HATS-17b is remarkably high for an object with its mass. For comparison, the mean density of Jupiter is 1.326 g/cm³.
Figure 2: Transit light curve indicating the presence of HATS-17b. Brahm et al. (2015)
Figure 3: Radial velocity curve indicating the presence of HATS-17b. The lower panel shows the residuals from the best-fit model. Brahm et al. (2015)
Gas giant planets are typically comprised almost entirely of hydrogen and helium, with heavier elements making up just a small proportion of the mass. This is not the case for HATS-17b. Among gas giant planets with less than twice the mass of Jupiter, HATS-17b is currently the densest known. The compact nature of HATS-17b means that the gravity on HATS-17b is almost 6 times the surface gravity on Earth.
Interior models of HATS-17b suggest that ~50 percent of the planet’s mass is comprised of a massive core of heavier elements. This works out to be ~200 times the mass of Earth. The massive core of HATS-17b is consistent with the high metallicity of its parent star. A star’s metallicity basically refers to its abundance of elements heavier than hydrogen and helium. The host star of HATS-17b has roughly twice the metallicity of the Sun and a more metal-rich protoplanetary disk can form massive cores more efficiently.
HATS-17b is the first warm Jupiter-mass planet detected by the HATSouth network - a network of 6 telescopes in South America, Africa, and Australia. The orbital period of HATS-17b is currently the longest known for any transiting planet detected from the ground. HATS-17b orbits its host star at a distance of 0.131 AU (i.e. 19.6 million km). At that distance, the level of irradiation HATS-17b receives from its host star is over 70 times more than what Earth receives from the Sun, and the temperature on HATS-17b is estimated to be 814 ± 25 K. HATS-17b belongs to the category of gas giant planets known as warm-Jupiters since it is still not hot enough to be considered a hot-Jupiter.
Figure 4: Left panel: mass-radius diagram of the known transiting gas giant planets. HATS-17b is the one of the smallest known gas giant planet for its mass. Right panel: density of gas giant planets as function of the planetary mass. HATS-17b lies at the upper envelope of this distribution. Black circles indicate gas giant planets with irradiation levels high enough for them to be classified as hot-Jupiters, while orange circles correspond to planets receiving lower levels of irradiation. Brahm et al. (2015)
Brahm et al. (2015), “HATS-17b: A Transiting Compact Warm Jupiter in a 16.3 Days Circular Orbit”, arXiv:1510.05758 [astro-ph.EP]