Properties of a Newly Discovered Super-Neptune

Bakos et al. (2015) present the discovery of HATS-7b, a transiting Super-Neptune with an orbital period of 3.185 days around a K dwarf star. The host star of HATS-7b has an effective temperature of 4,990 K, 85 percent the Sun’s mass, 82 percent the Sun’s diameter and shines with 37 percent the luminosity of the Sun. Being a K dwarf star, it is somewhat cooler and less luminous than the Sun - a G dwarf star. The detection of HATS-7b was made using the HATSouth network, comprised of a number of fully automated telescopes in the Southern Hemisphere. The primary goal of the HATSouth network is to search for transiting exoplanets.

Figure 1: Artist’s impression of a Neptune-like planet.

By measuring how much HATS-7b dims its host star when it transits in front, the planet is estimated to be 0.563 times the size of Jupiter. As HATS-7b circles its host star, it also gravitationally perturbs its host star, causing its host star to wobble back and forth. The magnitude of wobbling depends on the planet’s mass. Radial velocity measurements of the host star’s wobbling motion indicate that HATS-7b has 0.120 times the mass of Jupiter, placing it in the mass regime of super-Neptunes. For comparison, the planet Neptune has 0.054 times the mass of Jupiter, or 17.147 times the mass of Earth.

Figure 2: Transit light curve of HATS-7b phase folded to the planet’s orbital period of 3.185 days. The lower panel zooms in on the transit. Bakos et al. (2015).

Figure 3: Radial velocity measurements for the host star of HATS-7b. The gravitational perturbation from HATS-7b induces radial velocity semi-amplitude of 18.4 ± 1.9 m/s on its host star. Bakos et al. (2015).

Knowing the size and mass of HATS-7b allows the planet’s bulk composition to be constrained. Interior models of HATS-7b indicate a hydrogen-helium (H2-He) mass fraction of 18 ± 4 percent if the planet has a rock-iron core and a H2-He envelope, or a H2-He mass fraction of 9 ± 4 percent if the planet has an ice core and a H2-He envelope. If HATS-7b has a rock-iron core and a hydrogen-helium envelope, the best fit models give a core mass of 31 ± 4 Earth-masses and an envelope mass of 7 ± 1.5 Earth-masses. If instead HATS-7b has a ice core and a hydrogen-helium envelope, the best fit models give a core mass of 34.5 ± 4 Earth-masses and an envelope mass of 3.5 ± 1.5 Earth-masses.

The composition of HATS-7b is broadly similar to that of Uranus and Neptune, but quite different from Jupiter and Saturn, which are both predominantly comprised of hydrogen and helium. Super-Neptunes like HATS-7b and the recently discovered HATS-8b (also by the HATSouth network) are important for understanding the transition from ice giants (i.e. Uranus and Neptune) to gas giants (i.e. Jupiter and Saturn). HATS-7b circles in a close-in orbit around its host star at a distance of only 6 million km. This is 25 times closer than Earth is from the Sun. The dayside of HATS-7b is heated to a temperature of over 1,000 K.

Figure 4: Mass-radius diagram of super-Neptunes (planets with less than 0.18 times Jupiter’s mass) and super-Earths with accurately measured masses and radii (less than 20 percent uncertainties). Colour indicates equilibrium temperature. HATS-7b is marked with a box and Neptune is marked with a blue triangle. Abbreviations are: K: Kepler, H: HAT, HS: HATSouth, C: Corot. Bakos et al. (2015).

References:
- Bakos et al. (2015), “HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star”, arXiv:1507.01024 [astro-ph.EP]
- Bayliss et al. (2015), “HATS-8b: A Low-Density Transiting Super-Neptune”, arXiv:1506.01334 [astro-ph.EP]