Figure 1: Artist’s impression of a transiting planet.
HATS-18b is a very short period massive gas giant planet with 1.980 ± 0.077 times the mass and ~1.337 times the radius of Jupiter. It orbits a Sun-like host star with an orbital period of only 20.1 hours. The host star of HATS-18b has ~1.037 times the mass and ~1.020 times the radius of the Sun, and effective temperature 5600 ± 120 K. Additionally, the host star of HATS-18b is measured to have a rotation period of only 9.8 ± 0.4 days. Such a rotation period is remarkably fast given that the star is estimated to be 4.2 ± 2.2 billion years old. Sun-like stars at that age tend to have rotation periods around ~30 days. The host star of HATS-18b is basically spinning too fast for its age.
The relatively large mass and close-in orbit of HATS-18b is the reason for the short rotation period of its host star. HATS-18b raises tides on its host star and because the planet orbits faster than the star rotates, the tidal bulge raised on the star lags the planet. This causes the orbit of HATS-18b to decay due to tidal dissipation. As a result, the angular momentum that is taken out as the planet's orbit shrinks is deposited in the star, causing the star to spin up. Because HATS-18b orbits so close to its host star, the planet's equilibrium temperature is estimated to be 2060 ± 59 K.
Figure 2: Transit light curve indicating the presence of HATS-18b. Penev et al. (2016)
Penev et al. (2016), "HATS-18 b: An Extreme Short-Period Massive Transiting Planet Spinning Up Its Star", arXiv:1606.00848 [astro-ph.EP]