HAT-P-18 is a K-type main sequence star about 530 light-years away. The star is very old and has a concentration of heavy elements similar to solar abundance. A survey in 2015 detected very strong starspot activity in HAT-P-18.

In 2010, a transiting hot planet the size of Saturn was detected. Its equilibrium temperature is 841 K.

In 2014, observations using the Rossiter-McLaughlin effect detected an exoplanet, HAT-P-18b, in a retrograde orbit, with an angle between the planet’s orbital plane and the equatorial plane of the parent star equal to 132±15°.

Measurements of transit time variation in 2015 did not detect additional planets in the system.

In 2016, the planet’s optical transmission spectra indicated that the atmosphere lacks detectable clouds or haze, and is blue due to the scattering of light from Rayleigh.La atmosphere seems to gradually evaporate, but at a slow rate: less than 2% of the planetary mass is lost every billion years.

The daytime temperature of HAT-P-18b was measured in 2019 at 1004+78−94K


Observations of HAT-P-18b

Collaborative projects