HAT-P-18
Star in the constellation Hercules
From Wikipedia, the free encyclopedia
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.[6] A survey in 2015 detected very strong starspot activity on HAT-P-18.[8]
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Hercules[1] |
| Right ascension | 17h 05m 23.1475s[2] |
| Declination | +33° 00′ 44.940″[2] |
| Apparent magnitude (V) | 12.759[3] |
| Characteristics | |
| Spectral type | K2V[4] |
| Astrometry | |
| Radial velocity (Rv) | -11.83 km/s |
| Proper motion (μ) | RA: −14.002(9) mas/yr[2] Dec.: −36.751(11) mas/yr[2] |
| Parallax (π) | 6.1863±0.0093 mas[2] |
| Distance | 527.2 ± 0.8 ly (161.6 ± 0.2 pc) |
| Details[5] | |
| Mass | 0.750+0.015 −0.014 M☉ |
| Radius | 0.7202+0.0095 −0.01 R☉ |
| Luminosity | 0.27±0.04[6] L☉ |
| Surface gravity (log g) | 4.599±0.013 cgs |
| Temperature | 4835+39 −35 K |
| Metallicity | 0.044+0.060 −0.051 |
| Rotation | 14.66±0.03 d[6] |
| Rotational velocity (v sin i) | 0.5±0.5[6] km/s |
| Age | 12.4+4.4 −6.4[6] Gyr |
| Other designations | |
| GSC 02594-00646, 2MASS J17052315+3300450, Gaia DR2 1334573817793362560[7] | |
| Database references | |
| SIMBAD | data |
Planetary system
In 2010 a transiting hot Saturn-sized planet was detected.[3] Its equilibrium temperature is 841 K.[9]
In 2014, observations utilizing the Rossiter–McLaughlin effect detected an exoplanet, HAT-P-18b, on a retrograde orbit, with an angle between orbital plane of the planet and the parent star equatorial plane equal to 132±15°.[10]
Transit-timing variation measurements in 2015 did not detect additional planets in the system.[11]
In 2016, the transmission optical spectra of the planet indicated that the atmosphere is lacking detectable clouds or hazes, and is blue in color due to Rayleigh scattering of light.[12] The atmosphere seems to gradually evaporate, but at a slow rate - less than 2% of planetary mass is lost per one billion years.[13] By contrast, spectra taken in 2022 showed extensive haze and clear evidence of water vapour, along with the tail of escaping helium.[14]
The dayside temperature of HAT-P-18b was measured in 2019 to be 1004+78
−94 K.[15]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination (°) |
Radius |
|---|---|---|---|---|---|---|
| b | 0.183+0.034 −0.032 MJ |
0.04649± | 5.508029±0.0000042 | 0.106+0.15 −0.084 |
88.79±0.21 | 0.947±0.044 RJ |