HAT-P-18

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.[3] A survey in 2015 detected very strong starspot activity on HAT-P-18.[4]

HAT-P-18
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension  17h 05m 23.1476s[1]
Declination +33° 00′ 44.9391″[1]
Apparent magnitude (V) 12.759[2]
Characteristics
Spectral type K2V
Astrometry
Radial velocity (Rv)-11.83 km/s
Proper motion (μ) RA: -14.073 mas/yr
Dec.: -36.689 mas/yr
Parallax (π)6.1669 ± 0.0231[1] mas
Distance529 ± 2 ly
(162.2 ± 0.6 pc)
Details[3]
Mass0.770±0.031 M☉
Radius0.749±0.037 R☉
Luminosity0.27±0.04 L☉
Surface gravity (log g)4.72 cgs
Temperature4857 K
Metallicity-0.05
Rotation14.66±0.03 d
Rotational velocity (v sin i)0.5±0.5 km/s
Age12.4+4.4
−6.4 Gyr
Other designations
Gaia DR2 1334573817793362560, GSC 02594-00646, 2MASS J17052315+3300450[1]
Database references
SIMBADdata

Planetary system

In 2010 a transiting hot Saturn-sized planet was detected.[2] Its equilibrium temperature is 841 K.[5]

In 2014, observations utilizing a 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°.[6]

The transit-timing variation measurements in 2015 did not detect additional planets in the system.[7]

In 2016, the transmission optical spectra of planet indicated that the atmosphere is lacking detectable clouds or hazes, and is blue in color due Rayleigh scattering of light.[8]

The dayside temperature of the HAT-P-18b was measured in the 2019 to be 1004+78
−94 K.[9]

Size comparison of HAT-P-18 b and Jupiter
The HAT-P-18 planetary system[2]
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

References
  1. HAT-P-18 -- Star
  2. HAT-P-18b and HAT-P-19b: Two Low-Density Saturn-Mass Planets Transiting Metal-Rich K Stars, 2010, arXiv:1007.4850
  3. Spectroscopy of Faint Kepler Mission Exoplanet Candidate Host Stars, 2013, arXiv:1305.0578
  4. FRIENDS OF HOT JUPITERS III: AN INFRARED SPECTROSCOPIC SEARCH FOR LOW-MASS STELLAR COMPANIONS, 2015, arXiv:1510.08062
  5. Planet HAT-P-18 b at exoplanets.eu
  6. The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b, 2014, arXiv:1403.6728
  7. Ground-based transit observations of the HAT-P-18, HAT-P-19, HAT-P-27/WASP40 and WASP-21 systems, 2015, arXiv:1508.06215
  8. Rayleigh scattering in the transmission spectrum of HAT-P-18b, 2016, arXiv:1611.06916
  9. INVESTIGATING TRENDS IN ATMOSPHERIC COMPOSITIONS OF COOL GAS GIANT PLANETS USING SPITZER SECONDARY ECLIPSES, 2019, arXiv:1908.00014

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