HD 20781 b is an extrasolar planet orbiting the K-type main-sequence star HD 20781 117 light years away in the southern constellation Fornax. It was discovered in 2011 during a survey for southern exoplanets conducted with HARPS[1] using doppler spectroscopy, the radial velocity variations caused by the gravitational pull of the planet. Its existence was confirmed in another survey during 2019.[3]

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HD 20781 b

An artist's impression of HD 20781 b orbiting its host star.
Discovery[1]
Discovered by	Mayor et al.
Discovery site	La Silla Observatory[2]
Discovery date	2011
Detection method	Doppler spectroscopy (HARPS)
Orbital characteristics[1]
Periastron	0.15 AU
Apoastron	0.19 AU
Semi-major axis	0.1647+0.0076 −0.0083 AU[3]
Eccentricity	0.11±0.06
Orbital period (sidereal)	29.15±0.02 d
Time of periastron	2,455,511±2 JD[3]
Argument of periastron	60.99°+30.79° −30.03°[3]
Physical characteristics
Mean radius	~3.18 R🜨[2]
Mass	≥10.61+1.20 −1.19 M🜨[3]

Properties

Orbit

HD 20781 b takes only 29 days[1] to orbit its host star, classifying it as a hot Neptune. However, its orbit is slightly eccentric, carrying it as close as 0.15 AU[1] and as far as 0.19 AU.[1] Despite it being the "b" planet, HD 20781 b is the second outermost planet after HD 20781 c.

Characteristics

With a minimum mass 10.6[3] times that of Earth, it is probably a sub-Neptune, a planet that is larger than Earth but smaller than Neptune. Due to indirect detection, most of its properties can't be studied such as inclination, density, and temperature. Due to HD 20781 b's mass, it is estimated to be about 3[2] times larger than Earth, hence its classification.

References