HD 217107 c

HD 217107 c is an extrasolar planet^[3] approximately 64 light-years away from Earth in the constellation of Pisces (the Fish). The planet was the second planet to be discovered orbiting the star HD 217107. HD 217107 c's existence was hypothesized in 1998 due to the eccentricity of the inner planet's orbit and confirmed in 2005 when radial velocity studies of the star indicated another, more distant and massive companion orbiting the star. The planet has an eccentric orbit lasting on order of a decade.

HD 217107 c

Discovery
Discovered by	Vogt et al.
Discovery site	United States
Discovery date	June 24, 2005
Detection method	Doppler spectroscopy
Orbital characteristics[1]
Semi-major axis	5.922+0.035 −0.044 AU
Eccentricity	0.3918+0.0064 −0.0067
Orbital period (sidereal)	5138±11 days
Inclination	88°+14° −12°
Time of periastron	2455911±15
Argument of periastron	205.0±1.1
Semi-amplitude	53.41±0.75[2]
Star	HD 217107
Physical characteristics[1]
Mass	4.37+0.13 −0.10 MJ
Temperature	198±3 K

Detection and discovery

A study of the radial velocity of HD 217107 carried out in 1998 revealed that its motion along the line of sight varied over a 7.1 day cycle, indicating the presence of a planet in orbit around the star. The planet was designated HD 217107 b, and was found to be somewhat heavier than Jupiter, and orbiting extremely close to the parent star in an orbit with quite a large eccentricity.^[4]

Most planets with orbital periods of less than 10 days have almost circular orbits, and its discoverers proposed that the high eccentricity of HD 217107 b's orbit could be due to the gravitational influence of a second planet in the system at a distance of several astronomical units (AU).^[5] Confirmation of the existence of the second planet followed in 2005, and it was designated HD 217107 c.

The parameters of this planet was initially very weakly constrained, with a period in excess of 8 years with a high eccentricity and a minimum mass of approximately two times the mass of Jupiter. Continued observations restrained the plausible solutions substantially, resulting in the current parameters published in 2008.^[6]

In 2025, the use of astrometry allowed the inclination and true mass of the planet to be found. The inclination is nearly edge-on, so the minimum mass is close to the real one.^[1]

See also

• HD 217107
• HD 217107 b