HD 217107

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HD 217107
Observation data
Epoch J2000.0
Constellation Pisces
Right ascension 22h 58m 15.54s
Declination −2° 23′ 43.39″
Apparent magnitude (V) +6.17
Spectral type G8IV
B-V colour index 0.744
U-B colour index 0.42
Variable type none
Radial velocity (Rv) −14.0 km/s
Proper motion (μ) RA: −6.05 mas/ yr
Dec.: −16.03 mas/ yr
Parallax (π) 50.71 ± 0.75 mas
Distance 64.3 ± 1 ly
(19.7 ± 0.3 pc)
Absolute magnitude (MV) 4.70
Mass 0.98 M
Radius 1.31 R
Luminosity 0.97 L
Temperature 5570 K
Metallicity 0.29
Rotation 39 days
Age 7.7 ×109 years
Other designations
HR 8734, BD -03°5539, HD 217107, SAO 146412, FK5 3836, HIP 113421.

HD 217107 is a yellow subgiant star in the constellation Pisces. Its mass is very similar to the Sun's, although it is considerably older. Two planets have been discovered orbiting the star: one is extremely close to the star and completes an orbit every seven days, while the other is much more distant, taking 8 years to complete an orbit.

Distance, age and mass

HD 217107 is fairly close to the Sun: the Hipparcos astrometric satellite measured its parallax as 50.71 milliarcseconds, which corresponds to a distance of 64 light years. Its apparent magnitude is 6.17, making it just barely visible to the naked eye under favourable conditions.

Spectroscopic observations show that its spectral type is G7 or G8, which means its temperature is about 5,000 K. Its mass is thought to be roughly the same as the Sun's, although its estimated age of 7.7 billion years is rather older than the Sun's 4.6 billion years, and it is thought to be beginning to evolve away from the main sequence, having consumed almost all the hydrogen in its core in nuclear fusion reactions.

Planetary system

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. The period and amplitude of this variation indicated that it was caused by a planetary companion in orbit around the star, with a minimum mass slightly greater than that of Jupiter . The companion planet was designated HD 217107 b.

While most planets with orbital periods of less than 10 days have almost circular orbits, HD 217107 b has a somewhat eccentric orbit, and its discoverers hypothesised that this could be due to the gravitational influence of a second planet in the system at a distance of several astronomical units (AU). Confirmation of the existence of a second planet followed in 2005, when long term observations of the star's radial velocity variations revealed a variation on a period of about 8 years, caused by a planet with a mass at least twice that of Jupiter in a very eccentric orbit with a semimajor axis of about 4 AU. . The second planet was designated HD 217107 c.

(In order from star)
( MJ)
Orbital period
Semimajor axis
( AU)
b >1.37 ± 0.14 7.1269 ± 0.00022 0.074 ± 0.002 0.13 ± 0.02
c >2.1 ± 1 3150 ± 1000 4.3 ± 2 0.55 ± 0.20
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