HD 217107

HD 217107 (6 G. Piscium) is a yellow subgiant star approximately 65 light-years away from Earth in the constellation of Pisces (the Fish). 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 and completes an orbit every seven days, while the other is much more distant, taking eight years to complete an orbit.

HD 217107
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pisces
Right ascension  22h 58m 15.5412s[1]
Declination –2° 23′ 43.3871″[1]
Apparent magnitude (V) +6.17[2]
Characteristics
Spectral type G8 IV-V[2]
B−V color index 0.744±0.006[2]
Astrometry
Radial velocity (Rv)−13.4±0.1[2] km/s
Proper motion (μ) RA: −7.108±0.125[1] mas/yr
Dec.: −14.777±0.071[1] mas/yr
Parallax (π)49.8170 ± 0.0574[1] mas
Distance65.47 ± 0.08 ly
(20.07 ± 0.02 pc)
Absolute magnitude (MV)4.68[2]
Details
Mass0.969[3] M☉
Radius1.2104±0.0195[3] R☉
Luminosity1.0951±0.0338[3] L☉
Temperature5391±40[3] K
Metallicity0.31[3]
Age11.9[3] Gyr
Other designations
6 G. Piscium, BD−03° 5539, FK5 3836, HD 217107, HIP 113421, HR 8734, SAO 146412, CCDM J22583-0224AB[4]
Database references
SIMBADdata

Distance, age, and mass

HD 217107 is fairly close to the Sun: the Gaia astrometric satellite measured its parallax as 49.8170 Milliarcseconds, which corresponds to a distance of 65.47 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.[5]

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.[5] 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 hypothesized that this could be due to the gravitational influence of a second planet in the system at a distance of several astronomical units (AU).[6] 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 eight 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 4.3 AU.[7] The second planet was designated HD 217107 c.[8]

The HD 217107 planetary system[9]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b >1.4135±0.0042 MJ 0.07505±0.00097 7.126846±0.000013 0.1283±0.0027 — —
c >4.513±0.072 MJ 6.074±0.080 5189±21 0.3848±0.0086 — —
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References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  3. Boyajian, Tabetha S.; et al. (July 2013), "Stellar Diameters and Temperatures. III. Main-sequence A, F, G, and K Stars: Additional High-precision Measurements and Empirical Relations", The Astrophysical Journal, 771 (1): 31, arXiv:1306.2974, Bibcode:2013ApJ...771...40B, doi:10.1088/0004-637X/771/1/40, 40. See Table 3.
  4. "HD 217107". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-02-26.
  5. Fischer, Debra A.; et al. (1999). "Planetary Companions around Two Solar-Type Stars: HD 195019 and HD 217107". Publications of the Astronomical Society of the Pacific. 111 (755): 50–56. arXiv:astro-ph/9810420. Bibcode:1999PASP..111...50F. doi:10.1086/316304.
  6. Fischer, Debra A.; et al. (2002). "Planetary Companions to HD 12661, HD 92788, and HD 38529 and Variations in Keplerian Residuals of Extrasolar Planets". The Astrophysical Journal. 551 (2): 1107–1118. Bibcode:2001ApJ...551.1107F. doi:10.1086/320224.
  7. Vogt, Steven S.; et al. (2005). "Five New Multicomponent Planetary Systems". The Astrophysical Journal. 632 (1): 638–658. Bibcode:2005ApJ...632..638V. doi:10.1086/432901.
  8. Wright, J. T.; et al. (2009). "Ten New and Updated Multi-planet Systems, and a Survey of Exoplanetary Systems". The Astrophysical Journal. 693 (2): 1084–1099. arXiv:0812.1582. Bibcode:2009ApJ...693.1084W. doi:10.1088/0004-637X/693/2/1084.
  9. Feng, Y. Katherina; et al. (2015). "The California Planet Survey IV: A Planet Orbiting the Giant Star HD 145934 and Updates to Seven Systems with Long-period Planets". The Astrophysical Journal. 800 (1). 22. arXiv:1501.00633. Bibcode:2015ApJ...800...22F. doi:10.1088/0004-637X/800/1/22.

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