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Kepler-21, also known as HD 179070, is a star with a closely orbiting exoplanet in the northern constellation of Lyra. At an apparent visual magnitude of 8.25 this was the brightest star observed by the Kepler spacecraft to host a validated planet until the discovery of an exoplanet orbiting HD 212657 in 2018. This system is located at a distance of 354 light-years (109 parsecs) from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −18.2 km/s.

The spectrum of HD 179070 presents as an evolving F-type subgiant star with a stellar classification of F6 IV. This suggests the star has exhausted the supply of hydrogen at its core and is evolving into a giant star. It is an estimated 2.6 billion years old and is spinning with a rotation period of 12.6 days. With 1.4 times the mass of the Sun it currently has 1.9 times the Sun's radius. The star is radiating five times the luminosity of the Sun from its photosphere at an effective temperature of 6,305 K.

A faint nearby source was detected in 2011 and determined to be a co-moving stellar companion in 2016. Designated HD 179070 B, it lies at an angular separation of 0.75″ along a position angle of 129° relative to the primary. At the distance of this star, this corresponds to a projected separation of 87 AU. It is possible that this companion star had a significant influence on the exoplanet formation and subsequent orbital evolution.

Planetary system

A candidate transiting exoplanet was discovered based on data from the first four months of photometry from the Kepler spacecraft. Confirmation was obtained in 2012 after extensive follow-up observations and analysis of the Kepler light curves.

The calculated density of the planet is approximately 6.4 g·cm, similar to Earth's 5.5 g·cm, which suggests a rocky composition. With an equilibrium temperature of 2,025 Kelvin, the top few-hundred kilometers of the planet is probably molten.

Calculations of the rate of orbital decay from tidal effects results in a decrease in the orbital period of 3.88 milliseconds per year, since this would be a change of only 4 seconds every thousand years it would be undetectable in any reasonable length of time.

Kepler-21 might have another exoplanet, a gas giant with at least 3.7 times the mass of Jupiter, named Kepler-21c.

References

1. Roman, Nancy G. (1987). "Identification of a Constellation From a Position". Publications of the Astronomical Society of the Pacific. 99 (617): 695–699. Bibcode:1987PASP...99..695R. doi:10.1086/132034. Vizier query form
2. ^ Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
3. ^ López-Morales, Mercedes; et al. (2016). "Kepler-21b: A Rocky Planet Around a V = 8.25 Magnitude Star". The Astronomical Journal. 152 (6) 204. arXiv:1609.07617. Bibcode:2016AJ....152..204L. doi:10.3847/0004-6256/152/6/204.
4. ^ Howell, Steve B.; et al. (2012). "Kepler-21b: A 1.6 R_Earth Planet Transiting the Bright Oscillating F Subgiant Star HD 179070". The Astrophysical Journal. 746 (2) 123. arXiv:1112.2165. Bibcode:2012ApJ...746..123H. doi:10.1088/0004-637X/746/2/123.
5. ^ Skrutskie, Michael F.; et al. (1 February 2006). "The Two Micron All Sky Survey (2MASS)". The Astronomical Journal. 131 (2): 1163–1183. Bibcode:2006AJ....131.1163S. doi:10.1086/498708. Vizier catalog entry
6. ^ Jönsson, Henrik; et al. (August 17, 2020). "APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South". The Astronomical Journal. 160 (3). American Astronomical Society: 120. arXiv:2007.05537. Bibcode:2020AJ....160..120J. doi:10.3847/1538-3881/aba592. ISSN 1538-3881. S2CID 220496397.
7. Holmberg, J.; et al. (July 2009). "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics". Astronomy and Astrophysics Supplement Series. 501 (3): 941–947. arXiv:0811.3982. Bibcode:2009A&A...501..941H. doi:10.1051/0004-6361/200811191. S2CID 118577511.
8. ^ Silva Aguirre, V.; et al. (2015). "Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology". Monthly Notices of the Royal Astronomical Society. 452 (2): 2127–2148. arXiv:1504.07992. Bibcode:2015MNRAS.452.2127S. doi:10.1093/mnras/stv1388.
9. ^ Ginski, C.; et al. (2016-04-01). "A lucky imaging multiplicity study of exoplanet host stars – II". Monthly Notices of the Royal Astronomical Society. 457 (2): 2173–2191. arXiv:1601.01524. Bibcode:2016MNRAS.457.2173G. doi:10.1093/mnras/stw049.
10. "HD 179070". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-06-10.
11. Mayo, Andrew W.; et al. (2018). "275 Candidates and 149 Validated Planets Orbiting Bright Stars in K2 Campaigns 0–10". The Astronomical Journal. 155 (3) 136. arXiv:1802.05277. Bibcode:2018AJ....155..136M. doi:10.3847/1538-3881/aaadff.
12. Borucki, William J.; et al. (2011). "Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data". The Astrophysical Journal. 736 (1) 19. arXiv:1102.0541. Bibcode:2011ApJ...736...19B. doi:10.1088/0004-637X/736/1/19.
13. Luna, S. H.; et al. (September 2020). "The dynamical evolution of close-in binary systems formed by a super-Earth and its host star". Astronomy & Astrophysics. 641 A109. arXiv:1907.10575. Bibcode:2020A&A...641A.109L. doi:10.1051/0004-6361/201936551.
14. ^ Beard, Corey; et al. (2024-10-01). "Utilizing Photometry from Multiple Sources to Mitigate Stellar Variability in Precise Radial Velocities: A Case Study of Kepler-21". The Astronomical Journal. 168 (4) 149. arXiv:2408.02873. Bibcode:2024AJ....168..149B. doi:10.3847/1538-3881/ad6b22.
15. Bonomo, A. S.; Dumusque, X.; et al. (April 2023). "Cold Jupiters and improved masses in 38 Kepler and K2 small-planet systems from 3661 high-precision HARPS-N radial velocities. No excess of cold Jupiters in small-planet systems". Astronomy & Astrophysics. arXiv:2304.05773. Bibcode:2023A&A...677A..33B. doi:10.1051/0004-6361/202346211. S2CID 258078829.

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