Location of Kapteyn's Star in the constellation Pictor
Epoch J2000 Equinox J2000
|Right ascension||05h 11m 40.58984s|
|Declination||−45° 01′ 06.3617″|
|Apparent magnitude (V)||8.853±0.008|
|Spectral type||sdM1 or M1.5V|
|U−B color index||+1.21|
|B−V color index||1.57±0.012|
|Variable type||BY Dra|
|Radial velocity (Rv)||245.05±0.13 km/s|
|Proper motion (μ)|| RA: +6,491.223 mas/yr |
Dec.: −5,708.614 mas/yr
|Parallax (π)||254.1986 ± 0.0168 mas|
|Distance||12.8308 ± 0.0008 ly |
(3.9339 ± 0.0003 pc)
|Absolute magnitude (MV)||10.89|
|Surface gravity (log g)||4.96±0.13 cgs|
|Metallicity [Fe/H]||−0.86±0.05 dex|
|Rotational velocity (v sin i)||9.15 km/s|
Kapteyn's Star is a class M1 red subdwarf about 12.83 light-years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With an apparent magnitude of nearly 9 it is visible through binoculars or a telescope.
Its diameter is 30% of the Sun's, but its luminosity just 1.2% that of the Sun. It may have once been part of the globular cluster Omega Centauri, itself a likely dwarf galaxy swallowed up by the Milky Way in the distant past. The discovery of two planets—Kapteyn b and Kapteyn c—was announced in 2014, but had a mixed history of rejections and confirmations, until a 2021 study refuted both planets. The "planets" are in fact artifacts of the star's rotation and activity.
History of observations
Attention was first drawn to what is now known as Kapteyn's Star by the Dutch astronomer Jacobus Kapteyn in 1898. Under the name CPD-44 612 it was included in the Cape photographic Durchmusterung for the equinox 1875 (−38 to −52) by David Gill and Jacobus Cornelius Kapteyn in 1897. This catalogue was based on Gill's observations from the Cape Observatory in 1885–1889 and was created in collaboration with Kapteyn. While he was reviewing star charts and photographic plates, Kapteyn noted that a star, previously catalogued in 1873 by B. A. Gould as C.Z. V 243, seemed to be missing. However, Robert T. A. Innes found an uncatalogued star about 15 arcseconds away from the absent star's position. It became clear that the star had a very high proper motion of more than 8 arcseconds per year and had moved significantly. Later, CPD-44 612 came to be referred to as Kapteyn's Star although equal credit should be accorded to Robert Innes. At the time of its discovery it had the highest proper motion of any star known, dethroning Groombridge 1830. In 1916, Barnard's Star was found to have an even larger proper motion. In 2014, two super-Earth planet candidates in orbit around the star were announced, but later refuted.
Based upon parallax measurements, Kapteyn's Star is 12.83 light-years (3.93 parsecs) from the Earth. It came within 7.0 ly (2.1 pc) of the Sun about 10,900 years ago and has been moving away since that time. Kapteyn's Star is distinctive in a number of regards: it has a high radial velocity, orbits the Milky Way retrograde, and is the nearest-known halo star to the Sun. It is a member of a moving group of stars that share a common trajectory through space, named the Kapteyn moving group. Based upon their element abundances, these stars may once have been members of Omega Centauri, a globular cluster that is thought to be the remnant of a dwarf galaxy that merged with the Milky Way. During this process, the stars in the group, including Kapteyn's Star, may have been stripped away as tidal debris.
Kapteyn's Star is between one quarter and one third the size and mass of the Sun and has a much cooler effective temperature at about 3500 K, with some disagreement in the exact measurements between different observers. The stellar classification is sdM1, which indicates that it is a subdwarf with a luminosity lower than that of a main-sequence star at the same spectral type of M1. The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 14% of the abundance in the Sun. It is a variable star of the BY Draconis type with the identifier VZ Pictoris. This means that the luminosity of the star changes because of magnetic activity in the chromosphere coupled with rotation moving the resulting star spots into and out of the line of sight with respect to the Earth.
The star has a mass of 0.27 M☉, a radius of 0.29 R☉ and has about 1.2% of the Sun's luminosity. It has a surface temperature of 3,550 K and is roughly 11 billion years old. In comparison, the Sun is about 4.6 billion years old and has a surface temperature of 5,778 K. Stars like Kapteyn's Star have the ability to live up to 100–200 billion years, ten to twenty times longer than the Sun will live.
Search for planets
In 2014, Kapteyn's Star was announced to host two planets, Kapteyn b and Kapteyn c, based on Doppler spectroscopy observations by the HARPS spectrometer which is housed at the European Southern Observatory's La Silla Observatory in Chile, at the Keck Observatory in Hawaii, and at the PFS Observatory, also in Chile. Kapteyn b was described as the oldest-known potentially habitable planet, estimated to be 11 billion years old, while Kapteyn c was described as beyond the host star's habitable zone.
Kapteyn b was thought to make a complete orbit around its parent star about every 48.62 days at a distance of 0.17 AU, with an eccentricity of 0.21, meaning its orbit is mildly elliptical. Kapteyn c was thought to orbit with a period of 121.5 days at a distance of 0.31 AU, with an eccentricity of 0.23. Both planets were thought to be super-Earths, with minimum masses of 4.8 and 7.0 M🜨, respectively.
The purported planets were thought to be close to a 5:2 period commensurability, but resonances could not be confirmed. Dynamical integration of the orbits suggested that the pair of planets are in a dynamical state called apsidal co-rotation, which usually implies that the system is dynamically stable over long time scales. Guinan et al. (2016) suggested that the present day star could potentially support life on Kapteyn b, but that the planet's atmosphere may have been stripped away when the star was young (~0.5 Gyr) and highly active. The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.
However, subsequent research by Robertson et al. (2015) found that the orbital period of Kapteyn b is an integer fraction (1/3) of their estimated stellar rotation period, and thus the planetary signal is most likely an artifact of stellar activity. The authors did not rule out the existence of Kapteyn c, calling for further observation. This refutation was questioned by the team that published the exoplanet discovery paper. Guinan et al. (2016) (as well as earlier authors) found a lower value for the stellar rotation, which lended support to the original planetary finding.
In 2021, a new analysis found no evidence for either planet, and found that the observed radial velocity signals are in fact artifacts of the star's rotation and activity, after the rotational period of the star was refined, with a rotational period very similar to that of candidate c. There is currently no evidence for planets orbiting Kapteyn's Star.
- 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.
- Guinan, Edward F.; Engle, Scott G.; Durbin, Allyn (April 2016), "Living with a Red Dwarf: Rotation and X-Ray and Ultraviolet Properties of the Halo Population Kapteyn's Star", The Astrophysical Journal, 821 (2): 14, arXiv:1602.01912, Bibcode:2016ApJ...821...81G, doi:10.3847/0004-637X/821/2/81, S2CID 119283541, 81.
- Koen, C.; et al. (April 2010), "UBV(RI)C JHK observations of Hipparcos-selected nearby stars", Monthly Notices of the Royal Astronomical Society, 403 (4): 1949–1968, Bibcode:2010MNRAS.403.1949K, doi:10.1111/j.1365-2966.2009.16182.x
- Demory, Brice-Olivier; Ségransan, Damien; Forveille, Thierry; Queloz, Didier; Beuzit, Jean-Luc; Delfosse, Xavier; Di Folco, Emmanuel; Kervella, Pierre; Le Bouquin, Jean-Baptiste; Perrier, Christian; Benisty, Myriam; Duvert, Gilles; Hofmann, Karl-Heinz; Lopez, Bruno; Petrov, Romain (October 2009). "Mass-radius relation of low and very low-mass stars revisited with the VLTI". Astronomy and Astrophysics. 505 (1): 205–215. arXiv:0906.0602. Bibcode:2009A&A...505..205D. doi:10.1051/0004-6361/200911976. S2CID 14786643.
- "V* VZ Pic -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, archived from the original on 2016-03-03, retrieved 2009-10-14.
- "VZ Pic", General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia, archived from the original on 2011-09-27, retrieved 2009-10-14
- Bortle, Anna; et al. (2021). "A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star". The Astronomical Journal. 161 (5): 230. arXiv:2103.02709. Bibcode:2021AJ....161..230B. doi:10.3847/1538-3881/abec89. S2CID 232110395.
- Houdebine, E. R. (September 2010), "Observation and modelling of main-sequence star chromospheres - XIV. Rotation of dM1 stars", Monthly Notices of the Royal Astronomical Society, 407 (3): 1657–1673, Bibcode:2010MNRAS.407.1657H, doi:10.1111/j.1365-2966.2010.16827.x
- "Kapteyn b and c: Two Exoplanets Found Orbiting Kapteyn's Star". Sci-News. Archived from the original on 3 August 2014. Retrieved 23 July 2014.
- Anglada-Escudé, Guillem; et al. (2014), "Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf", Monthly Notices of the Royal Astronomical Society: Letters, 443: L89–L93, arXiv:1406.0818, Bibcode:2014MNRAS.443L..89A, doi:10.1093/mnrasl/slu076, S2CID 67807856
- Kapteyn, J. C. (1898), "Stern mit grösster bislang bekannter Eigenbewegung", Astronomische Nachrichten, 145 (9–10): 159–160, Bibcode:1897AN....145..159K, doi:10.1002/asna.18981450906.
- Dictionary of Nomenclature of Celestial Objects. CPD entry Archived 2015-09-25 at the Wayback Machine. SIMBAD. Centre de Données astronomiques de Strasbourg.
- "Resultados del Observatorio Nacional Argentino, vol. 7, pg.98". Archived from the original on 2019-04-21. Retrieved 2019-04-21.
- Kaler, James B. (2002), "Kapteyn's Star", The Hundred Greatest Stars, Copernicus Books, pp. 108–109.
- Gill, D. (1899). "On the Discovery of a Certain Proper Motion". The Observatory: 99–101.
- Barnard, E. E. (1916), "A small star with large proper motion", Astronomical Journal, 29 (695): 181, Bibcode:1916AJ.....29..181B, doi:10.1086/104156.
- Kotoneva, E.; et al. (2005), "A study of Kapteyn's star", Astronomy & Astrophysics, 438 (3): 957–962, Bibcode:2005A&A...438..957K, doi:10.1051/0004-6361:20042287.
- Bailer-Jones, C. A. L. (March 2015), "Close encounters of the stellar kind", Astronomy & Astrophysics, 575: 13, arXiv:1412.3648, Bibcode:2015A&A...575A..35B, doi:10.1051/0004-6361/201425221, S2CID 59039482, A35.
- Woolf, V. M.; Wallerstein, G. (2004), "Chemical abundance analysis of Kapteyn's Star", Monthly Notices of the Royal Astronomical Society, 350 (2): 575–579, Bibcode:2004MNRAS.350..575W, doi:10.1111/j.1365-2966.2004.07671.x, S2CID 15907478.
- Eggen, O. J. (December 1996), "The Ross 451 Group of Halo Stars", Astronomical Journal, 112: 2661, Bibcode:1996AJ....112.2661E, doi:10.1086/118210
- Wylie-de Boer, Elizabeth; Freeman, Ken; Williams, Mary (February 2010), "Evidence of Tidal Debris from ω Cen in the Kapteyn Group", The Astronomical Journal, 139 (2): 636–645, arXiv:0910.3735, Bibcode:2010AJ....139..636W, doi:10.1088/0004-6256/139/2/636, S2CID 119217292
- "Backward star ain't from round here", New Scientist, November 4, 2009, archived from the original on May 25, 2015, retrieved September 2, 2017
- Woolf, Vincent M.; Wallerstein, George (January 2005), "Metallicity measurements using atomic lines in M and K dwarf stars", Monthly Notices of the Royal Astronomical Society, 356 (3): 963–968, arXiv:astro-ph/0410452, Bibcode:2005MNRAS.356..963W, doi:10.1111/j.1365-2966.2004.08515.x, S2CID 15664454
- The abundance is given by taking the metallicity to the power of 10. From Woolf and Wallerstein (2005), [M/H] ≈ –0.86 dex. Thus:
- 10−0.86 = 0.138
- Cain, Fraser (16 September 2008). "How Old is the Sun?". Universe Today. Archived from the original on 18 August 2010. Retrieved 19 February 2011.
- Cain, Fraser (September 15, 2008). "Temperature of the Sun". Universe Today. Archived from the original on 29 August 2010. Retrieved 19 February 2011.
- Adams, Fred C.; Laughlin, Gregory; Graves, Genevieve J. M. "Red Dwarfs and the End of the Main Sequence". Gravitational Collapse: From Massive Stars to Planets. Revista Mexicana de Astronomía y Astrofísica. pp. 46–49. Bibcode:2004RMxAC..22...46A.
- Wall, Mike (3 June 2014). "Found! Oldest Known Alien Planet That Might Support Life". Space.com. Retrieved 10 January 2015.
- David Dickinson, Discovered: Two New Planets for Kapteyn’s Star (June 4, 2014).
- Kapteyn's Star at SolStations.com.
- Michtchenko, Tatiana A.; et al. (August 2011), "Modeling the secular evolution of migrating planet pairs", Monthly Notices of the Royal Astronomical Society, 415 (3): 2275–2292, arXiv:1103.5485, Bibcode:2011MNRAS.415.2275M, doi:10.1111/j.1365-2966.2011.18857.x, S2CID 85449365
- "Sad Kapteyn", Science fiction story released with the announcement of planetary system, Jun 4, 2014, archived from the original on June 6, 2014, retrieved 2014-06-04
- Robertson, Paul; Roy, Arpita; Mahadevan, Suvrath (June 2015), "Stellar activity mimics a habitable-zone planet around Kapteyn's star", The Astrophysical Journal Letters, 805 (2): 6, arXiv:1505.02778, Bibcode:2015ApJ...805L..22R, doi:10.1088/2041-8205/805/2/L22, S2CID 117871083, L22.
- Anglada-Escudé, G.; Tuomi, M.; Arriagada, P.; Zechmeister, M.; Jenkins, J. S.; Ofir, A.; Dreizler, S.; E. Gerlach; Marvin, C. J. (2016). "No Evidence for Activity Correlations in the Radial Velocities of Kapteyn's Star". The Astrophysical Journal. 830 (2): 74. arXiv:1506.09072. Bibcode:2016ApJ...830...74A. doi:10.3847/0004-637X/830/2/74. hdl:2299/17695. ISSN 0004-637X. S2CID 14348277.
- Luyten, W. J. (1927), "Note on the magnitude and spectrum of Kapteyn's star", Harvard College Observatory Bulletin, 843: 3–4, Bibcode:1927BHarO.843....3L.
- MacConnell, D. J. (1973), "The spectrum and colors of Kapteyn's star", Bulletin of the American Astronomical Society, 5: 346, Bibcode:1973BAAS....5..346M.
- Murdin, Paul, ed. (2001), "Kapteyn's Star", Encyclopedia of Astronomy and Astrophysics, Bristol: Institute of Physics Publishing, doi:10.1888/0333750888/5156.
- Perryman, Michael (2010), The Making of History's Greatest Star Map, Astronomers' Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8.
- Wing, R. F.; Dean, C. A.; MacConnell, D. J. (1976), "The temperature, luminosity, and spectrum of Kapteyn's star", The Astrophysical Journal, 205: 186–193, Bibcode:1976ApJ...205..186W, doi:10.1086/154263.