Habitability of binary star systems

Planets in binary star systems may be candidates for supporting extraterrestrial life.[1] Habitability of binary star systems is determined by many factors from a variety of sources.[2] Typical estimates often suggest that 50% or more of all star systems are binary systems. This may be partly due to sample bias, as massive and bright stars tend to be in binaries and these are most easily observed and catalogued; a more precise analysis has suggested that the more common fainter stars are usually singular, and that up to two thirds of all stellar systems are therefore solitary.[3]

Schematic of a binary star system with one planet on an S-type orbit and one on a P-type orbit.

The separation between stars in a binary may range from less than one astronomical unit (au, the "average" Earth-to-Sun distance) to several hundred au. In latter instances, the gravitational effects will be negligible on a planet orbiting an otherwise suitable star, and habitability potential will not be disrupted unless the orbit is highly eccentric (see Nemesis, for example). In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. If the separation is significantly close to the planet's distance, a stable orbit may be impossible.

Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges.[4]

Non-circumbinary planet (S-Type)

In non-circumbinary planets, if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.[5] Whether planets might form in binaries at all had long been unclear, given that gravitational forces might interfere with planet formation. Theoretical work by Alan Boss at the Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars.[6]

Studies of Alpha Centauri, the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 au distance at closest approach (23 au mean), and both have stable habitable zones.[2][7] A study of long-term orbital stability for simulated planets within the system shows that planets within approximately three au of either star may remain stable (i.e. the semi-major axis deviating by less than 5%). The habitable zone for Alpha Centauri A extends, conservatively estimated, from 1.37 to 1.76 au[2] and that of Alpha Centauri B from 0.77 to 1.14 au[2]—well within the stable region in both cases.[8]

Circumbinary planet (P-Type)

For a circumbinary planet, orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.

The minimum stable star-to-circumbinary-planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.[9]

For example, Kepler-47c is a gas giant in the circumbinary habitable zone of the Kepler-47 system.

If Earth-like planets form in or migrate into the circumbinary habitable zone they are capable of sustaining liquid water on their surface in spite of the dynamical and radiative interaction with the binary star.[10]

The limits of stability for S-type and P-type orbits within binary as well as triple stellar systems have been established as a function of the orbital characteristics of the stars, for both prograde and retrograde motions of stars and planets.[11]

See also

References
  1. "Earth-Sized 'Tatooine' Planets Could Be Habitable" (Press release). NASA Jet Propulsion Laboratory, California Institute of Technology. April 2017.
  2. Eggl, S. (2018). Habitability of Planets in Binary Star Systems. Handbook of Exoplanets. Springer. pp. 1–27. Bibcode:2017haex.bookE..61E. doi:10.1007/978-3-319-30648-3_61-1. ISBN 978-3-319-30648-3.
  3. "Most Milky Way Stars Are Single" (Press release). Harvard-Smithsonian Center for Astrophysics. January 30, 2006. Archived from the original on 2007-08-13. Retrieved 2007-06-05.
  4. Elisa V. Quintana, Jack J. Lissauer (2007). "Terrestrial Planet Formation in Binary Star Systems". Extreme Solar Systems. 398: 201. arXiv:0705.3444. Bibcode:2008ASPC..398..201Q.
  5. "Stars and Habitable Planets". www.solstation.com. Sol Company. Retrieved 2007-06-05.
  6. "Planetary Systems can from around Binary Stars" (Press release). Carnegie Institution. January 2006. Archived from the original on 2011-05-15. Retrieved 2007-06-05.
  7. Eggl, S.; Haghighipour, N.; Pilat-Lohinger, E. (2013). "Detectability of Earth-like planets in circumstellar habitable zones of binary star systems with sun-like components". The Astrophysical Journal. 764 (2): 130. arXiv:1212.4884. Bibcode:2013ApJ...764..130E. doi:10.1088/0004-637X/764/2/130.
  8. Wiegert, Paul A.; Holman, Matt J. (April 1997). "The stability of planets in the Alpha Centauri system". The Astronomical Journal. 113 (4): 1445–1450. arXiv:astro-ph/9609106. Bibcode:1997AJ....113.1445W. doi:10.1086/118360.
  9. Recent Kepler Results On Circumbinary Planets, William F. Welsh, Jerome A. Orosz, Joshua A. Carter, Daniel C. Fabrycky, (Submitted on 28 Aug 2013)
  10. Popp, M.; Eggl, S. (2017). "Climate variations on Earth-like circumbinary planets". Nature Communications. 8: 14957. Bibcode:2017NatCo...814957P. doi:10.1038/ncomms14957. PMC 5384241. PMID 28382929.
  11. Busetti, Franco; Beust, Hervé; Harley, Charis (2018). Stability of planets in triple star systems. Astronomy & Astrophysics, 619, A91.
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