Pulsar planet

Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. The first such planets to be discovered were around a millisecond pulsar and were the first extrasolar planet to be confirmed as discovered.

An artist's conception of PSR B1257+12's system of planets

History

Pulsar planets are discovered through pulsar timing measurements, to detect anomalies in the pulsation period. Any bodies orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near-constant speed, any changes can easily be detected with the help of precise timing measurements. The discovery of pulsar planets was unexpected; pulsars or neutron stars have previously gone supernova, and it was thought that any planets orbiting such stars would have been destroyed in the explosion.

In 1991, Andrew G. Lyne announced the first-ever pulsar planet discovered around PSR 1829–10.[1] However, this was later retracted,[2] just before the first real pulsar planets were announced.

In 1992, Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planet planetary system around the millisecond pulsar PSR 1257+12.[3] These were the first two extrasolar planets confirmed to be discovered, and thus the first multi-planet extrasolar planetary system discovered, and the first pulsar planets discovered. There was doubt concerning the discovery because of the retraction of the previous pulsar planet, and questions about how pulsars could have planets. However, the planets proved to be real.[4] Two additional planets of lower mass were later discovered by the same technique, although one has since been retracted.

In 2000, the millisecond pulsar PSR B1620−26 was found to have a circumbinary planet (PSR B1620−26 b) that orbits both it and its companion white dwarf, WD B1620–26. This was announced as the oldest planet ever discovered, at 12.6 billion years old.[5] It is currently believed to have originally been the planet of WD B1620–26 before becoming a circumbinary planet, and therefore, while discovered through the pulsar timing method, it did not form the way that PSR B1257+12's planets are thought to have.

In 2006, the magnetar 4U 0142+61, located 13,000 light-years (1.2×1017 km) from Earth, was found to have a circumstellar disk. The discovery was made by a team led by Deepto Chakrabarty of MIT using the Spitzer Space Telescope.[6] The disk is thought to have formed from metal-rich debris left over from the supernova that formed the pulsar roughly 100,000 years ago and is similar to those seen around Sun-like stars, suggesting it may be capable of forming planets in a similar fashion. Pulsar planets would be unlikely to harbour life as we know it, because the high levels of ionizing radiation emitted by the pulsar and the corresponding paucity of visible light.

In 2011, a planet that is theorized to be the remaining core of a star that orbited a pulsar was announced. It orbits millisecond pulsar PSR J1719−1438, and represents a path to planetary status by evaporation of a star.[7][8] The planet is estimated to have a density of at least 23 times that of water, a diameter of 55,000 km, a mass near that of Jupiter's, and a 2-hour, 10-minute orbital period at 600,000 km. It is thought to be the diamond crystal core remaining from the evaporated white dwarf, with an estimated weight of 2.0×1027 kg (1×1031 carats).[9][10]

There are three types of pulsar planets known so far. The PSR B1257+12 planets were formed out of the debris of a destroyed companion star that used to orbit the pulsar.[11] In PSR J1719-1438, the planet most likely is the companion, or what's left of it after being almost entirely blasted away by the extreme irradiation from the nearby pulsar. PSR B1620–26 b is most likely a captured planet.

List of pulsar planets

Confirmed planets
Pulsar Planetary object Mass Semimajor axis
(AU)		 Orbital period Discovered
PSR B1957+20 PSR B1957+20 b 22 MJ − 0.38 days 1988
PSR B1620-26 PSR B1620-26 b 2.5 MJ 23 100 years 2003
PSR B1257+12 PSR B1257+12 A 0.020 M⊕ 0.19 25.262±0.003 days 1994
PSR B1257+12 B 4.3 M⊕ 0.36 66.5419±0.0001 days 1992
PSR B1257+12 C 3.90 M⊕ 0.46 98.2114±0.0002 days 1992
PSR B0943+10 PSR B0943+10 b 2.8 MJ 1.8 730 d 2014
PSR B0943+10 c 2.6 MJ 2.9 1460 d 2014
PSR B0329+54 PSR B0329+54 b 1.97 ± 0.19 M⊕ 10.26 ± 0.07 27.76 ± 0.03 years 2017
PSR J2322-2650 PSR J2322-2650 b 0.7949 MJ 0.0102 0.322963997 d 2017

Candidate planets
Pulsar Planetary object Mass Semimajor axis
(AU)		 Orbital period Announced
PSR J1719-1438 PSR J1719-1438 b ~1 MJ 0.004 2.176951032 hours 2011 August 25
PSR J1544+4937 PSR J1544+4937 b 18 MJ 0.00537 0.1207729895±0.0000000001 d 2013
PSR B1937+21 PSR B1937+21 b ~0.0001 M⊕?
 2.71 1999

Doubtful planets
Pulsar Planetary object Mass Semimajor axis
(AU)		 Orbital period Announced

Protoplanetary disks (fallback disks)
Pulsar Protoplanetary disk Discovered
4U 0142+61 debris disk 2006
1E 2259+586 candidate debris disk 2009[12]

Disproven planets
Pulsar Planet Mass
Geminga Geminga b 1.7 M⊕
PSR B1829-10 PSR 1829-10 A 10 M⊕
PSR B1257+12 PSR B1257+12 D[13] 0.0004 M⊕
PSR B1828-11 PSR B1828-11 A 3 M⊕
PSR B1828-11 B 12 M⊕
PSR B1828-11 C 8 M⊕
PSR B0329+54 PSR B0329+54A 0.3 M⊕
PSR B0329+54B 2.2±0.002 M⊕

See also

References
  1. Nature vol.352 no.6333 : A planet orbiting the neutron star PSR1829–10
  2. Nature vol.355 no.6357 : No planet orbiting PS R1829–10
  3. Wolszczan, A.; Frail, D. (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature. 355 (6356): 145–147. Bibcode:1992Natur.355..145W. doi:10.1038/355145a0. S2CID 4260368.
  4. Wolszczan, A. (1994). "Confirmation of Earth Mass Planets Orbiting the Millisecond Pulsar PSR B1257+12". Science. 264 (5158): 538–542. Bibcode:1994Sci...264..538W. doi:10.1126/science.264.5158.538. PMID 17732735. S2CID 19621191.
  5. Britt, Robert Roy. "Primeval Planet: Oldest Known World Conjures Prospect of Ancient Life". Space.com. Retrieved 2020-10-28.
  6. "Scientists crack mystery of planet formation". CNN.com. April 5, 2006. Archived from the original on 2006-04-08. Retrieved 2006-04-05.
  7. Max Planck Institute, "A Planet made of Diamond" Archived 2011-09-14 at the Wayback Machine, 25 August 2011
  8. Reuters, "Astronomers discover planet made of diamond", Ben Hirschler, 25 August 2011
  9. National Geographic, "Diamond" Planet Found; May Be Stripped Star", Andrew Fazekas, 25 August 2011
  10. New Scientist, "Astrophile: The diamond as big as a planet", David Shiga, 25 August 2011
  11. Very long baseline interferometry astrometry of PSR B1257+12, a pulsar with a planetary system
  12. Kaplan, David L.; Chakrabarty, Deepto; Wang, Zhongxiang; Wachter, Stefanie (July 2009). "A Mid-Infrared Counterpart to the Magnetar 1E 2259+586". Astrophysical Journal. 700 (1): 149–154. arXiv:0906.1604. Bibcode:2009ApJ...700..149K. doi:10.1088/0004-637X/700/1/149. ISSN 0004-637X. S2CID 9937378.
  13. Wolszczan, Alex (January 2012). "Discovery of pulsar planets". New Astronomy Reviews. Elsevier. 56 (1): 2–8. Bibcode:2012NewAR..56....2W. doi:10.1016/j.newar.2011.06.002.
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