Coordinates: Sky map 18h 56m 14.32s, +44° 31′ 05.3″
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Artist's impression of Kepler-37b.
Discovery siteKepler Space Observatory
Discovery dateFebruary 20, 2013[1]
Transit (Kepler Mission)
Orbital characteristics
0.1003 AU (15,000,000 km)[2]
13.367308[1] d
Physical characteristics
Mean radius
0.354 (± 0.014) REarth[3][4][5][6][a]
Temperature700 K (427 °C; 800 °F)[7]

Kepler-37b is an extrasolar planet (exoplanet) orbiting Kepler-37 in the constellation Lyra.[8] As of February 2013 it is the smallest planet discovered around a main-sequence star, with a radius slightly greater than that of the Moon and slightly smaller than that of Mercury. [9] The measurements do not constrain its mass, but masses above a few times that of the Moon give unphysically high densities.[6]


A size comparison of the planets in the Kepler-37 system and objects in the Solar System

Mass, radius and temperature[edit]

Kepler-37b is a sub-Earth, an exoplanet with a radius and mass smaller than Earth. Its surface temperature is 700 K (427 °C; 800 °F). Because of this, it is not expected to have an atmosphere.[7] Its radius is approximately 0.35 REarth (about a diameter of 3,900 kilometres (2,400 mi)), slightly larger than the Moon[10] (0.27 REarth), but a little smaller than Mercury (0.38 REarth). Due to its small size, it is very likely Kepler-37b is a rocky planet with a solid surface.[7] Furthermore, it is too hot to support liquid water on its surface.[7]

Host star[edit]

The planet orbits a (G-type) star similar to the Sun, named Kepler-37, orbited by a total of four planets. The star has a mass of 0.80 M and a radius of 0.79 R. It has a temperature of 5417 K and is 5.66 billion years old. In comparison, the Sun is 4.6 billion years old[11] and has a temperature of 5778 K.[12]

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 9.71. Therefore, it is too dim to be seen with the naked eye.


Kepler-37b orbits its parent star at a distance of about 15 million kilometers (9.3 million miles), with a period of roughly 13 days at a distance of 0.1 AU (compared to Mercury's distance from the Sun, which is about 0.38 AU).[9] The outer two planets in the system have orbital periods[1][2] within one percent of the 8:5 and 3:1 resonances with Kepler-37b's period.


Kepler-37b, along with two other planets, Kepler-37c and Kepler-37d, were discovered by the Kepler space telescope, which observes stellar transits.[1][7] After observing transits of Kepler-37b, astronomers had to compare it with the size of the parent star.

The size of the star was obtained using asteroseismology;[10] Kepler-37 is currently the smallest star to be studied using this process.[7] This allowed the size of Kepler-37b to be determined "with extreme accuracy".[7]

To date, Kepler-37b is the smallest planet discovered around a main-sequence star[b] outside the Solar System.[9] Detection of Kepler-37b was possible due to its short orbital period, relative brightness, and low activity of its host star, allowing brightness data to average out quickly.[13] The discovery of Kepler-37b has led Jack Lissauer, a scientist at NASA's Ames Research Center, to conjecture that "such little planets are common".[7]

See also[edit]


  1. ^ Minimum mass is very likely to be greater than that of the Moon.
  2. ^ The pulsar planet PSR B1257+12 A has a comparable mass. The actual size of PSR B1257+12 A is unknown, but is likely comparable to Kepler-37b.


  1. ^ a b c d Barclay, T.; Rowe, J. F.; Lissauer, J. J.; Huber, D.; Fressin, F.; Howell, S. B.; Bryson, S. T.; Chaplin, W. J.; Désert, J. M. (2013-02-20). "A sub-Mercury-sized exoplanet". Nature. 494 (7438): 452–4. arXiv:1305.5587. Bibcode:2013Natur.494..452B. doi:10.1038/nature11914. ISSN 0028-0836. PMID 23426260. S2CID 205232792.
  2. ^ a b c "Kepler-37 System". Archived from the original on May 16, 2013. Retrieved February 21, 2013.
  3. ^ Johnson, Michele (31 March 2015). "Kepler and K2 Missions". Archived from the original on 18 December 2014. Retrieved 5 October 2017.
  4. ^ "Kepler-37 b". Extrasolar Planets Encyclopaedia. Retrieved 28 January 2023.
  5. ^ "NASA Exoplanet Archive". Retrieved 2022-12-01.
  6. ^ a b Marcy, Geoffrey W.; Isaacson, Howard; Howard, Andrew W.; Rowe, Jason F.; Jenkins, Jon M.; Bryson, Stephen T.; Latham, David W.; Howell, Steve B.; Gautier, Thomas N.; Batalha, Natalie M.; Rogers, Leslie; Ciardi, David; Fischer, Debra A.; Gilliland, Ronald L.; Kjeldsen, Hans; Christensen-Dalsgaard, Jørgen; Huber, Daniel; Chaplin, William J.; Basu, Sarbani; Buchhave, Lars A.; Quinn, Samuel N.; Borucki, William J.; Koch, David G.; Hunter, Roger; Caldwell, Douglas A.; Van Cleve, Jeffrey; Kolbl, Rea; Weiss, Lauren M.; Petigura, Erik; et al. (2014). "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets". The Astrophysical Journal Supplement Series. 210 (2): 20. arXiv:1401.4195. Bibcode:2014ApJS..210...20M. doi:10.1088/0067-0049/210/2/20. S2CID 10760418.
  7. ^ a b c d e f g h "NASA's Kepler Mission Discovers Tiny Planet System". NASA. February 20, 2013. Retrieved February 21, 2013.
  8. ^ "Smallest Alien Planet Kepler-37b Explained (Infographic)". 20 February 2013. Retrieved 5 October 2017.
  9. ^ a b c "Tiniest Planet Yet Discovered by NASA Outside our Solar System". February 21, 2013. Retrieved February 21, 2013.
  10. ^ a b "Astronomers Find the Tiniest Exoplanet Yet". Slate. February 20, 2013. Retrieved February 21, 2013.
  11. ^ Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
  12. ^ Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
  13. ^ "Small Planets Confirm Kepler's Capabilities". Retrieved 5 October 2017.
Preceded by Least massive exoplanet[citation needed]
Succeeded by
Preceded by Smallest-radius exoplanet
Succeeded by