G 139-21 / GJ 1214

On December 16, 2009, a team of astronomers announced the discovery of a super-Earth in an inner orbit using the "transit method" of planetary detection using "a fleet of ground-based telescopes no larger than those many amateur astronomers have in their backyards" as part of the MEarth Project (CfA news release). With a diameter of 2.68 ± 0.13 times that of Earth, the mass of the planet was derived from radial-velocity measurements using the ESO 3.6-meter telescope and its HARPS spectrograph) (ESO press release; Charbonneau et al 2009; Geoffrey Marcy, 2009; Rogers and Seager, 2009; Dennis Overbye, New York Times, December 16, 2009; and Ivan Seminuik, New Scientist, December 16, 2009). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

Luis Calçada, ESO



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animation slides.



A likely "hot-ocean planet,"
with around 6.6 Earth-masses
and some 2.7 times Earth's
diameter, has a "torch" orbit
around GJ 1214 (more).

The high proper motion of this star was detected during the Lowell Proper Motion Survey of the Northern Hemisphere begun in 1957 by Henry Lee Giclas (1910-2007), Robert Burnham, Jr. (1931-93), and Norman G. Thomas at Lowell Observatory and designated as G 139-021 in the Giclas catalogues of faint proper motion stars (Giclas et al, 1971). It was also included in the 1979 Luyten Half-Second Catalogue as LHS 3275 by Willem Jacob Luyten (1899-1994). The star was added to the 1979 update of the Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) by Wilhelm Gliese (1915-93) and Hartmut Jahreiss (Gliese and Jahreiss, 1979) as GJ 1214.

Digitized Sky Survey 2,
ESO






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Field image around GJ 1214,
a very dim, red dwarf star
not visible with the naked
eye (more).

The Star

Gliese 176 is a cool and dim, main sequence red dwarf of spectral and luminosity type M4.5 V (NASA Star and Exoplanet Database, based on Hawley et al, 1996). The star has 15.7 ± 1.9 percent of Sol's mass, 21.1 (± 0.97 percent of its diameter, and under 0.02 percent of its visual and over 0.328 percent of its bolometric luminosity (Rogers and Seager, 2009). GJ 1214 is around six billion years old. Some other useful star catalogue designations include: GJ 1214, G 139-21, LHS 3275, USNO 256, and 2MASS J17151894+0457496.

Planet "b"

On December 16, 2009, a team of astronomers (including David Charbonneau, Zachory K. Berta, Jonathan Irwin, Christopher J. Burke, Philip Nutzman, Lars A. Buchhave, Christophe Lovis, Xavier Bonfils, David W. Latham, Stéphane Udry, Ruth A. Murray-Clay, Matthew J. Holman, Emilio E. Falco, Joshua N. Winn, Didier Queloz, Francesco Pepe, Michel Mayor, Xavier Delfosse, and Thierry Forveille) announced the discovery of a planet "b" of 6.55 ± 0.98 Earth-masses in an inner orbit using the "transit method" of planetary detection using "a fleet of ground-based telescopes no larger than those many amateur astronomers have in their backyards" as part of the MEarth Project (CfA news release). With a diameter of 2.68 ± 0.13 times that of Earth, the mass of the planet was derived from radial-velocity measurements using the ESO 3.6-meter telescope and its HARPS spectrograph) (ESO press release; Charbonneau et al 2009; Geoffrey Marcy, 2009; Rogers and Seager, 2009; Dennis Overbye, New York Times, December 16, 2009; and Ivan Seminuik, New Scientist, December 16, 2009). It revolves around GJ 1214 at an average distance of 0.014 AU, in a roughly circular orbit (e<0.27) which it completes in 1.6 days (38 hours), and so the planet must have a very hot "surface" temperature -- at around 450° Kelvin, 317° Fahrenheit, or 177° Celsius. With respect to Earth's line of sight, the planet's orbit is inclined at 88.6 degrees.

David A. Aguilar, CfA




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Planet "b" has a thick
atmosphere above a
possibly very deep,
hot-ocean of water
and ice under crushing
pressure around a
rocky core (more).

The planet's mass and diameter are consistent with the hypothesis that it has a low average density due to an inferred composition of three-fourths water (possbily 47 percent) and other ices and one-fourth rock and iron in the core, within a atmosphere of hydrogen and helium that may be 200 kilometers (124 miles) thick. Totalling no more than one percent of the planet's mass, the atmosphere "is probably escaping hydrodynamically" into space, suggesting that the planet has lost most of its primordial atmosphere since formation. An interesting possibility is that GJ 1214 b is a "hot-ocean planet" with a "steam atmosphere that transitions continuously to a superfluid without passing through the liquid phase" (Charbonneau et al 2009; Geoffrey Marcy, 2009; Rogers and Seager, 2009).

Theoretical modelling indicate that a "hot-ocean planet" could form if it formed in a colder orbit farther from GJ 1214, where lower temperatures would have created an ice-rock composition similar to Jupiter's moon Ganymede. Over time, the planet could have migrated inward into a closer orbit and melted into a water world with a steamy atmosphere. Alternative explanations include a small rocky planet with an implausibly large atmosphere that could be replenished by volcanic activity and a mini-Neptune with a much less massive gas atmosphere than Neptune or Uranus in the Solar System (Rogers and Seager, 2009). Based on its visual luminosity, a planet may be able to hold water on its surface around 0.057 AU of GJ 1214, with an orbital period slightly over 12.6 days. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)