Gliese 436 b explained

Gliese 436 b / Awohali
Discoverer:Butler, Vogt,
Marcy et al.
Discovery Site:California, USA
Discovered:August 31, 2004
Discovery Method:Radial velocity, Transit
Alt Names:Awohali
Apsis:astron
Semimajor: AU
Period:2.643904±0.000005 d
Inclination:85.8
Star:Noquisi
Mean Radius:4.327 ± 0.183[1]
Density:1.51g/cm3
Surface Grav:1.18 g
Single Temperature:712K

Gliese 436 b (sometimes called GJ 436 b,[2] formally named Awohali) is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty (in 2007) and was among the smallest-known transiting planets in mass and radius, until the much smaller Kepler exoplanet discoveries began circa 2010.

In December 2013, NASA reported that clouds may have been detected in the atmosphere of GJ 436 b.[3] [4] [5] [6]

Nomenclature

In August 2022, this planet and its host star were included among 20 systems to be named by the third NameExoWorlds project.[7] The approved names, proposed by a team from the United States, were announced in June 2023. Gliese 436 b is named Awohali and its host star is named Noquisi, after the Cherokee words for "eagle" and "star".

Discovery

Awohali was discovered in August 2004 by R. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley, respectively, using the radial velocity method. Together with 55 Cancri e, it was the first of a new class of planets with a minimum mass (M sini) similar to Neptune.

The planet was recorded to transit its star by an automatic process at NMSU on January 11, 2005, but this event went unheeded at the time.[8] In 2007, Michael Gillon from Geneva University in Switzerland led a team that observed the transit, grazing the stellar disc relative to Earth. Transit observations led to the determination of its exact mass and radius, both of which are very similar to that of Neptune, making Awohali at that time the smallest known transiting extrasolar planet. The planet is about four thousand kilometers larger in diameter than Uranus and five thousand kilometers larger than Neptune and slightly more massive. Awohali orbits at a distance of four million kilometers or one-fifteenth the average distance of Mercury from the Sun.

Physical characteristics

The planet's surface temperature is estimated from measurements taken as it passes behind the star to be .[9] This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star, which was prior to this measurement, estimated at 520 K. Whatever energy tidal effects deliver to the planet, it does not affect its temperature significantly.[10] A greenhouse effect would result in a much greater temperature than the predicted 520–620 K.

Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms,[11] which would remain solid despite the high temperatures, because of the planet's gravity.[12] The planet could have formed further from its current position, as a gas giant, and migrated inwards with the other gas giants. As it approached its present position, radiation from the star would have blown off the planet's hydrogen layer via coronal mass ejection.[13]

However, when the radius became better known, ice alone was not enough to account for the observed size. An outer layer of hydrogen and helium, accounting for up to ten percent of the mass, was needed on top of the ice to account for the observed planetary radius.[14] This obviates the need for an ice core. Alternatively, the planet may consist of a dense rocky core surrounded by a lesser amount of hydrogen.[15]

Observations of the planet's brightness temperature with the Spitzer Space Telescope suggest a possible thermochemical disequilibrium in the atmosphere of this exoplanet. Results published in Nature suggest that Awohali’s dayside atmosphere is abundant in CO and deficient in methane (CH4) by a factor of ~7,000. This result is unexpected because, based on current models at its temperature, atmospheric carbon should prefer CH4 over CO.[16] [17] [18] [19] In part for this reason, it has also been hypothesized to be a possible helium planet.[20]

In June 2015, scientists reported that the atmosphere of Awohali was evaporating,[21] resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 9order=flipNaNorder=flip long.[22]

Orbital characteristics

One orbit around the star Noquisi takes only about two days, 15.5 hours. Awohali orbit is likely misaligned with Noquisi’s rotation. The eccentricity of Awohali’s orbit is inconsistent with models of planetary system evolution. To have maintained its eccentricity over time requires that it be accompanied by another planet.[23]

A study published in Nature found that the orbit of Awohali is nearly perpendicular (inclined by 103.2 degrees) to the stellar equator of Noquisi and suggests that the eccentricity and misalignment of the orbit could have resulted from interactions with a yet undetected companion. The inward migration caused by this interaction could have triggered the atmospheric escape that sustains its giant exosphere.

See also

Selected media articles

Notes and References

  1. Confirmed, Transit infrared spectroscopy of the hot neptune around GJ 436 with the Hubble Space Telescope . 2008. Pont . F.. Gilliland . R. L.. Knutson . H.. Holman . M.. Charbonneau . D.. 10.1111/j.1745-3933.2008.00582.x. . 393. 1. L6–L10 . free . 0810.5731. 2009MNRAS.393L...6P . 3746845.
  2. Beust, Hervé . etal . Dynamical evolution of the Gliese 436 planetary system - Kozai migration as a potential source for Gliese 436b's eccentricity . August 1, 2012 . Astronomy. 1208.0237 . 10.1051/0004-6361/201219183 . 2012A&A...545A..88B . 545 . A88. 10253533 .
  3. Web site: Harrington . J.D. . Weaver . Donna . Villard . Ray . Release 13-383 - NASA's Hubble Sees Cloudy Super-Worlds With Chance for More Clouds . December 31, 2013 . . January 1, 2014 . January 2, 2014 . https://web.archive.org/web/20140102191652/http://www.nasa.gov/press/2013/december/nasas-hubble-sees-cloudy-super-worlds-with-chance-for-more-clouds/ . live .
  4. Moses . Julianne . Extrasolar planets: Cloudy with a chance of dustballs . January 1, 2014 . . 505 . 31–32 . 10.1038/505031a . 2014Natur.505...31M . 7481 . 24380949. 4408861 .
  5. Knutson, Heather . etal . A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b . January 1, 2014 . . 505 . 66–68 . 10.1038/nature12887 . 1401.3350 . 2014Natur.505...66K . 7481 . 24380953. 4454617 .
  6. Kreidberg, Laura . etal . Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b . January 1, 2014 . . 505 . 69–72 . 10.1038/nature12888 . 1401.0022 . 2014Natur.505...69K . 7481 . 24380954. 1721.1/118780 . 4447642 .
  7. Web site: List of ExoWorlds 2022 . 8 August 2022 . nameexoworlds.iau.org . . 27 August 2022 . 8 March 2023 . https://web.archive.org/web/20230308103542/https://www.nameexoworlds.iau.org/2022exoworlds . live .
  8. New observations and a possible detection of parameter variations in the transits of Gliese 436b. 2008. Coughlin . Jeffrey L.. Stringfellow . Guy S.. Becker . Andrew C.. Mercedes Lopez-Morales. Fabio Mezzalira. Tom Krajci. 10.1086/595822. The Astrophysical Journal. 689. 2. L149–L152. 0809.1664 . 2008ApJ...689L.149C. 14893633.
  9. Spitzer Transit and Secondary Eclipse Photometry of GJ 436b. 2007. Drake Deming. Joseph Harrington. Gregory Laughlin. Sara Seager. Navarro . Sarah B.. Bowman . William C.. Karen Horning. 10.1086/522496. The Astrophysical Journal. 667. 2. L199–L202. 0707.2778. 2007ApJ...667L.199D. 13349666.
  10. Tidal Heating of Extra-Solar Planets. 2008. Brian Jackson. Richard Greenberg. Rory Barnes. 10.1086/587641. The Astrophysical Journal. 681. 2. 1631–1638 . 0803.0026. 2008ApJ...681.1631J. 42315630.
  11. News: New Scientist . Strange alien world made of "hot ice" . 6 May 2007 . David . Shiga . 2007-05-16 . dead . https://web.archive.org/web/20080706143705/http://space.newscientist.com/article/dn11864-strange-alien-world-made-of-hot-ice-and-steam.html . July 6, 2008 .
  12. News: Hot "ice" may cover recently discovered planet . Maggie . Fox . Scientific American.com . Science News . May 16, 2007 . 2008-08-06 . 2012-09-07 . https://archive.today/20120907134721/http://www.reuters.com/article/scienceNews/idUSN1621607620070516 . live .
  13. Geophysical Research Abstracts . 9 . 7850 . 2007 . The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters . H. Lammer . etal . 2008-08-18 . 2019-12-15 . https://web.archive.org/web/20191215090631/https://www.cosis.net/abstracts/EGU2007/07850/EGU2007-J-07850.pdf?PHPSESSID=1eb3a7a98603083dda25d18001ea2a33 . live . By analogy with Gliese 876 d.
  14. A Hubble Space Telescope transit light curve for GJ 436b. Bean, J.L.. etal. Astronomy & Astrophysics. 2008. 10.1051/0004-6361:200810013. 2008A&A...486.1039B. 486. 3. 1039–1046. 0806.0851. 6351375. 2008-08-07. 2010-04-08. https://web.archive.org/web/20100408055429/http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=%2Farticles%2Faa%2Fabs%2F2008%2F30%2Faa10013-08%2Faa10013-08.html. live.
  15. Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres . E. R. Adams . S. Seager . L. Elkins-Tanton . . 673 . 2 . 1160–1164 . February 2008 . 10.1086/524925 . 2008ApJ...673.1160A. 0710.4941 . 6676647 .
  16. Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b . Nature. 464. 1161–1164. 22 April 2010. 1010.4591 . 2010Natur.464.1161S . 10.1038/nature09013 . 20414304 . 7292 . Stevenson . KB . Harrington . J . Nymeyer . S . 4416249. etal.
  17. http://planets.ucf.edu/research/exoplanets/GJ436b-Wheres-the-methane GJ436b - Where's the methane?
  18. A Spitzer Transmission Spectrum for the Exoplanet GJ 436b . 2011. Knutson . Heather A.. 10.1088/0004-637X/735/1/27. Astrophysical Journal. 735, 27 . 1. 1104.2901 . 2011ApJ...735...27K . 27. 18669291.
  19. Thermochemical and Photochemical Kinetics in Cooler Hydrogen Dominated Extrasolar Planets . 2011. LINE . Michael R. . VASISHT . Gautam. CHEN . Pin. ANGERHAUSEN . D.. YANG . Yuk L.. Astrophysical Journal. 1104.3183. 10.1088/0004-637X/738/1/32. 738, 32. 1. 2011ApJ...738...32L . 32. 15087062., abstract in the arXiv titled "Thermochemistry and Photochemistry in Cooler Hydrogen Dominated Extrasolar Planets: The Case of GJ436b"
  20. News: Helium-Shrouded Planets May Be Common in Our Galaxy . SpaceDaily . 16 June 2015 . 3 August 2015 . 11 August 2019 . https://web.archive.org/web/20190811113518/http://www.spacedaily.com/reports/Helium_Shrouded_Planets_May_Be_Common_in_Our_Galaxy_999.html . live .
  21. A Giant Comet-like Cloud of Hydrogen Escaping from the warm Neptune-mass Exoplanet GJ 436b . D. Ehrenreich . V. Bourrier . P. Wheatley . A. Lecavelier des Etangs . G. Hébrard . S. Udry . X. Bonfils . X. Delfosse . J.-M. Désert . D. K. Sing . A. Vidal-Madjar . . 522 . 7557 . 459–461 . 25 June 2015 . 10.1038/nature14501 . 2015Natur.522..459E. 1506.07541 . 26108854. 4388969 .
  22. News: Bhanoo . Sindya N. . A Planet with a Tail Nine Million Miles Long . 25 June 2015 . . 25 June 2015 . 25 June 2015 . https://web.archive.org/web/20150625153820/http://www.nytimes.com/interactive/projects/cp/summer-of-science-2015/latest/exoplanet-tail . live .
  23. Observational Consequences of the Recently Proposed Super-Earth Orbiting GJ436. Astronomy & Astrophysics. 487. 2. L25–L28. 0806.3270. 2008. Bean . Jacob L.. Andreas Seifahrt. 10.1051/0004-6361:200810278. 2008A&A...487L..25B. 14811323.

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