Gliese 436 b/ˈɡliːzə/ (sometimes calledGJ 436 b,[6] formally namedAwohali[2]) is aNeptune-sizedexoplanet orbiting thered dwarfGliese 436 (Noquisi), the only known planet in its system.[4] It was the firsthot Neptune discovered with certainty (in 2007) and was among the smallest-knowntransiting planets in mass and radius, until the much smallerKepler exoplanet discoveries begancirca 2010.
The planet was recorded totransit its star by an automatic process atNMSU on January 11, 2005, but this event went unheeded at the time.[11] In 2007, Michael Gillon fromGeneva University inSwitzerland 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 Gliese 436 b 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. Gliese 436 b orbits at a distance of four million kilometers or one-fifteenth the average distance ofMercury from theSun.[12]
In August 2022, this planet and its host star were included among 20 systems to be named by the thirdNameExoWorlds project.[13] The approved names, proposed by a team from theUnited States, were announced in June 2023. Gliese 436 b is named Awohali and its host star is named Noquisi, after theCherokee words for "eagle" and "star". As stated in the IAU naming citation, the name refers to a Cherokee legend, alluding to the planet's evaporating atmosphere:[2]
“Awohali” (ᎠᏬᎭᎵ, Ah-Wo-Ha-Lee) is one of the Cherokee words for “eagle”. According to Cherokee legend, an eagle flew to the Sun to deliver a prayer for a warrior. In delivering the prayer to the Sun, the Sun kissed the tail feather of the eagle and had him return the feather to the warrior as a symbol of the connection between his people and the Great Spirit. Awohali’s sun-kissed tail feather alludes to the comet-like cloud of evaporating atmosphere detected around the exoplanet GJ 436 b.
The planet's surface temperature is estimated from measurements taken as it passes behind the star to be 712 K (439 °C; 822 °F).[14] 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.[15] Agreenhouse effect would result in a much greater temperature than the predicted 520–620 K.[12]
Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms,[12][16] which would remain solid despite the high temperatures, because of the planet's gravity.[17] 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 viacoronal mass ejection.[18]
However, when the radius became better known, ice alone was not enough to account for the observed size. An outer layer ofhydrogen andhelium, accounting for up to ten percent of the mass, was needed on top of the ice to account for the observed planetary radius.[14][19] 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.[20]
Observations of the planet'sbrightness temperature with theSpitzer 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.[21][22][23][24] In part for this reason, it has also been hypothesized to be a possiblehelium planet.[25]
In June 2015, scientists reported that the atmosphere of Awohali was evaporating,[26] resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14×10^6 km (9×10^6 mi) long.[27]
Artist impression of Gliese 436b shows the enormous comet-like cloud of hydrogen boiling off.[28]
One orbit around the star takes only about twodays, 15.5hours. Awohali orbit is likely misaligned with its star's rotation.[23] 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.[14][29]
A study published inNature found that the orbit of Awohali is nearly perpendicular (inclined by 103.2+12.8 −11.5 degrees)[30] 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.[31]
^abcdefTrifonov, Trifon; Kürster, Martin; Zechmeister, Mathias; Tal-Or, Lev; Caballero, José A.; Quirrenbach, Andreas; Amado, Pedro J.; Ribas, Ignasi; Reiners, Ansgar; et al. (2018). "The CARMENES search for exoplanets around M dwarfs. First visual-channel radial-velocity measurements and orbital parameter updates of seven M-dwarf planetary systems".Astronomy and Astrophysics.609. A117.arXiv:1710.01595.Bibcode:2018A&A...609A.117T.doi:10.1051/0004-6361/201731442.S2CID119340839.
^Coughlin, Jeffrey L.; Stringfellow, Guy S.; Becker, Andrew C.; Mercedes Lopez-Morales; Fabio Mezzalira; Tom Krajci (2008). "New observations and a possible detection of parameter variations in the transits of Gliese 436b".The Astrophysical Journal.689 (2):L149–L152.arXiv:0809.1664.Bibcode:2008ApJ...689L.149C.doi:10.1086/595822.S2CID14893633.
^abcDrake Deming; Joseph Harrington; Gregory Laughlin; Sara Seager; Navarro, Sarah B.; Bowman, William C.; Karen Horning (2007). "Spitzer Transit and Secondary Eclipse Photometry of GJ 436b".The Astrophysical Journal.667 (2):L199–L202.arXiv:0707.2778.Bibcode:2007ApJ...667L.199D.doi:10.1086/522496.S2CID13349666.
^LINE, Michael R.; VASISHT, Gautam; CHEN, Pin; ANGERHAUSEN, D.; YANG, Yuk L. (2011). "Thermochemical and Photochemical Kinetics in Cooler Hydrogen Dominated Extrasolar Planets".Astrophysical Journal. 738, 32 (1): 32.arXiv:1104.3183.Bibcode:2011ApJ...738...32L.doi:10.1088/0004-637X/738/1/32.S2CID15087062., abstract in the arXiv titled "Thermochemistry and Photochemistry in Cooler Hydrogen Dominated Extrasolar Planets: The Case of GJ436b"
^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 (25 June 2015). "A Giant Comet-like Cloud of Hydrogen Escaping from the warm Neptune-mass Exoplanet GJ 436b".Nature.522 (7557):459–461.arXiv:1506.07541.Bibcode:2015Natur.522..459E.doi:10.1038/nature14501.PMID26108854.S2CID4388969.
^Bourrier, V.; Zapatero Osorio, M. R.; Allart, R.; Attia, O.; Cretignier, M.; Dumusque, X.; Lovis, C.; Adibekyan, V.; Borsa, F.; Figueira, P.; Hernández, J. I. González; Mehner, A.; Santos, N. C.; Schmidt, T.; Seidel, J. V.; Sozzetti, A.; Alibert, Y.; Casasayas-Barris, N.; Ehrenreich, D.; Lo Curto, G.; Martins, C. J. A. P.; Di Marcantonio, P.; Mégevand, D.; Nunes, N. J.; Palle, E.; Poretti, E.; Sousa, S. G. (2022), "The polar orbit of the warm Neptune GJ 436b seen with VLT/ESPRESSO",Astronomy & Astrophysics,663: A160,arXiv:2203.06109,Bibcode:2022A&A...663A.160B,doi:10.1051/0004-6361/202142559,S2CID247139822
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