HELIUM ATMOSPHERES ON WARM NEPTUNE- AND SUB-NEPTUNE-SIZED EXOPLANETS AND APPLICATIONS TO GJ 436b
Name
Hu-2015-HELIUM ATMOSPHERES O.pdf
Size
1.88 MB
Format
Adobe PDF
Checksum (MD5)
fc550332ec9f5c0f1f1d77b62fc6bbc1
Author(s)
Seager, Sara
Yung, Yuk L.
Hu, Renyu, Ph. D. Massachusetts Institute of Technology
Date Issued
June 2015
Journal
The Astrophysical Journal
Publisher
IOP Publishing
Citation
Hu, Renyu, Sara Seager, and Yuk L. Yung. “HELIUM ATMOSPHERES ON WARM NEPTUNE- AND SUB-NEPTUNE-SIZED EXOPLANETS AND APPLICATIONS TO GJ 436b.” The Astrophysical Journal 807, no. 1 (June 24, 2015): 8. © 2015 The American Astronomical Society
Version
Final published version
Abstract
Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than Mercury's are thought to have experienced extensive atmospheric evolution. Here we propose that a potential outcome of this atmospheric evolution is the formation of helium-dominated atmospheres. The hydrodynamic escape rates of Neptune- and sub-Neptune-sized exoplanets are comparable to the diffusion-limited escape rate of hydrogen, and therefore the escape is heavily affected by diffusive separation between hydrogen and helium. A helium atmosphere can thus be formed—from a primordial hydrogen–helium atmosphere—via atmospheric hydrodynamic escape from the planet. The helium atmosphere has very different abundances of major carbon and oxygen species from those of a hydrogen atmosphere, leading to distinctive transmission and thermal emission spectral features. In particular, the hypothesis of a helium-dominated atmosphere can explain the thermal emission spectrum of GJ 436b, a warm Neptune-sized exoplanet, while also being consistent with the transmission spectrum. This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen, with the predominance of CO over CH[subscript 4] as the main form of carbon. With our atmospheric evolution model, we find that if the mass of the initial atmosphere envelope is 10[superscript −3] planetary mass, hydrodynamic escape can reduce the hydrogen abundance in the atmosphere by several orders of magnitude in ~10 billion years. Observations of exoplanet transits may thus detect signatures of helium atmospheres and probe the evolutionary history of small exoplanets.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Terms of Use
Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.
Persistent DSpace Link
DOI of Published Version
https://doi.org/10.1088/0004-637X/807/1/8
