Yuwei Wang1, Yonggang Liu1, Feng Tian2,3, Jun Yang1, Feng Ding4, Linjiong Zhou5, Yongyun Hu1
1. Laboratory for Climate and Ocean-Atmosphere Sciences, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, 100871, China
2. Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth System, Science, Tsinghua University, Beijing, 100084, China
3. Joint Center for Global Change Studies (JCGCS), Beijing, 100875, China
4. The Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
5. NOAA/Geophysical Fluid Dynamics Laboratory, and Atmospheric Oceanic Sciences Program, Princeton University, Princeton, NJ 08544, USA
Most of previous studies on how obliquity affects planetary habitability focused on planets around Sun-like stars. Their conclusions may not be applicable to habitable planets around M dwarfs, due to the tidal-locking feature and associated insolation pattern of these planets. Here we use a comprehensive 3-dimensional (3D) atmospheric general circulation model to investigate this issue. We find that the climates of planets with higher obliquity are generally warmer, consistent with previous studies. The mechanism of warming is, however, completely different. Significant reduction of low cloud, instead of sea ice cover, within the substeller region (which moves if obliquity is non-zero) is the key in warming the M-dwarf planets with high obliquity. For a total insolation of 1237 W m-2, the climate warms by 21 K when the obliquity increases from 0° to 90°. Correspondingly, the runaway greenhouse inner edge of the habitable zone shifts outward from 2500 W m-2 to 2100 W m-2. The moist greenhouse inner edge, based on our crude estimation, shifts less, from 2180 W m-2 to 2075 W m-2. Near the outer edge, in contrast, the climates of planets with higher obliquity are colder, due to their reduced ability to maintain a hotspot at the surface. Therefore, the outer edge moves inward when obliquity is increased, opposite to the finding of previous studies on planets around Sun-like stars. Our results thus indicate that the habitable zone for M dwarfs narrows if the obliquity of their planets increases.
Citation: Wang, Y., Liu, Y., Tian, F., Yang, J., Ding, F., Zhou, L., & Hu, Y. (2016). Effects of Obliquity on the Habitability of Exoplanets around M Dwarfs. The Astrophysical Journal Letters, 823(1), L20.