Peking University, Jan. 13, 2014: A paper entitled “Role of ocean heat transport in climates of tidally locked exoplanets around M-dwarf Stars” is published in Proceedings of the National Academy of Sciences on December 30, 2013 (Early Edition). The authors of the article are Professor Yongyun Hu and his former graduate student, Jun Yang (currently a postdoctoral research scientist at the University of Chicago) from the Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University. In this paper, they demonstrated the importance of exo-oceanography in determining climate states, habitability and the width of the Habitable Zone around M-Dwarf stars.

The habitable zone around M stars is close to such stars because of their weak luminosity. In consequence habitable planets orbiting M-type stars are likely to be tide-locked to their primary. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the night side. For a planet with an extensive ocean, habitability also involves the distribution of open water vs. ice, and the question of whether the planet becomes locked in a globally glaciated Snowball state. Including only atmospheric heat transports, climate states have been found in which there is a circular patch of open water centered on the substellar point (“Eyeball superEarth”). Yongyun Hu and Jun Yang carried out the first simulation of this problem with a fully coupled dynamic ocean-atmosphere model, and found that ocean heat transports substantially extend the area of open water along the equator, and can even lead to complete deglaciation of the nightside. They demonstrated further that when ocean heat transports are taken into account, open water can be maintained near the substellar point even in the outer reaches of the habitable zone, and even at very low atmospheric concentrations of CO2. This study provides the first demonstration of the importance of exo-oceanography in determining the habitability state of an exoplanet.

Figure caption: Simulation results of sea-ice coverage (left) and surface temperature (right). Upper maps are for the case of a slab ocean, and the lower maps are for a dynamic ocean. For left-hand maps, blue indicates open ocean, and gray denotes ice. For the right-hand maps, the unit of temperature color bars is °C.

The attached figure here compares sea-ice coverage and surface air temperatures between two-types of simulations: slab ocean and dynamic ocean. With a slab ocean, the open-ocean region is a round area, while all rest parts are covered by ice and nightside temperature is below -70 °C. The exoplanet looks like an “Eyeball”. In contrast, as a dynamic ocean is included to the model, the spatial patterns of both open-ocean and warm surface temperatures are like a “Lobster”, with two “claws” at both sides of the equator and a long tail along the equator, which extends to the nightside. Moreover, the nightside surface temperature greatly increased due to ocean heat transport from the dayside to the nightside.

M-dwarf stars are the most common type of stars in the Universe. It is thus very likely that habitable exoplanets are first discovered around M Dwarfs in future. Climate patterns caused by ocean circulations, as shown in this study, may have observational consequences in both the infrared and visible phase curves of the system, which could be visible in future observational missions.

Open access is at: http://www.pnas.org/content/early/2013/12/26/1315215111.full.pdf+html

Edited by: Zhang Jiang