Significance Tidal-locking planets receive very uneven stellar heating because their one side permanently faces their stars and the other remains dark. While the dayside can be warm enough to sustain liquid water, the nightside could be so cold that any gases would condense out. Here, we perform simulations with a coupled atmosphere–ocean model to demonstrate the importance of exooceanography in determining the habitability of tidal-locking exoplanets around M dwarfs. We show that ocean heat transport can substantially extend the dayside habitable area and efficiently warm the nightside, so that atmosphere collapse does not occur. As greenhouse effect or stellar radiation is sufficiently strong, ocean heat transport can even cause global deglaciation. Ocean heat transport also likely narrows the width of M dwarfs’ habitable zone.

Abstract The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere–ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an “eyeball.” For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs’ habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets.

M dwarf stars are the most common type of star in the Universe (1). The habitable zone (HZ) around M stars is close to such stars because of their weak luminosity (2). In consequence, habitable exoplanets orbiting M stars are likely to be tidally locked to their primary stars, so that one side of tidal-locking exoplanets permanently faces stars, and the other side remains dark. Previous studies have demonstrated the role of atmospheric heat transport in preventing atmospheric collapse on the nightside of terrestrial exoplanets located in the HZ of M stars (3⇓⇓⇓⇓⇓⇓–10). For a planet with an extensive ocean, its climate and habitability also involves ocean heat transport, which is known to be important in Earth’s climate (11). None of the existing studies has considered the role of ocean heat transport. Moreover, the climate also involves the spatial distribution of open water versus ice and the question of whether the planet becomes locked in a globally glaciated Snowball state. Simulation with a comprehensive Earth atmospheric general circulation model (AGCM) coupled to a slab ocean, without dynamic ocean heat transport, revealed an “eyeball” climate state, with a round area of open ocean centered at the substellar point and complete ice coverage on the nightside, even for very high CO 2 concentrations (4). In the presence of sea ice, ocean heat transport is likely to be especially important, because it is known from studies of the Snowball Earth phenomenon in Earth-like conditions that ocean heat transport is very effective in holding back the advance of the sea-ice margin (12⇓–14). The distribution of sea ice on tidally locked exoplanets is only an issue for M stars, because planets with sufficiently dense atmospheres orbiting hotter stars in orbits close enough to yield tidal locking are likely to be too hot to permit ice and may even be too hot to retain water. The purpose of the present paper is to study how ocean heat transport and sea-ice processes influence the climate and habitability of tidal-locking exoplanets in the HZ around M stars by carrying out simulations with a fully coupled atmospheric–oceanic general circulation model (AOGCM).

Methods The AOGCM used here is the Community Climate System Model version 3 (CCSM3) (15), which was originally developed at the National Center for Atmospheric Research for studying Earth climate. The model includes a dynamic ocean with both wind-driven and deep-ocean circulations. The sea-ice component of CCSM3 consists of energy-conserving thermodynamics and elastic–viscous–plastic dynamics, and the model freezing point of sea water is set to −1.8 °C. The atmosphere component has 26 vertical levels from the surface to the model top of 2.6 hPa and horizontal resolution of 3.75 by 3.75° in latitude and longitude. The ocean component has 25 vertical levels, longitudinal resolution of 3.6°, and variable latitudinal resolutions of about 0.9° near the equator. Eddy parameterization used in the ocean model is the Gent–McWilliams parameterization (16). The model is modified with planetary parameters the same as that of Gliese 581g (Gl 581g) (17). The synchronous rotating period, radius, gravity, and incident stellar radiation are 36.7 Earth days, 1.5 times Earth radius, 13.5 m s−2, and 866 W m−2, respectively. Although Gl 581g is not a confirmed exoplanet, its planetary parameters represent a hypothetical Super-Earth in the HZ of an M star. The stellar spectrum used here is an M star spectrum, with an effective temperature of 3,400 K. Unlike the solar spectrum, most of the M star luminosity is emitted in the near-infrared. Near-infrared radiation is much less reflected by ice and snow compared with visible light. Thus, ice–albedo feedback is weaker on exoplanets orbiting M stars than on Earth (18). Here, sea ice and snow albedos are set to 0.3 and 0.6, respectively. The substellar point is located at the equator and 180° in longitude, and both the eccentricity and obliquity are set to zero. The geothermal heat flux is also set to zero. We assume that the exoplanet is an aquaplanet with a uniform ocean depth of 4,000 m, which is the mean depth of Earth’s oceans. The influence of CO 2 concentration on climate is probed with five different CO 2 levels: 3.6, 355, 10,000, 100,000, and 200,000 ppmv. It has been speculated that the atmosphere of aquaplanets could have high CO 2 levels because silicate weathering could be ceased by water coverage (19). On the other hand, it has been conjectured that CO 2 concentrations high enough to maintain open water in the outer portions of the HZ might be incompatible with limitations imposed by sea-floor weathering (12). All other atmospheric compositions are kept the same as present-day Earth. The radiative transfer module of the model is approximately valid for atmospheres with CO 2 concentration lower than 200,000 ppmv and water-vapor column amount less than 1,200 kg m−2, although the simulated climate is very slightly colder than it should be (20, 21). For CO 2 levels above 200,000 ppmv, the effects of pressure broadening and collision-induced CO 2 absorption become significant (22, 23). In addition, we also examine climate states for exoplanets closer to both inner and outer edges of the HZ by performing simulations with a sequence of stellar radiation fluxes of 700, 866, 1,200, and 1,400 W m−2 and with constant CO 2 concentration of 355 ppmv. The model is initialized with present-day Earth atmosphere conditions and globally uniform vertical profiles of present-day Earth ocean temperature and salinity. All AOGCM simulations are run for about 2,200 Earth years, and the results shown here are based on averages over the last 100 y. To distinguish the role of ocean heat transport, we also carry out simulations with an AGCM coupled with a 50-m slab ocean. The AGCM used here is the atmospheric component of the AOGCM. All AGCM simulations are run for 80 Earth years. Results shown here are averaged over the last 5 y.

Discussion and Conclusions We have demonstrated that ocean heat transports play critically important roles in determining the climate and habitability of tidally locked exoplanets in the HZ around M-type stars. In the presence of a dynamic ocean, the open-ocean area displays a lobster-like spatial pattern, instead of the “eyeball” state. For sufficiently high greenhouse-gas concentrations or strong stellar radiation, ocean heat transport is more efficient than atmospheric heat transport and can cause ice free on the nightside, greatly enlarging the habitable area of tidally locked Super-Earths (but no photosynthesis is possible on the nightside). Sea-ice thickness simulated by AOGCM here is much thinner than that simulated by models without a dynamic ocean. Furthermore, our simulations suggest that a dynamic ocean likely narrows the width of the HZ around M-type stars. The phase shift of surface temperatures between the hottest spots and the substellar point caused by ocean circulations may have observational consequences in both the infrared and visible phase curves of the system, which could be visible in future observational missions. Exoplanets are likely to have a wide range of ocean depth or even have continents. Results in Fig. 2 suggest that ocean depth has important influences on ocean heat transports because the strong equatorial current and the warm ocean layer can both extend downward to about 2 km for a strong greenhouse effect. Thus, both zonal and meridional ocean heat transports would largely decrease if ocean depth is shallower by one order. On the other hand, if ocean depth is thicker by one order, ocean heat transport would increase. Given that the ocean is separated by continents, especially by meridional continental barriers, continents would block ocean zonal heat transports from the dayside to the nightside. In such a case, the importance of ocean heat transport would be largely reduced, and the climate state would be more like the “eyeball” state although the open-ocean region is not a round area due to ocean heat transports over ocean-domain scales. In addition to ocean heat transport, ice–albedo feedback and sea-ice dynamics also have contributions to climates of tidally locked exoplanets. Detailed analysis of these effects is beyond the scope of the present paper. We will report them in a separate paper. It is worth pointing out that it is difficult to make a full exploration on how ocean heat transport would alter the HZ width with current general circulation models (GCMs). Defining the outer and inner HZ boundaries involves high levels of CO 2 and water vapor, respectively, and radiation transfer modules of current GCMs are not valid in resolving these problems.

Acknowledgments We thank R. T. Pierrehumbert who sharpened our ideas and helped us greatly in revising early drafts of the paper. We also thank D. N. C. Lin for fruitful discussion on this subject. We are grateful to Y. Liu who helped us set up the model. This project was made possible through the support of a grant from the John Templeton Foundation. The funds from the John Templeton Foundation were awarded in a grant to the University of Chicago which also managed the program in conjunction with the National Astronomical Observatories, Chinese Academy of Sciences. This work is also supported by the National Natural Science Foundation of China (41025018) and by the National Basic Research Program of China (973 Program, 2010CB428606).

Footnotes Author contributions: Y.H. and J.Y. designed research; Y.H. and J.Y. performed research; Y.H. and J.Y. analyzed data; and Y.H. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

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