Small red stars, known by astronomers as “M-dwarfs”, are the most abundant type of star in the sky and are also the most long-lived of all stars. This means there are plenty nearby of M-dwarfs to search for possible habitable planets, and many current and planned exoplanet surveys emphasize the search for potential worlds orbiting within the habitable zone of these low-mass stars. Astrobiologists often use the term “habitability” to indicate a planet’s ability to sustain liquid water on its surface, thereby providing conditions where life might be able to develop and thrive. The corresponding “habitable zone” describes the range of orbital distances that can support these clement conditions and not lose the water to a rapid runaway greenhouse (from too close an orbit) or a cool condensing atmosphere (from too far an orbit).
The problem with planets orbiting M-dwarfs is that they are prone to fall into “synchronous rotation” so that one side of the planet always faces the star, while the other side remains in perpetual darkness. Synchronous rotation can occur as a result of tidal forces from gravitational interactions between two orbiting bodies (Earth’s moon is an example of an object in synchronous rotation, so that we only ever see one side from the ground). For a planet orbiting an M-dwarf, the “sub-stellar point” beneath a constant stream of starlight is ceaselessly warmed, while the opposing “anti-stellar point” receives no starlight at all and resides in total darkness. One potential problem is that the atmosphere may condense into large ice caps on the frigid night side of these planets, which could result in total atmosphere collapse and the loss of habitable conditions.
Fortunately, the large-scale motions of the atmosphere help to redistribute this energy and, in many studies with climate models, can help avoid this atmospheric freeze-out. In this investigation, we use a three-dimensional computer climate model to examine the role of geothermal heating on planets orbiting M-dwarfs. Geothermal heating is another consequence of tidal forces from a close orbit, and this additional surface warming can help to amplify the asymmetric distribution of energy transport toward the night side of the planet. This can help to induce the melting of ice near the anti-stellar point and create additional habitable area surrounding the night-time hemisphere.
We also examine the large-scale dynamical circulations on these synchronous rotating planets in comparison to the general circulation patterns on Earth. We demonstrate that the direction of of the meridional (i.e. north-south) circulation changes directions from one side of the sub-stellar point to the other. That is, a global average of the meridional circulation provides an incomplete picture of the large-scale dynamics because the eastern and western hemispheres each show strong motion but in opposite directions that cancel when summed together. Additionally, we examine the presence of a cross-polar circulation that transports energy and mass from the sub-stellar to anti-stellar point across the northern and southern poles. This also contributes toward maintaining climate stability and avoiding atmospheric freeze-out with a circulation pattern atypical of those observed on Earth.
Our study emphasizes the need for careful analysis when considering how the atmospheric dynamics of a synchronously rotating terrestrial planet may differ from our own. The study of Earth-like exoplanets must begin with analogies to observations on Earth, and studies like ours help to apply Earth system models toward more general planetary system. As research into planetary habitability continues, through theory as well as observations, we will indeed continue to observe how even basic physical principles can manifest in very different ways on these alien worlds.