habitable zone

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Red dwarf stars outnumber yellow dwarf stars like our sun by over a factor of ten. Observations of exoplanets have also shown that rocky, and potentially habitable, planets are just as common around red dwarfs as yellow dwarfs. But if these much smaller stars are more commonplace, then why do we find ourselves around a yellow star like the sun, instead of a red dwarf?

My co-authors and I attempt to address this question of selection bias in a recent paper titled “Why do we find ourselves around a yellow star instead of a red star?” and published in International Journal of Astrobiology. We take a statistical approach to thinking about the region around all stars where life is most likely to develop. The liquid water habitable zone provides the best observational constraint on where we would expect to find planets that could support conscious observers like us, and this study examines the probability of finding oneself on a planet in the habitable zone of a yellow dwarf star, compared to a red dwarf. The results show that even though red dwarfs are much more numerous, they have a narrower habitable zone than yellow dwarfs, so our existence around a star like the sun is actually to be expected.

This study also considers that red dwarf stars will be even more numerous in the distant future of the universe, due to their much longer lifetimes than other stars. If these red dwarf stars will eventually become the predominant place for conscious observers to develop, then why do we not instead find ourselves around a red dwarf star billions or trillions of years into the future? The statistics for this aspect of the problem suggest that our existence around a yellow dwarf star today, compared to a red dwarf star in the future, might be a slight statistical anomaly—perhaps on the order of finding oneself born ambidextrous or with perfect pitch. But this statistical unlikelihood might also suggest that life is wholly impossibly around red dwarf stars, or else any type of conscious observers that do develop around such stars will be drastically different from our type of conscious life.

The outer edge of the habitable zone is traditionally defined as the outermost orbital distance at which a planet could sustain liquid water on its surface. At this distance orbit, Earth-like planets with plate tectonics (or a similar process for recycling volatiles) should build up dense carbon dioxide atmospheres that help offset the reduction in starlight. Carbon dioxide released from volcanoes provides additional greenhouse warming, although rainwater dissolves some of this. The amount of carbonic acid that dissolves in rainwater and reaches the ground depends upon the temperature: the colder it gets, the less carbon dioxide gets rained out of the atmosphere. This feedback is part of the carbonate-silicate cycle, which regulates an Earth-like planet’s carbon dioxide over geologic (million year) time scales.

In a recent paper published in The Astrophysical Journal, titled “Limit cycles can reduce the width of the habitable zone,” my co-authors and I examine the propensity of this carbonate-silicate cycle to cause a planet to oscillate between completely frozen and completely ice-free climate states. We update a simplified climate model to account for the increase in weathering that occurs as a planet builds up a dense carbon dioxide atmosphere. Beginning with a planet in completely ice-covered conditions, we allow volcanic outgassing of carbon dioxide to continue until the planet melts from the enhanced greenhouse effect. However, under certain conditions, the planet will then start to rain out and weather the atmospheric carbon dioxide at such a fast rate that the greenhouse effect decreases and the planet again plummets into global glaciation.

This type of climate cycle between glacial and ice-free states is not likely to occur on Earth today, but such cycles might have been possible on early Earth during the Hadean eon. Extrasolar planets may also be prone to this type of climate cycling, although predicting whether or not this should occur depends upon knowing a planet’s volcanic outgassing rate. Our climate calculations place boundaries on the conditions under which we should expect such climate cycles to occur for Earth-like planets orbiting a range of different stars.

Planets in the habitable zone of low-mass, cool stars are expected to be in synchronous rotation, where one side of the planet always faces the host star (the substellar point) and the other side experiences perpetual night (the anti-stellar point). Previous studies using three-dimensional climate models have shown that slowly rotating plants orbiting these low-mass stars should develop thick water clouds form at substellar point, at the point at which the star is directly overhead, which should increase the reflectivity, and thus stabilize the planet against increased warming at the inner edge of the habitable zone.

However these studies did not use self-consistent orbital and rotational periods for synchronously rotating planets placed at different distances from the host star, which are a requirement from Kepler’s laws of motion. We address this issue in a new study led by Dr. Ravi Kopparapu, on which I am a co-author, titled “The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models.” In this study, we use correct relations between orbital and rotational periods to show that the inner edge of the habitable zone around low mass, cool stars is not as close as the estimates from previous studies. We also discuss how the stellar composition, or ‘metallicity,’ can affect the orbital distance of the habitable zone.

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 a paper published in the Monthly Notices of the Royal Astronomical Society, my co-author and I 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.