Inferring the Climates of Red Dwarf Planets

Planets orbiting red dwarf stars are unique compared to other star systems because such planets are prone to falling into synchronous rotation, so that one side experiences perpetual day and the opposite side resides in permanent night. Such planets could still be habitable, sustaining liquid water and perhaps even life, so such systems continue to be targeted in the search for signs of life on exoplanets.

One starting point to looking for life on such worlds is to infer properties of an exoplanet climate from astronomical data. Eric Wolf, Ravi Kopparapu, and myself examine this problem in a paper titled “Simulated phase-dependent spectra of terrestrial aquaplanets in M dwarf systems” and published in The Astrophysical Journal. Infrared emission and reflected stellar light from a planet changes as it orbits its host star. We should that observations of these orbital changes in thermal energy could provide important information on the circulation state of the planet, the location of major cloud decks, and the abundance of water vapor. As the next generation of space telescope are designed and launched, methods such as these will become important tools for understanding the potential of M-dwarf systems to support life.

More on Mars Climate Cycles

Fluvial features on Mars seem to indicate that liquid water once flowed on the surface, yet climate theorists remain divided among how the red planet was able to sustain warm enough conditions in the distant past when the sun was fainter. Popular ideas include a dense greenhouse atmosphere of carbon dioxide, hydrogen, and other gases permitted a lengthy period of warmth. Another option suggests that periodic impacts caused enough warming to carve the features in a shorter time.

My co-authors and I have argued in previous papers that climate cycles on early Mars could have been driven by oscillations in the carbonate-silicate cycle, which would have provided transient warming from the accumulation of greenhouse gases by volcanoes and subsequent loss by weathering. In a new paper, we respond to a critique of the limit cycle hypothesis in our “Reply to Shaw.”

We acknowledge that the biggest obstacle to any explanation for warming early Mars with carbon dioxide is their ultimate fate: are there carbonate rocks buried underneath the martian regolith? If not, where did all the carbon dioxide go? Even so, we maintain that the early Mars climate cycle hypothesis remains consistent with observable geologic evidence and could have played at least a partial role in providing warm conditions on early Mars.

Circulation States of Synchronous Rotators

Some planets around low mass stars are expected to be in synchronous rotation, so that the star is continually fixed upon one side. This not only causes one hemisphere to experience permanent day and the other to reside in permanent night (with perpetual twilight along the “terminator” at the edges), but this also drives the climate into a regime fundamentally unlike any seen in the atmosphere of Earth.

In a recent paper titled, “Demarcating circulation regimes of synchronously rotating terrestrial planets within the habitable zone,” my co-authors and I analyze a set of climate model calculations to examine the dependence upon stellar effective temperature of the atmospheric dynamics of planets as they move closer to the inner edge of the
habitable zone. These results show that the surface temperature contrast between day and night hemispheres decreases with an increase in incident stellar flux. This trend is opposite that seen on gas giants, where the same forcing shows an increase in the day-night atmosphere temperature contrast.

We define three dynamical regimes in terms of the dynamical quantities known as the Rossby deformation radius (the ratio of buoyancy to rotation) and the Rhines length (the maximum extent of turbulent structures). The slow rotation regime is characterized by a mean zonal circulation that spans from the day to night side. Slow rotation requires that both the Rossby deformation radius and the Rhines length exceed planetary radius, which occurs for planets with rotation rate > 20 days. Rapid rotators show a mean zonal circulation that only partially spans a hemisphere, with banded cloud formation beneath the substellar point. The rapid rotation regime is defined by the Rossby deformation radius being less than planetary radius, which occurs for planets with rotation rate < 5 days. In between is the Rhines rotation regime, which retains a mean zonal circulation from day to night side but also features midlatitude turbulence-driven zonal jets. Rhines rotators are expected for planets with rotation rate between 5 to 20 days, where the Rhines length is less than planetary radius but the Rossby deformation radius is greater than planetary radius. The dynamical state can be inferred from observations of orbital period and spectral type of the host star as well as from comparing the morphology of the thermal emission phase curves of synchronously rotating planets. Such phase curves will be potentially useful tools for characterizing planets with the next generation of space telescopes.

Does Intelligence Depend on Star Color?

Our civilization on Earth resides around a yellow dwarf star, but orange and red dwarf stars are much more numerous in the galaxy. Does this suggest that we should orient the search for extraterrestrial intelligence (SETI) toward these smaller and redder star systems? Or are certain star types better abodes for intelligent life than others?

Myself and Ravi Kopparapu address these questions in a chapter titled, “The Drake equation as a function of spectral type and time,” which appears in the book Habitability of the Universe Before Earth. Our approach invokes the Drake equation, which is a probabilistic device for estimating the number of communicative civilizations in the galaxy. The conventional terms of the Drake equation are the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, the probability of life, the probability of intelligence, the probability of communicative technology, and the average lifetime of such civilizations. The product of all these terms yields the number of civilizations in the galaxy that SETI could conceivably discover.

Our book chapter analyzes calculations of the liquid water habitable zone for plants orbiting yellow, orange, and red dwarf stars. These habitable zone calculations provide constraint on the terms of the Drake equation. In particular we consider the dependence of each term of the Drake equation on both the stellar type (color) of the star, as well as the change in each parameter over time since the galaxy formed. We suggest that the habitability of red dwarf systems may peak in the far future, while the present time is optimal for habitability around yellow and orange dwarf stars.