It’s easy to see why interest in planets around red dwarfs is growing. The low mass of such a star makes finding smaller planets feasible. It also produces orbits closer to the star, another aid to their detection. We know that planets can form near the habitable zone of such stars because we have the example of Gliese 581, where two planets orbit close to if not just within that region. But is a habitable planet always habitable? If not, what could make these conditions change?
I’m looking at a paper that examines tidal effects, an important factor when dealing with M dwarfs. Planets in the habitable zone around these stars experience effects that can cause both their orbital distance and orbital eccentricity to decrease [see comments below re my original misstatement of the eccentricity change, now corrected]. The paper, by Rory Barnes (University of Arizona, Tucson), Sean Raymond (University of Colorado, Boulder) and team, examines an interesting parameter: The habitable lifetime. The authors define this as the time needed for a planet to move from a habitable region to one that is not. More massive stars have habitable zones far enough away that this tidal evolution does not occur, but M dwarfs below about 0.35 solar masses can be affected.
The result is striking and potentially devastating to life: A planet with a large enough orbital eccentricity (larger than about 0.5) around a low-mass star can, because of this tidal effect, be pulled out of the habitable zone in less than a billion years. Given the fact that M dwarfs account for over 75 percent of the stars in our galaxy, and given the fraction of known exoplanets with high eccentricity, the authors suggest that tidal effects may be a noteworthy constraint on the total number of habitable planets. The definition of habitability used in this study is the classic one, based upon the presence of liquid water upon the surface.
The results for Gliese 581 c are intriguing. This is the planet that caused such a stir when researchers announced that it was within the habitable zone of its star, a finding that has since been sharply questioned. Looking at the interactions between Gliese 581 c and inner planet 581 b, the authors state:
Tidal theory suggests that planet c orbited with larger values of semi-major axis and eccentricity in the past. Therefore, it may have been habitable in the past, but tides subsequently moved the planet into an uninhabitable orbit. If planet c was the only planet in the system, plausible physical properties indicate that it was habitable. However, when constraints from the mutual interactions of the additional planets are considered, planet c has likely never been habitable.
The details of these interactions are complicated, but what I want to focus on is the deeper implication here:
The detection of a terrestrial planet around a low-mass star is insufficient to determine that planet’s past and future habitability. The tidal forces between planet and star can significantly change the orbits and hence limit the habitable lifetime. Planets detected in the HZ with large eccentricities may be bound for hotter temperatures and, ultimately, a global extinction. On the other hand, planets detected interior to the HZ may have been habitable in the past. Gliese 581 c was most likely not habitable in the past, but if its companion planets were on different orbits, past habitability would have been possible.
As if we didn’t have reason enough for caution about these matters, we’re now reminded that just finding a rocky world at a particular distance from its parent star is no guarantee of its long-term habitability, particularly when we’re dealing with M dwarfs. Because we’re talking about major orbital evolution over a span comparable to the age of the Earth, it’s clear that sustained complex life demands planets that form with low eccentricities. The good news is that most exoplanets are thought to have been formed with relatively low eccentricities.
So now we’re looking at a true science fiction scenario, a planet that once supported life but moved ultimately inside the habitable zone of its star. What might future exobiologists find among the wreckage? The authors of this paper note that we should extend our work on habitable atmospheres and their evolution to include the possibility of detecting the signatures of extinct life on planets like this around M dwarfs. In many ways, the thought is poignant, and it’s easy to agree with this statement:
Perhaps the most distressing aspect of this work (from a SETI perspective) is the prediction that planets can be habitable long enough for complex life to develop, but then that life is extinguished by tides. Yet this work suggests that such a “tidal extinction” may occur on some planets around low-mass stars.
The paper is Barnes et al., “Tides and the Evolution of Planetary Habitability,” accepted by Astrobiology and available online. Be aware as well of Jackson, Greenberg and Barnes, “Tidal Heating of Extra-Solar Planets,” accepted by the Astrophysical Journal (abstract). An earlier Centauri Dreams story on that paper is here. Thanks to Adam Crowl for his assistance on this story.