SETI, Astrobiology and Red Dwarfs

If you’ve been following the KIC 8462852 story, you’ll want to be aware of Paul Carr’s Dream of the Open Channel blog, as well as his Wow! Signal Podcast, both of which make for absorbing conversation. In his latest blog post, Carr offers sensible advice about how to look at anomalies in our astronomical data. Dysonian SETI tries to spot such anomalies in hopes of uncovering the activities of an extraterrestrial civilization, but as Carr makes clear, this is an enterprise that needs to be slowly and patiently done, without jumping to any unwarranted assumptions.

Let me quote Carr on this important point:

…we will have to be patient, since we will be almost certainly be wrong at first, or perhaps just unlucky in our search. We don’t need to nail it exactly, but we will need to develop rough models of ET activity that distinguishes it from nature. These models would more or less fit the data that we think anomalous, would make testable predictions, and would show how to rule out at least known natural phenomena. Such a family of models may be available next year, or it may be in 100 years, but the more anomalous data we have, the more the models can be constrained.

This paragraph gets it right, taking it as a given that we have no idea whether there are extraterrestrial civilizations or, for that matter, life of any kind around other stars. We certainly have no idea how widespread either form of life might be, and in the case of Dysonian SETI, we would be looking at technologies so far in advance of ours that recognizing them for what they are (or might be) creates myriad challenges. So while we try to distinguish natural phenomena from the possibility of intelligent activity, we need to keep these profound limitations in mind.

Tabby’s Star, then, is a wonderful case in point, certainly a motivator for this kind of research (and, as we’ve seen, one capable of being sustained at least modestly by public funding), but we should also consider it in a broader perspective. The goal will be to build a catalog of unusual phenomena that can be consulted as we begin to differentiate among such targets. We may discover that all of these can be accounted for by natural processes, and if so, then we have learned something valuable about the universe. No small accomplishment, that.

Red Dwarfs and Astrobiology

Looking beyond SETI to more fundamental questions of astrobiology, we find ourselves in that unsettling period when we have instruments in the pipeline that can tell us much about the exoplanets we observe, but we’re not yet receiving the data that can make a definitive call on the existence of life elsewhere. Astrobiology will accumulate data at increasingly fine levels of detail as we move from missions like Kepler to searches around closer stars. Meanwhile, we have to tune up our models for detecting biosignatures as we wait for the technology to test them.

Here the Transiting Exoplanet Survey Satellite (TESS) comes to mind, as does PLATO (PLAnetary Transits and Oscillations of stars), and of course the James Webb Space Telescope. TESS is due for a 2017 launch, JWST for 2018 and PLATO for 2024. WFIRST (Wide Field Infrared Survey Telescope), scheduled for the mid-2020s, is likewise going to provide key exoplanet observations, and let’s not neglect the small photometric platform CHEOPS (CHaracterising ExOPlanet Satellite), which will sharpen the target lists of future ground-based observatories. We need to continue refining our answers to this question: What does life do to a planet that offers a key observable, and what are the best instruments to detect it.

Red dwarfs make excellent targets if we’re studying a planetary atmosphere to learn whether or not there are biomarkers there, and now we have a new paper from Avi Loeb (Harvard-Smithsonian Center for Astrophysics) that asks whether such stars may ultimately become home to the vast majority of cosmic civilizations. Working with Rafael Batista and David Sloan (both at Oxford University), Loeb acknowledges the obvious: We don’t know if stars like these can support life, and the authors call for building the datasets to find out. But if they can, then the implications are that most life in deep space will eventually be around such stars.

I say ‘eventually’ because M-dwarfs have lifetimes measured in the trillions of years, much greater than the 10 billion years or so that G-class dwarfs like our Sun can expect. And of course, around our own star life gets problematic within about a billion years. We have a planet that cannot be expected to remain habitable all the way to the last days of the Sun.

If life can form on planets around red dwarfs, then the probability of life grows much higher as we go further and further into the future, for these small stars are the most common kind of star in the galaxy, comprising as much as 80 percent of the stellar population. That would mean we are early to the dance, and a densely populated galaxy has simply not had time to develop. Loeb’s paper calculates the relative formation probability per unit time of habitable Earth-like planets within a fixed comoving volume of the Universe and finds red dwarfs favored:

“If you ask, ‘When is life most likely to emerge?’ you might naively say, ‘Now,'” says Loeb. “But we find that the chance of life grows much higher in the distant future.”

Image: This artist’s conception shows a red dwarf star orbited by a pair of habitable planets. Because red dwarf stars live so long, the probability of cosmic life grows over time. As a result, Earthly life might be considered “premature.” Credit: Christine Pulliam (CfA).

Hence the importance of a biosignature detection. If we find such markers in the atmosphere of a red dwarf, we have learned something not only about that particular star, but about the prospect of life in later cosmic eras up to the ten trillion year lifetime of the average red dwarf. The universe we see has had 13.7 billion years to produce life, but we can only imagine what kinds of life might emerge in the future. As for the probability of our own emergence, let me quote from the paper:

One can certainly contend that our result presumes our existence, and we therefore have to exist at some time. Although our result puts the probability of finding ourselves at the current cosmic time within the 0.1% level, rare events do happen. In this context, we reiterate that our results are an order of magnitude estimate based on the most conservative set of assumptions within the standard ΛCDM model.

Conservative indeed, and if we tweak the assumptions, it gets more extreme:

If one were to take into account more refined models of the beginning of life and observers, this would likely push the peak even farther into the future, and make our current time less probable. As an example, one could consider that the beginning of life on a planet would not happen immediately after the planet becomes ‘habitable’. Since we do not know the circumstances that led to life on Earth, it would be more realistic to assume that some random event must have occurred to initiate life, corresponding to a Poisson process [in probability theory, used to model random points in time and space]. This would suppress early emergence and thus shift the peak probability to the future.

Are we truly premature, or are we simply going to learn that life is not possible around stars in an M-dwarf habitable zone? We’ve considered all the possibilities many times in these pages. Tidally locked to its star, a planet like this would experience constant day on one side, constant night on the other, with ramifications for climate and habitability that remain controversial. Extreme radiation from solar flares in young M-dwarfs may scour the surface of life (or, on the other hand, act as an evolutionary spur). And such planets may be home to volcanic activity that can lead to runaway greenhouse effects (see A Mini-Neptune Transformation?).

In other words, life’s chances around G-class stars may be profoundly greater than around M-dwarfs, in which case the chance of life emerging does not increase as we move into the distant future. For these reasons, using our upcoming space missions to search for life around small red stars can help us place ourselves in the cosmic hierarchy. We need to learn what conditions a planet in the habitable zone of an M-dwarf can support, and the discovery of biosignatures there would cause us to re-evaluate our thoughts on ‘average’ life and its existence around Sun-like stars.

The paper is Loeb, Batista and Sloan, “Relative Likelihood for Life as a Function of Cosmic Time,” accepted for publication in Journal of Cosmology and Astroparticle Physics (preprint). A CfA news release is also available. Ben Guarino writes up Loeb’s findings in a helpful essay for the Washington Post.