An essay of mine called Distant Ruins is now available from Aeon Magazine, looking at a field that is increasingly becoming known as ‘interstellar archaeology.’ Rather than looking for radio or optical signals flagging an extraterrestrial culture, some scientists have asked whether a sufficiently advanced civilization might not have left evidence of its existence in the form of huge engineering projects, mining asteroids or breaking up entire planets to build Dyson spheres. Or perhaps so-called ‘blue straggler’ stars are evidence of a culture tinkering with its own sun.

I speculate in Aeon that what we may someday detect in our rapidly growing astronomical databases is evidence not of living but long-vanished cultures, whose mega-engineering may stand as enigmatic evidence of beings that died before our Sun was born. We don’t, after all, know how long technological civilizations live, and there is no reason to think them immortal.

All of this plays into today’s post because one of the key elements of the Drake Equation is the term L, which stands for the lifetime of a technological civilization. On this matter we simply have no knowledge, other than to say that our own culture has managed to survive with technology until now. Do civilizations inevitably destroy themselves at some point through misuse of their tools, and is this the ‘Great Filter’ that a culture has to make it through to reach maturity?

An Alternative to Drake


We can ponder these issues as the various forms of SETI proceed, but we should remember that the hunt for biological — not necessarily technological — markers is ongoing. At MIT, Sara Seager is offering a new take on the Drake Equation that opts to look not at intelligent life but at the presence of life itself. It’s a smart decision because we’re coming up on an era when we’ll be able to probe the atmospheres of potentially habitable planets around small M-class dwarf stars. Not only is the TESS (Transiting Exoplanet Survey Satellite) mission in the works, but we also have the James Webb Space Telescope. If TESS can find candidate planets around stars, JWST can study them to learn whether the biosignature gases that mark life are there.

Image: MIT exoplanet hunter Sara Seager.

Seager’s equation is a sharp break from Drake’s, and I’ll pull it right out of this Astrobiology Magazine interview, to which I refer you for more background::



N = the number of planets with detectable signs of life
N* = the number of stars observed
FQ = the fraction of stars that are quiet
FHZ = the fraction of stars with rocky planets in the habitable zone
FO = the fraction of those planets that can be observed
FL = the fraction that have life
FS = the fraction on which life produces a detectable signature gas

What’s being left out is immediately obvious when compared with the famous Drake approach. Here’s Drake’s original formulation:

N = R* fp ne fl fi fc L


N = The number of communicative civilizations
R* = The rate of formation of suitable stars (stars such as our Sun)
fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun.)
ne = The number of Earth-like worlds per planetary system
fl = The fraction of those Earth-like planets where life actually develops
fi = The fraction of life sites where intelligence develops
fc = The fraction of communicative planets (those on which electromagnetic communications technology develops)
L = The “lifetime” of communicating civilizations

You can see that Seager’s approach focuses solely on biosignature gases, which we are usefully able to study because the atmosphere of a planet transiting its host star will absorb some of the starlight. So we’re looking for photons of starlight shining through the atmosphere of a planet, and we’re also looking for stars quiet enough that flare activity and other disruptions don’t mask the data we need to gather from the transiting planet. Some figures in Seager’s equation can be calculated: The fraction of M-dwarfs with planets in the habitable zone, based on Kepler statistics, is roughly 0.15 for quiet stars. Other terms are, as Seager says, just guesses, including the fraction that have life and the fraction that produce a detectable signature gas.


Image: Habitable zone relative to size of star. Credit: Wikimedia Commons.

Much could be said about biosignatures themselves. On the Earth, oxygen, ozone, methane and carbon dioxide are produced biologically, but could also occur naturally in the atmosphere of a planet that was devoid of life. So it’s not so much a single gas but a combination that tells the tale. A biosignature would be the simultaneous presence of these gases in quantities telling us that life must be part of what is keeping them in production. On that score, Seager’s last term — the fraction of planets on which life produces a detectable signature gas — is cunning because it leads to the basic issues that will have to be resolved as we broaden the hunt for life. Says Seager:

I carefully crafted the last term of this equation so one could actually add more information in. Does life produce a detectable signature? Are there systematic effects that rule out some biosignature gases being detected in some planets? Can we not find the signature for technical reasons? We just don’t know how many planets have life that is producing biosignature gases that are detectable by us.

None of this de-emphasizes the current SETI effort, which proceeds with Drake’s Equation very much in mind. But Seager’s new equation is a nice addition to the exoplanet toolkit. After all, we have no idea whether or when a SETI project will pull in evidence of an extraterrestrial civilization. But in Seager’s view, there is at least “a remote shot” that we’ll detect a biosignature within the next ten years. Inferring some kind of life on a distant world isn’t like being handed the password to the Encyclopedia Galactica, but it would tell us that life is not confined to our own world.

How striking to think that the first discovery of life elsewhere may come from the light of a distant exoplanet rather than from an object in our own Solar System! But ponder: Seager is talking about a possible biosignature detection within a mere ten years. Are we likely to have unambiguous evidence of life on Mars, Europa or any other nearby object as soon as that?