The Seager Equation: The Search For Life

The Seager Equation: The Search For Life


Thousands of years ago, our nearest celestial neighbor, the Moon was believed to be inhabited. We have since learned it is not. Hundreds of years ago we discovered several of the stars in our night sky were actually planets, and speculation grew.  Percival Lowell popularized the idea that Mars, our next neighbor to the outer reaches of the solar system was inhabited by a race starved for water who built canals for the purpose of irrigation and supplying their civilization. While life may have existed there once, briefly, alas it has not harbored life in our lifetime (if ever) beyond perhaps a few lingering bacteria.

Venus, Jupiter, Mercury, even the Sun has had it’s speculative inhabitants. Now we send our surrogate eyes and ears to our nearest neighbors and find cold and forbidding desolations or hot and violent land or cloudscapes. While we have not yet ruled out potential for extremeophiles living in the ice cloaked oceans of places like Europa, we still have only speculation in those areas.

Only decades ago we discovered the first planet orbiting a Main Sequence star, Pegasi 51. While we are not yet able to make detailed enough observations of those planets to determine if they are capable of supporting life, we can make many educated guesses based on what we understand of the life and environments we know. We have found a few planets we believe capable of supporting life.

We continue to make advances in exploring the stars, we continue to hope to find life in orbit around one of those stars. No matter how you look at it, it seems like the more we learn about the universe around us, the further away the potential for life to exist becomes. At the same time the potential places for life is expanding.

How do we resolve that seeming contradiction? In 1961, Frank Drake formulated an equation of pure variables to estimate the number of civilizations that might exist.

The Drake equation:

The equation is usually written:
N = R* • fp • ne • fl • fi • fc • L

N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone);
R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space[8]

We continue to revise our estimates of a few of these factors with our searches of the local galaxy.

Now there is a new equation, refined by exo-planet hunter Sara Seager,

Sara Seager, expert planet hunter. Credit: MIT

using our current understanding to help us narrow down the possibilities. Not so much looking for civilizations, as Drake’s equation does, but merely life.

Instead of aliens with radio technology, Seager has revised the Drake equation to focus on simply the presence of any alien life. Her equation can be used to estimate how many planets with detectable signs of life might be discovered in the coming years. Presented at a meeting earlier this year, the Seager equation looks like this:


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

Focusing on M stars, the most common stars our neighborhood that are smaller and less luminous than our Sun, Seager plugged in values for each term. Her calculation suggested that two inhabited planets could reasonably turn up during the next decade.

The fact remains that the Seager equation is also only a string of variables to which we apply guesswork. But our guesswork is getting better as we continue to observe.

Seager was interviewed at Astrobiology Magazine

Q: What was the inspiration behind this equation?

Sara Seager (SS): People have been thinking about trying to find signs of life for a hundred years. This equation is a purposeful take-off on the Drake equation, which was about the search for intelligent extraterrestrial life. Frank Drake wrote that equation because he was using radio telescopes to look for life. It was relevant then and still is. SETI has been going on now for 50 years.

I wanted to explain that we have a new search in progress. We’ll use TESS [the Transiting Exoplanet Survey Satellite] to find rocky planets transiting small stars. Then we’ll use the James Webb Space Telescope to observe the atmospheres of those planets, during transits or secondary eclipses. The punchline here is that if we’re really lucky and everything works in our favor, we will be able to infer signs of life on those planets. We have a shot—I’d call it a remote shot—of finding life within the next decade.

Q: Is your approach specific to intelligent life as well?

SS: No. The equation focuses on the search for planets with biosignature gases, gases produced by life that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes. If we find gases that we might attribute to life we will not know if the gases are produced by intelligent life or simple bacteria.

You can read the full interview at Astrobiology Magazine

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Henry Paterson