It’s been over half a century since Frank Drake developed an equation to estimate the probability of finding intelligent life in our galaxy. We’ve learned a lot since then, prompting an astrophysicist from MIT to come up with her own take on the equation. Here’s how the new formula works — and how it could help in the search for alien life.
The new formula was devised by Sara Seager, a professor of planetary science and physics at the Massachusetts Institute of Technology. I contacted her to learn more about the new equation and why the time was right for a rethink.
Assessing the Probability of Intelligent Life
Back in 1961, Frank Drake proposed a probabilistic formula to help estimate the number of active, radio-capable extraterrestrial civilizations in the Milky Way Galaxy. It goes like this:
- N is the number of civilizations in our galaxy with which we might hope to be able to communicate
- R* is the average rate of star formation in our galaxy
- fp is the fraction of those stars that have planets
- ne is the average number of planets that can potentially support life per star that has planets
- fl is the fraction of the above that actually go on to develop life at some point
- fi is the fraction of the above that actually go on to develop intelligent life
- fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L is the length of time such civilizations release detectable signals into space
People have plugged in a variety of values over the past 50 years — all of them purely speculative. Values for N have ranged anywhere from one (i.e. here's looking at you kid) up to the millions.
“The original Drake Equation just gave us the format with which to see what the different ingredients would be,” Seager told io9. “No one had ever quantitatively organized our thoughts before. That’s the revolutionary nature of the equation.”
But it can never give us a quantitative answer, she says, and we shouldn’t expect the equation to be a real equation in the sense that we can have precise definitions for each term.
“It’s a wonderful, amazing, innovative way for us to think about intelligent life — or the existence of intelligent life,” she says, “But there are just so many unknowns that can’t be quantified.”
But things have changed since 1951. Thanks to the Kepler Space Telescope, we now know that there's an absolute plethora of exoplanets out there. What’s more, they come in all sorts of shapes and sizes, they orbit a diverse array of stars, and they reside in solar systems that scarcely resemble our own. Our sense of the galaxy is changing dramatically with each new discovery — as is our sense of its potential to harbor life.
Given all this new information, Seager felt that the time was right to rethink the Drake Equation.
The Seager Equation
The new equation looks like this:
- N is the number of planets with detectable biosignature gases
- N* is the number of stars within the sample
- FQ is the fraction of quiet stars
- FHZ is the fraction with rocky planets in the habitable zone
- FO is the fraction of observable systems
- FL is the fraction with life
- FS is the fraction with detectable spectroscopic signatures
Now, it’s important to note that this equation is not an update to the Drake Equation per se — it’s more like a parallel equation that can work in tandem with the original version. Rather than come up with a formula to predict the predominance of intelligent life, Seager is interested in predicting our chances of detecting any kind of life within the next ten years.
And the reason for this is purely practical: It’s rooted in the real world of what we already know — or are soon to know.
“We’re not throwing out the Drake Equation, which is really a different topic,” she explains. “Since Drake came up with the equation, we have discovered thousands of exoplanets. We as a community have had our views revolutionized as to what could possibly be out there. And now we have a real question on our hands, one that’s not related to intelligent life: Can we detect any signs of life in any way in the very near future?”
Seager is uncomfortable in referring to the new equation as an update, instead suggesting that we call it the “parallel Drake Equation,” or the “revised Drake Equation.” Personally, I think we should call it for what it is, “The Seager Equation,” as its purpose is distinguished from what Drake was trying to achieve. Instead of trying to assess our chances of finding radio capable civilizations, the new equation evaluates our chances of detecting signs of life on exoplanets by signs of biosignature gases.
Indeed, Seager’s N represents the number of planets with detectable biosignature gases.
Detectable being the key word.
“We’re actually on a different track, where we’re trying to find signs of life on another planet,” she says, “and the only way we know how to do this right now is by remote sensing.”
In other words, spectroscopic imaging — the process of splitting the light up from a planet or any star and trying to identify what gases are present by what they have removed or added to the light.
“Just like on Earth where we have satellites that look down to measure gas concentrations, we can use space telescopes to look at the atmospheres of planets far away,” she explains. “We’re going to look for gases that essentially don’t belong — gases that may be produced by life.”
Seager gives the example of oxygen on Earth.
“Oxygen fills our atmosphere to 20% by volume. But without life we actually wouldn’t have oxygen at all — we’d have about 10 billion times less oxygen,” she says. “So plants and photosynthetic bacteria are creating oxygen in our atmosphere, and so, if aliens were to look at us from far away, and using optical wavelength telescopes — rather than radio telescopes — they would see all this excess oxygen and they would hopefully know that that it doesn’t belong here — that it should be attributed to life.”
Quiet and Active Stars
Another unique element of Seager’s equation is the addition of so-called quiet stars.
Stars vary in terms of their activity. Our sun, for example, is currently in a solar maximum phase, so it’s giving off more solar flares than usual. But some active stars can be super active, and in ways that are not good.
“Active stars are way more active than quiet stars, and there’s a kind of concern that certain active stars could be harmful to life,” Seager told io9.
Another problem is that active stars vary in brightness, which often makes it hard to find exoplanets. Flare stars pose another problem.
Very active stars also have high ultraviolet radiation flux — and that’s a problem for biosignature gases. UV radiation sets off a chain of chemical reactions that often ends up destroying a lot of gases. Thus, it’s hard for biosignature gases to accumulate on those planets.
Hence the focus on quiet stars.
N = 2?
According to Seager’s own calculations, the value of N equals two — which is not as pessimistic as it sounds.
Keep in mind that her equation is strictly trying to determine the probability of our ability to detect planets with biosignature gases using the spectroscopic method. This means that we should be able detect at least a pair of planets with biosphere gases in the relatively near future — so her estimate is actually pretty damned exciting.
Indeed, Seager’s equation will become increasingly relevant and useful after the launch of the James Webb Space Telescope in 2018. MIT’s Tess Mission (Translating ExtraSolar planet Survey mission) will look at 500,000 stars spread out across the sky looking for transiting planets that are rocky.
“Once we have a pool of those planets, we hope to follow them up by looking at their atmospheres with the James Webb Telescope,” says Seager. “It’s that kind of two-pronged approach that we’ve adopted. So the equation is real in the sense that it’s talking about what we can accomplish in the next decade.”
In regards to the search for alien life, Seager says her team is working on this for real.
“We’re the first generation that gets to help answer this question.”
Top image: Stephane Guisard/ESO.