Thursday, April 28, 2016

The Probability of Detecting Signs of Life from Exoplanets

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.

The Drake Equation (probability of intelligent life)
  • 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,” said Sara Seager, a professor of planetary science and physics at the Massachusetts Institute of Technology. “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.

“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.

The Seager Equation (probability of detectable signs of life)
  • 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

“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.”

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.

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.

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.”

More information:
» Imgur: "This is how small you are..."

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