Exoplanets
Posted: Fri May 31, 2013 7:05 pm
Drake equation for alien life gets an upgrade
22 May 2013 by Lisa Grossman
The planet-spotting Kepler telescope seems doomed, but its discoveries along with a new version of the famous Drake equation will sharpen the hunt for ET
AN ICONIC tool in the search for extraterrestrial life is getting a 21st-century reboot – just as our best planet-hunting telescope seems to have died. Though the loss of NASA's Kepler telescope is a blow, the reboot could mean we find signs of life on extrasolar planets within a decade.
The new tool takes the form of an equation. In 1961 astronomer Frank Drake scribbled his now-famous equation for calculating the number of detectable civilisations in the Milky Way. The Drake equation includes a number of terms that at the time seemed unknowable – including the very existence of planets beyond our solar system.
But the past two decades have seen exoplanets pop up like weeds, particularly in the last few years thanks in large part to the Kepler space telescope. Launched in 2009, Kepler has found more than 130 worlds and detected 3000 or so more possibles. The bounty has given astronomers the first proper census of planets in one region of our galaxy, allowing us to make estimates of the total population of life-friendly worlds across the Milky Way.
With that kind of data in hand, Sara Seager at the Massachusetts Institute of Technology reckons the Drake equation is ripe for a revamp. Her version narrows a few of the original terms to account for our new best bets of finding life, based in part on what Kepler has revealed. If the original Drake equation was a hatchet, the new Seager equation is a scalpel.
Seager presented her work this week at a conference in Cambridge, Massachusetts, entitled "Exoplanets in the Post-Kepler Era". The timing could not be more prescient. Last week Kepler suffered a surprise hardware failure that knocked out its ability to see planetary signals clearly. If it can't be fixed, the mission is over.
"When we talked about the post-Kepler era, we thought that would be three to four years from now," co-organiser David Charbonneau of the Harvard-Smithsonian Center for Astrophysics said last week. "We now know the post-Kepler era probably started two days ago."
But Kepler has collected data for four years, slightly longer than the mission's original goal, and so far only the first 18 months' worth have been analysed. That means it may have already gathered enough information to give alien-hunters a fighting chance.
The original Drake equation includes seven terms, which multiplied together give the number of intelligent alien civilisations we could hope to detect (see diagram). Kepler was supposed to pin down two terms: the fraction of stars that have planets, and the number of those planets that are habitable.
To do that, Kepler had been staring unflinchingly at some 150,000 stars near the constellation Cygnus, looking for periodic changes in brightness caused by a planet crossing, or transiting, a star's face as seen from Earth. This method tells us a planet's size and its rough distance from its host star.
Size gives a clue to a planet's composition, which tells us whether it is rocky like Earth or gassy like Neptune. Before Kepler, only a few exoplanets had been identified as small enough to be rocky, because other search methods were better suited to spotting larger, gas giant worlds.
"Kepler is the single most revolutionary project that has ever been undertaken in exoplanets," says Charbonneau. "It broke open the piggybank and rocky planets poured out." A planet's distance from its star is also crucial, because that tells us whether the temperature is right for liquid water – and so perhaps life – to exist.
But with Kepler's recent woes, hopes of finding enough potentially habitable planets, or Earth twins, to satisfy the Drake equation have dimmed. The mission was supposed to run for three-and-a-half years, which should have been enough to pinpoint Earth-sized planets with years of a similar length. After the telescope came online, the mission team realised that other sun-like stars are more active than ours, and they bounce around too much in the telescope's field of view. To find enough Earths, they would need seven or eight years of data.
It was a relief when the mission was extended until 2016, and that much more of a blow when the telescope abruptly failed last week. NASA has a few last-ditch ideas for reviving the mission (see "Kepler's emergency surgery"), but chances are the telescope is dead.
Kepler's principal investigator William Borucki is optimistic that a few Earth twins around sun-like stars lurk in the existing data. Sun-like stars are not the only ones that can host habitable planets, though. The Seager equation focuses in on red dwarf stars, which are smaller and cooler than the sun. That makes it easier to detect rocky planets around them at the right distance for life, because the planets have tighter, briefer orbits. What's more, red dwarfs are the most common stars in our galaxy: projections based on Kepler data suggest that the nearest habitable Earth-sized world could orbit a red dwarf as close as 6.5 light years away.
Even better, it will be easier to probe these planets for gases associated with life, because tighter orbits mean that more of the star's light will filter through a planet's atmosphere on the way to us, picking up telltale clues to its composition. Seager's goal is to find the fraction of habitable Earth-sized worlds in our galactic neighbourhood with detectable atmospheric biosignatures – in other words, inhabited worlds. She has already put the number of inhabited planets that the James Webb space telescope might see at less than 10.
"Just like with the Drake equation, some of the terms are always speculative," Seager says.
So is it possible the work will lead us to discover aliens next door? "Of course I think it's possible. Why else would I be working so hard?" she says.
If Seager or someone else detected biosignatures, we would spend more time looking in those places for hints of intelligence, says Jill Tarter of the SETI Institute. "You'd know that's an inhabited world, not just a habitable world. And then you can ask the question, did they develop any technology we might detect?"
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