AN ICONIC tool in the search for extraterrestrial life is getting a 21st-century reboot 鈥� just as our best planet-hunting telescope seems to have died. Though the loss of NASA鈥檚 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 includes a number of terms that at the time seemed unknowable 鈥� 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.
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With that kind of data in hand, 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 鈥溾��. 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鈥檛 be fixed, the mission is over.
鈥淲hen we talked about the post-Kepler era, we thought that would be three to four years from now,鈥� co-organiser of the Harvard-Smithsonian Center for Astrophysics said last week. 鈥淲e now know the post-Kepler era probably started two days ago.鈥�
But Kepler has collected data for four years, slightly longer than the mission鈥檚 original goal, and so far only the first 18 months鈥� 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鈥檚 face as seen from Earth. This method tells us a planet鈥檚 size and its rough distance from its host star.
Size gives a clue to a planet鈥檚 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.
鈥淜epler is the single most revolutionary project that has ever been undertaken in exoplanets,鈥� says Charbonneau. 鈥淚t broke open the piggybank and rocky planets poured out.鈥� A planet鈥檚 distance from its star is also crucial, because that tells us whether the temperature is right for liquid water 鈥� and so perhaps life 鈥� to exist.
鈥淭he Kepler mission was revolutionary. It broke open the piggybank and rocky planets poured out鈥�
But with Kepler鈥檚 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鈥檚 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 鈥�Kepler鈥檚 emergency surgery鈥�), but chances are the telescope is dead.
Kepler鈥檚 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鈥檚 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鈥檚 light will filter through a planet鈥檚 atmosphere on the way to us, picking up telltale clues to its composition. Seager鈥檚 goal is to find the fraction of habitable Earth-sized worlds in our galactic neighbourhood with detectable atmospheric biosignatures 鈥� in other words, inhabited worlds. She has already put the number of inhabited planets that the might see at less than 10.
鈥淛ust like with the Drake equation, some of the terms are always speculative,鈥� Seager says.
So is it possible the work will lead us to discover aliens next door? 鈥淥f course I think it鈥檚 possible. Why else would I be working so hard?鈥� she says.
鈥淥f course I think it鈥檚 possible to find signs of life. Why else would I be working so hard?鈥�
If Seager or someone else detected biosignatures, we would spend more time looking in those places for hints of intelligence, says of the SETI Institute. 鈥淵ou鈥檇 know that鈥檚 an inhabited world, not just a habitable world. And then you can ask the question, did they develop any technology we might detect?鈥�
Kepler鈥檚 emergency surgery
The Kepler space telescope might be down, but it鈥檚 not out 鈥� yet. Its engineering team has a few ideas that could revive the mission.
To see the tiny dips in starlight that signal a planet crossing in front of its star, Kepler must keep an unwavering view of its targets. It maintained its orientation in space using three reaction wheels. A spare fourth wheel stopped turning last July. One of the remaining three failed last week.
The first line of attack is rather low-tech: run the recently stopped wheel backwards. 鈥淟ike with any stuck wheel you might be familiar with on the ground, we could try jiggling it back and forth in both directions, forcing it through the resistance that鈥檚 holding it up,鈥� says Kepler team member Charlie Sobek.
They could also try restarting the spare that failed in July. That wheel was showing unsafe friction levels, scraping metal against metal, so the team turned it off as a precaution. After nine months idle, the wheel may now be re-lubricated and functional once again.
Another option is to put Kepler鈥檚 sails to the wind. While at work, it had to constantly fight the solar wind, the stream of charged particles from the sun that push against its solar panels. It may be possible to use that pressure in place of a lost wheel, allowing Kepler to soldier on.
This article appeared in print under the headline 鈥淎lien hunting for the 21st century鈥�
