In 1960 radio astronomer Frank Drake began the first systematic search for intelligent signals emanating from space. He was not successful, but his pioneering efforts paved the way for numerous other programs, including SETI (Search for Extra-Terrestrial Intelligence). The Drake Equation is named for him, and put simply, gives an estimate of the number of active, extraterrestrial civilizations with methods of communication in our own galaxy. Drake postulated the equation as a way to get the scientific community engaged in the search for life beyond our home planet.

The Drake equation is:

where:

N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone); and

R* = the average number of star formation per year in our galaxy

f_p = the fraction of those stars that have planets

n_e = the average number of planets that can potentially support life per star that has planets

f_l = the fraction of planets that could support life that actually develop life at some point

f_i = the fraction of planets with life that actually go on to develop intelligent life (civilizations)

f_c = 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

Now, based on recent discoveries of hundreds of extra-solar planets, or exoplanets (those beyond our solar system), by the Kepler space telescope and other Earth-bound observatories, researchers are fine-tuning the original Drake Equation for the 21st century.

From the New Scientist:

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.

Read the entire article here.

Image courtesy of the BBC. Drake Equation courtesy of Wikipedia.