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he search for intelligent life onother worlds began nearly 50years ago. This quest hasn't suc-ceeded yet, but the "Drake Equa-

tion" can help astronomers narrowdown how many planets in the galaxymay harbor communicating life.

Frank Drake (then an astronomer atCornell University) began the searchin 1960 with an 8s-foot radio tele-scope at the National Radio Astron-omy Observatory in Green Bank, WestVirginia. He looked for artificial radioSignals emanating from two nearbystars, Tau (~) Ceti and Epsilon (E) Eri-darii. This observing run, dubbed"Project Ozrna,' lasted just 2 monthsand turned up empty-handed. B~tii:.,launched the search for extraterresir'i~Cintelligence (SET!). In his brief, pi~;·.;·:·nee ring effort, Drake showed that one·could use the technology availabie '.,;,:,(then to look - or "listen" - across tHedistances between stars for signs of .alien civilizations,

To put SET! on a firmer scientific.footing, the National Academy of Sci-ences asked Drake 10 convene a smallmeeting to assess the odds of detect-ing lntelligent life beyond Earth.Drake served as the entire scientific

organizing committee of the 1961meeting at Green Bank. He invitedeveryone in the world he knew whowas interested in the subject - "all12 of them," he recalls. "And all 12 ofthem showed up:'

To serve as an agenda, Drakedevised an equation for estimatinghow many communicating civilize-tions might lurk in the galaxy. Thatnumber, N according to Drake's for-mulation, is simply the product ofseven other numbers. So if we couldnail down those seven terms.wed havean answer to the age-old question', "is,'anybody out there?" .. ..

"Everyone has long forgo.fte~Wqu,(·that Green Bank meetingrs~Y~:.B.I';i~~iwho IS the only attendee still ali ."But for some reason they , ..ugotten about this equation

The "Drake Equation,"]called, has turned out to belong-lived, as well as us~tulJill Tarter, Drake's colleaguInstitute. So what is the e'lu;;;:,for' And are we any closer to sit today than we were when Drake. ,.::drafted his meeting agenda h~'lfrten;~:;tury ago? Here's the Drak~ EqithiiBri,;";;N=R'xf. Xn xj,xf.xf.xL.·· .. ··J·",·

f' t fir :i'. 1:,\,·

N= R *fpnefifit L

udoing anything 'smart: even while combining to create some-thing more complex:'

During the next 20 years. notes McKay, the challenge ofplanetary science in our solar system is to see if the number ofinhabited planets is greater than one, just as the past 20 yearshas seen the number of habitable planets move from one tofour. NASA's proposed Terrestrial Planet Finder - as well as theEuropean Space Agency's Darwin mission - could examineearthlike planets around other stars and take the spectra of theiratmospheres to see if there's oxygen, signaling the presence ofphotosynthesis or other hints oflife.

fi the fraction of life-bearinq planets where

to intelligent life emergesDrake assumed in 1961 thatf is dose to one,

. meaning tha~ some sort of intelligence likely willevolve on: planets where life appears. Recent findings supportthat claim. Lori Marino (Emory University). Daniel McShea(Duke), and Mark Uhen (Smithsonian Institution) looked atfossilized craniums of dolphins and toothed whales from a 50-million-year period to see if there is a noticeable increase inbrain size and, by inference, intelligence.

Rather than seeing an inexorable drive toward increasedintelligence, they found a more general "fanning out" process:The intelligence of some animal groups increased while othersdeclined. Although the average intelligence may not haveIncreased, the researchers did observe steady gains at the upperend of the spectrum. They see this pattern as more or less nec-essary for any biology that favors diversity. assuming there is nolimit or "upper bound" on intelligence. .

Even single-celled organisms like amoebas exhibit signs ofintelligence. says Marino, because they can respond to the envi-ronment and make adjustments. Every unicellular organism weknow of has a membrane. and membranes are precursors toneurons or nerve cells, she says. Once we have multicellular life,the establishment of a nervous system is virtually inevitable.

"All it takes is an environment that puts pressure on anorganism, and an organism that can adapt to that pressure,and the development of intelligence becomes almost certain,"argues Marino, whose research places f close to one, just asDrake had Intuited. .

fi the fraction of those planets on which

C technology capable of producingdetectable signals develops

..-;: Drake's original estimate forf was 0.1, meaning thatcommunicative civilizations will emerge on 10 percent of theplanets with intelligent life, The only real evidence we have ison Earth, but Drake finds Ihat compelling. "Technology devel-oped independently in so many different places here:' he says."OnICc you have enough intelligence in a creature whose anat-'omy allows the use of tools, you should get technology."

"It seems obvious that if a species has the brainpower forspeech, along with the sort of appendages that can manipulate apair of pliers, it will eventually blunder into science, technology,and radio," arglles Sli'Tl Institutc astronomer Seth Shostak.

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Star·formlng reglonl, like those found in the Carina Nebula, dot our gal-axy. Such regions contribute to the creation of, on average; about 10 stars ayear. Here ultraviolet radiation from hot newborn stars erodes nearby gas.

~ = R*fp nefifi/cLN is the number of communicative civilizations in the

Milky Way,R JIt is the average rate at which suitable stars - those that

could potentially harbor life ~ are born each year in• our galaxy,

!p is the fraction of those stars that have planets.n, is the number of planets per star that are, in principle,

suitable for life,fs"is the fraction of habitable planets on which life actually

originates./, is the fraction oflife-bearing planets where intelligent life

emerges./, is the fraction of those planets on which technology

capable of producing detectable signals develops,L is the average lifetime of a'communicating civilization.

Every factor in the equation is equally irr:tporta~1t, Drakesays. Yet the terms become progressively more uncertain asone moves from the left to the right, which is why some peoplehave called the Drake Equation "a shorthand for our igno-ranee," Drake, however, sees it in a more positive light. callingit "a way of organizing the search that actually gives us someinsights into what we need to know,"

Steve Nadls Is an Astronomycontriburing editor:

He's not alone in that assessment. "I'm often at meetingsabout life in the universe that use the seven factors.of the DrakeEquation to organize the whole conference:' recalls astronomerDan werthimer of the University of California at Berkeley, "It's.a way of compartmentalizing the problem," Take a blg, almostunapproachable, problem and break it into seven smaller ques-tions that we might make some headway on,

And scientists are making headway, Werthimer affirms."We're slowly whittling away at the Drake Equation, It used tobe we had no idea what any of the factors were except the firstone." Now researchers are making advances on multiple fronts- a testament to achievements in astronomy, planetary sci-ence, origin of life research, and investigations into the evolu-tion of intelligence.

Because the whole point of the Drake Equation is to reduceone huge, multifaceted question into seven distinct ones, per-haps the best way to evaluate progress toward "solving>! theequation is to look at each of these factors separately before put-ting them together,

R* the average rate at which suitablestars are born each year In our galaxyIn 1961, Drake assumed that, on average, 10

stars form in the galaxy each year. which still seems like a rea-son able guess, If you start with some 200 billion stars in the,Milky Way and divide that number bythe galaxy's lifetime(approximately 10 billion years), you get about 20 Stars per year,

But the really big stars burn out fast - In just millions ofyears - which would probably not allow time for intelligentspecies to evolve. If we throw out the fast-burning stars. we stillhave about 19 starsa year: And ofthose 19, only about 4 are likethe Sun, The other IS are smaller stars, known as M dwarfs,whose suitability for intelligent life is open to debate,

, Previous theories held that M dwarfs would 'be inhospitableto life, For a small, dim M star to warm a planet, the planet 'would be so close it would become "tldallylocked" - alwaysfacing the same direction, just as the Moon is to' Earth, Theatmosphere on the cold side would freeze and collapse, therebycrushing any hopes for a viable biosphere,

But more recent models suggest "tidal locking is not a show-stopper," says Berkeley astronomer Lueianne Walkowicz."These models show that it's possible to have an-atmospherethat circulates heat around:' thereby avoiding the predicted col-lapse, M dwarfs are so numerous (constituting 70 percent of the'stars In the galaxy) "that even if some are not suitable, there aremany others that potentially might be:' adds Walkowicz. Sodependin,g.on what we conclude about M dwarfs, Drakeexplains, Wcould be 4 to 19, with 10 somewhere In the middle,

fi the fraction of suitable stars

P that have planets '\', At Green Bank, Drake originally picked. value of

0,5, meaning that out of all the stars that form, halfwill have planets, But he now believes that number could be

, higher, Indeed, Berkeley astronomer Geoff Marcy estimatesthat about 90 percent of all stars have planets, WhenDrake

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first came up with his famous equation. no one eou1d say for acidity and temperatures well above the boiling paint. as well assure that any stars Other than the Sun had planets, Since that survive in solidice.time, astronomers have discovered more than 400 extrasolar Researchers now believe that ", for our solar system isatplanets, with Marcy and his associates responsible for about least four, according to Chris MCKay of NASA's Ames Research170 of those worlds, . Center, In addition to Earth, planetary scientists have found

Astronomers know there are many more undiscovered evidence of liquid water on Mars, Jupiter's moon Europa. andplanets - possibly hundreds of billions in this' galaxy alone, Saturn's moon Enceladus. "The nice thing about n,:' saysThey've missed mal\Y because techniques available until McKay, "is that as we learn more, it can only get bigger:' We'verecently could not reveal slow-moving planets or those much now seen that conditions for life might-exist on planets or satel-smaller than Neptune, But new technology should give them' a lites far, from the$un and outsidethe normal habitable zonemuch better idea of the fraction of stars with planets, Most because they rely on other energy sources _ geothermal, tidal,promising at the moment is. the Kepler mission .•which or radioactive c-, to keep some water in liquid form.launched in March 2009 to find "terrestrial!' or rocky, planetslike Earth. Marcy calls Kepler "the most fantastic mission of Fthe upcoming decade."

Kepler's l-rneter telescope is taking pictures of the constella- , 1tion Cygnus. capturing light from 100,000 stars, When a planetcrosses in front of its star, the detected light will dim, If Kepler 'detects a star's dimming by just 20 parts in a million, saysMarcy, "earth like planets will stick out like a sore thumb,"

Armed with data from Kepler and other ventures,researchers will soon know more about the preva-lence of earthlike planets in the universe. "That'sthe primary motivation of my work:' saysMarcy, "and it's also one of the key terms of theDrake Equation,"

26 AstronomY'Aprll2010 ,

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ne the number of planetsper star that are, inprinciple, suitable for lifeDrake guessed in 1961 that

among stars with planetary systems, twoplanets on average can sustain life. Based onwhat we knew at the time. of course, n t forour solar system was one. Earth was the onlyplanet we knew that met the conditionsdeemed necessary for life, such as liquidwater on the surface.

Over the years, there's been a gradualrealization thot the traditional notion of a"habitable zone" - just the rIght distancefrom a star for lIquid surface water to exist- may be too restrictive, Life is moreresilient than we once believed, On Earth.for example, scientists recently cameacross hardy microbes called "extreme-philes" that can tolerate water of extreme

NASA's Kepl.r mission looks for planets largeand sm?!11around stellar habitable zones, Thespacecraft has the sensitivity to detect an Earth-sized world orbiting another star. NASA

the fraction of habitable planets onwhich life actually originatesIn 196.1, Drake assumed that!, is close to one,meaning t~at life should develop 'on planets that have

the eight conditions. Evidence acquired in theintervening years only solidifies that view. We'velearned that water is almost ubiquitous throughout,

the galaxy, And we've seen that the other basic .ingredients of organic chemistry - includingcarbon and hydrogen - are widespread withinthe galactic clouds out of which stars form,

In 2009, University of Manchester· chemistJohn Sutherland and colleagues showed howthose ingredients may have given rise to life onEarth - a process that would apply whereversimilar conditions are found, Sutherland says,"Since the rules ere the· same everywhere,chemistry should give you the same product."

Researchers now believe that life as weknow it is based on RNA - a close cousin toDNA but simpler to make, Sutherland and his'collaborators showed how common ingredi-ents - heated and then held at roughly

room temperature and i~ the presence ofultraviolet radiation (or sunlight) - canspontaneously assemble themselves toform nucleotides, which are the buildingblocks of RNA,

"If you find the same conditionsaround another star, it will happen," saysSutherland, "Spontaneous assembly isnot magic. Chemistry is simply a 'description of how molecules behave,and the molecules in this case are not

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