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March 01, 2016

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Scientists propose: let’s search part of sky where aliens could have seen us

March 1, 2016
Courtesy of the Max Planck In­sti­tute 
for So­lar Sys­tem Re­search
and World Science staff

Two as­tro­no­mers are pro­pos­ing a way to dras­tic­ally nar­row down the search for ex­tra­ter­res­tri­al life.

Con­cen­trate on a small ar­ea of the sky where ex­tra­ter­res­tri­als—if any—could have al­ready seen us with­out too much dif­fi­cul­ty, they say. Those crea­tures might be try­ing to con­tact us al­ready.

This a­rtist's concep­tion illus­trates what the tran­sit of the Earth in front of the Sun would look like to an ob­serv­er who is not too far off. (© NA­SA/Ax­el Quetz (MPIA))


The idea is that al­iens could have seen our plan­et the same way we de­tect many oth­ers: when they pass in front of their host star, dim­ming its light a bit in the pro­cess. Al­ien civ­il­iz­a­tions could have found Earth the same way—the “tran­sit” meth­od­—but only if they lie in the right di­rec­tions. Earth, Sun and their own world would all have to lie on the same plane.

Hu­man sci­en­tists have iden­ti­fied more than 2,000 plan­ets around dis­tant stars. More than half of these “exoplan­ets” have been found us­ing the “tran­sit” meth­od. None of these dis­cov­ered worlds host life, as far as we know.

But may­be that’s be­cause we’re not us­ing the best search strat­e­gy, say the re­search­ers, from the Max Planck In­sti­tute for So­lar Sys­tem Re­search in Göt­tin­gen, Germany, and from Mc­Mas­ter Un­ivers­ity in Can­a­da. 

They sug­gest that fu­ture searches fo­cus on a thin band of the sky in which dis­tant ob­servers can no­tice the yearly trans­it of the Earth in front of the Sun. There, there’s a bet­ter chance that al­iens are al­ready try­ing to con­tact us, they ar­gue. And the strat­e­gy re­duces the search re­gion to about one five-hundredth of the sky, al­low­ing for more de­tailed and faster search­ing.

A dia­gram of the "tran­sit zone," in which dis­tant ob­ser­vers could see the Earth pass in front of the Sun. (© Ax­el Quetz (MPIA) / Axel Mell­inger, Central Michigan U.) 


The strat­e­gy “con­fines the search ar­ea to a very small part of the sky. As a con­se­quence, it might take us less than a hu­man life span to find out wheth­er or not there are ex­tra­ter­res­tri­al as­tro­no­mers who have found the Earth. They may have de­tected Earth’s bi­o­gen­ic [life-revealing] at­mos­phere and started to con­tact who­ev­er is home,” said René Hell­er from the Max Planck In­sti­tute.

The re­search­ers iden­ti­fied the re­gion in the sky from which an al­ien would see Earth trans­iting the Sun not too far from the cen­ter of the Sun—no more than a quar­ter of a Sun-width away from the cen­ter, to be ex­act.

Another point to consider: not every star is equally well suit­ed as a home for life. That’s just as true in the small strip of sky that Hell­er is in­ter­est­ed in, as an­y­where else. The heav­i­er a star, the shorter it lives, pro­vid­ing less time for in­tel­li­gence to evolve. There­fore the re­search­ers al­so made a list of stars that are not only in the ad­van­ta­geous part of the sky but al­so of­fer good chances of host­ing in­tel­li­gent be­ings.

The re­search­ers com­piled a list of 82 near­by Sun-like stars that sat­is­fy their cri­te­ria. That can now serve as an im­me­di­ate search tar­get list, they ar­gue. But they al­so es­ti­mate that there could be roughly 100,000 oth­er, dim­mer stars in our ga­la­xy with si­m­i­lar po­ten­tial.

A part of these plan­ets might be dis­cov­ered with the PLA­TO mis­sion of the Eu­ro­pe­an Space Agen­cy, sched­uled for 2024, they add. Hell­er is al­so in­volved in this mis­sion. PLA­TO is to use the trans­it meth­od to find small plan­ets, some of them pos­sibly Earth-like, around bright stars.

“PLA­TO might even de­tect the trans­its of exoplan­ets, whose pos­sible in­hab­i­tants would be able to see the Earth trans­iting the Sun,” Hell­er added. “Such a cra­zy set­up would of­fer both them and us the pos­sibil­ity of stu­dy­ing each oth­ers’ plan­ets with the trans­it meth­od.”


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Two astronomers are proposing a way to drastically narrow down the search for extraterrestrial life. Concentrate on a small area of the sky where extraterrestrials—if any—could have already seen us without too much difficulty, they say. Those creatures might be trying to contact us already. The idea is that aliens could have seen our planet the same way we detect many others: when they pass in front of their host star, dimming its light a bit in the process. Alien civilizations could have found Earth the same way—the “transit” method—but only if they lie in the right directions. Earth, Sun and their own world would all have to lie on the same plane. Human scientists have identified more than 2,000 planets around distant stars. More than half of these “exoplanets” have been found using the “transit” method. None of these discovered worlds host life, as far as we know. But maybe that’s because we’re not using the best search strategy, say the researchers, from the Max Planck Institute for Solar System Research in Göttingen, Germany, and from McMaster University in Canada. They suggest that future searches focus on a thin band of the sky in which distant observers can notice the yearly transit of the Earth in front of the Sun. There, there’s a better chance that aliens are already trying to contact us, they argue. And the strategy reduces the search region to about one five-hundredth of the sky, allowing for more detailed and faster searching. The strategy “confines the search area to a very small part of the sky. As a consequence, it might take us less than a human life span to find out whether or not there are extraterrestrial astronomers who have found the Earth. They may have detected Earth’s biogenic [life-revealing] atmosphere and started to contact whoever is home,” said René Heller from the Max Planck Institute. The researchers identified the region in the sky from which an alien would see Earth transiting the Sun not too far from the center of the Sun—no more than a quarter of a Sun-width away from the center, to be exact. Also to be taken into account: not every star is equally well suited as a home for life. That’s just as true in the small strip of sky that Heller is interested in, as anywhere else. The heavier a star, the shorter it lives, providing less time for intelligence to evolve. Therefore the researchers also made a list of stars that are not only in the advantageous part of the sky but also offer good chances of hosting intelligent beings. The researchers compiled a list of 82 nearby Sun-like stars that satisfy their criteria. That can now serve as an immediate search target list, they argue. But they also estimate that there could be roughly 100,000 other, dimmer stars out there with similar potential. A part of these planets might be discovered with the PLATO mission of the European Space Agency, scheduled for 2024, they add. Heller is also involved in this mission. PLATO is to use the transit method to find small planets, some of them possibly Earth-like, around bright stars. “PLATO might even detect the transits of exoplanets, whose possible inhabitants would be able to see the Earth transiting the Sun,” Heller added. “Such a crazy setup would offer both them and us the possibility of studying each others’ planets with the transit method.” have seen us