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E.T. might be detectable through his city lights, study proposes

Nov. 3, 2011
Courtesy of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics
and World Science staff

A pair of as­t­ro­phys­i­cists is pro­pos­ing a new tech­nique for track­ing down al­ien civ­il­iz­a­tions: look for city lights on their plan­ets.

“Look­ing for al­ien cit­ies would be a long shot,” and would re­quire fu­ture genera­t­ions of tele­scopes, said Avi Loeb of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. On the oth­er hand, he added, it “would­n’t re­quire ex­tra re­sources” be­yond the in­stru­ments peo­ple are likely to build an­y­way in the years ahead. “And if we suc­ceed, it would change our per­cep­tion of our place in the uni­verse.”

If an al­ien civ­i­li­za­tion builds brightly-lit cit­ies like those shown in this artist's con­cep­tion, fu­ture gen­er­a­tions of tele­scopes might al­low us to de­tect them, two sci­en­tists pro­pose. (Cred­it: Da­vid A. Aguilar (CfA))


Al­though per­haps not all in­tel­li­gent al­iens build cit­ies, they could rea­sonably be ex­pected to de­vel­op ar­ti­fi­cial lights for use at night, added Loeb, who de­vel­oped the pro­pos­al with Ed­win Turn­er of Prince­ton Uni­vers­ity. Large num­bers of such lights could cre­ate a de­tect­a­ble sig­nal, ac­cord­ing to the sci­en­tists, who sub­mit­ted a pa­per de­tail­ing the pro­pos­al to the jour­nal As­tro­bi­ol­o­gy.

How easy would it be to spot such light? First, it would have to be dis­tin­guish­able from the glare from the par­ent star. This star­light, re­flected off a plan­et, would dis­play a char­ac­ter­is­tic blend of col­ors probably dis­tinct from those in ar­ti­fi­cial light, as is the case on Earth. 

Loeb and Turn­er sug­gest look­ing at the change in light from a plan­et as it cir­cles its star. As the plan­et or­bits, it goes through phases si­m­i­lar to those of the Moon. When the plan­et is in a dark phase, Loeb and Turn­er rea­son, more ar­ti­fi­cial light from the night side would be vis­i­ble from Earth than re­flected light from the day side. Cur­rent tele­scopes can­not de­tect such a small sig­nal, they added, but the tech­nique could be tested clos­er to home­—us­ing ob­jects at the edge of our so­lar sys­tem, even though they most likely lack any life.

Loeb and Turn­er cal­cu­late that to­day’s best tele­scopes ought to be able to see the light gen­er­at­ed by a Tokyo-sized me­trop­o­lis at the dis­tance of the Kuiper Belt—the re­gion oc­cu­pied by Plu­to, Er­is, and thou­sands of smaller icy bod­ies. So if there are any cit­ies out there, we ought to be able to see them now. Just by look­ing, as­tro­no­mers can hone the tech­nique and be ready to apply it when the first Earth-sized worlds are found around dis­tant stars in our gal­axy. 

“It’s very un­likely that there are al­ien cit­ies on the edge of our so­lar sys­tem, but the prin­ci­ple of sci­ence is to find a meth­od to check,” Turn­er said. “Be­fore Gal­i­le­o, it was con­ven­tion­al wis­dom that heav­i­er ob­jects fall faster than light ob­jects, but he tested the be­lief and found they ac­tu­ally fall at the same rate.” 

Anoth­er al­ien-de­tection ef­fort un­der con­sid­era­t­ion in­volves check­ing wheth­er Earth-bound tele­scopes could pick up al­ien ra­di­o or tel­e­vi­sion sig­nals. How­ev­er, an al­ien civ­il­iz­a­tion might use such tech­nol­o­gy less in­ten­sively over time, per­haps for the same rea­sons our own tech­nol­o­gy has shifted from ra­di­o and TV broad­casts to ca­ble and fi­ber op­tics. That might leave ar­ti­fi­cial lights as be the best way to spot E.T. from afar, Loeb and Turn­er say.


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A pair of astrophysicists is proposing a new technique for tracking down alien civilizations: look for city lights on their planets. “Looking for alien cities would be a long shot,” and would require future generations of telescopes, said Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. On the other hand, he added, it “wouldn’t require extra resources” beyond the instruments people are likely to build anyway in the years ahead. “And if we succeed, it would change our perception of our place in the universe.” Although perhaps not all intelligent aliens build cities, they could reasonably be expected to develop artificial lights for use at night, added Loeb, who developed the proposal with Edwin Turner of Princeton University. Large numbers of such lights could be enough to create a detectable signal, according to the scientists, who submitted a paper detailing the proposal to the journal Astrobiology. How easy would it be to spot such light? First, it would have to be distinguishable from the glare from the parent star. This starlight, reflected off a planet, would display a characteristic blend of colors probably distinct from those in artificial light; as is the case on Earth. Loeb and Turner suggest looking at the change in light from a planet as it circles its star. As the planet orbits, it goes through phases similar to those of the Moon. When the planet is in a dark phase, Loeb and Turner reason, more artificial light from the night side would be visible from Earth than reflected light from the day side. Current telescopes cannot detect such a small signal, they added, but the technique could be tested closer to home—using objects at the edge of our solar system, even though they most likely lack cities. Loeb and Turner calculate that today’s best telescopes ought to be able to see the light generated by a Tokyo-sized metropolis at the distance of the Kuiper Belt—the region occupied by Pluto, Eris, and thousands of smaller icy bodies. So if there are any cities out there, we ought to be able to see them now. Just by looking, astronomers can hone the technique and be ready to apply it when the first Earth-sized worlds are found around distant stars in our galaxy. “It’s very unlikely that there are alien cities on the edge of our solar system, but the principle of science is to find a method to check,” Turner said. “Before Galileo, it was conventional wisdom that heavier objects fall faster than light objects, but he tested the belief and found they actually fall at the same rate.” Another alien-detection effort under consideration involves checking whether Earth-bound telescopes could pick up alien radio or television signals. However, an alien civilization might use such technology less intensively over time, perhaps for the same reasons our own technology has shifted from radio and TV broadcasts to cable and fiber optics. That might leave artificial lights as be the best way to spot E.T. from afar, Loeb and Turner say.