"Long before it's in the papers"
January 06, 2016


Could star clusters nurture interstellar civilizations?

Jan. 6, 2016
Courtesy of the Har­vard-Smith­son­ian 
Cen­ter for As­t­ro­phys­ics
and World Science staff

An ide­al place to look for space­far­ing civ­il­iz­a­tions may be with­in bunch­es of stars called glob­u­lar clus­ters, a study claims—be­cause they’re tightly packed with old stars.

The idea is that the stars are with­in com­mu­ni­cat­ing dis­tance of each oth­er, and old enough to host plan­ets that have evolved ad­vanced life forms.

The Globular Cluster 47 Tucanae in a Hubble Space Telescope image.

“A glob­u­lar clus­ter might be the first place in which in­tel­li­gent life is iden­ti­fied in our gal­axy,” said Ros­anne Di Ste­fano of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics, lead au­thor of a re­port on the find­ings. 

She pre­sented the work Jan. 6 at a press con­fer­ence at a meet­ing of the Amer­i­can As­tronomical So­ci­e­ty in Kis­sim­mee, Fla.

Our Milky Way gal­axy hosts about 150 glob­u­lar clus­ters, most of them or­bit­ing in the ga­lac­tic out­skirts, Di Ste­fano said. They formed about 10 bil­lion years ago on av­er­age. 

One prob­lem: be­ing very old, these stars are poor in the heavier el­e­ments, like iron and sil­i­con, needed to make plan­ets. This is be­cause it takes ear­li­er-gen­er­a­t­ion stars to form these el­e­ments. But in the case of the clus­ters, there are no earlier gen­er­a­tions of stars—the ex­ist­ing stars are the first gen­er­a­tion.

Some sci­en­tists have ar­gued that this makes glob­u­lar clus­ter stars less likely to host plan­ets. In fact, only one plan­et has been found in a glob­u­lar clus­ter to date.

But Di Ste­fano and her col­league Alak Ray of the Tata In­sti­tute of Fun­da­men­tal Re­search in Mum­bai ar­gue say it’s prem­a­ture to count out glob­u­lar clus­ter plan­ets. Plan­ets have been iden­ti­fied around stars only one-tenth as heavy-el­e­ment-rich as our Sun. And smaller plan­ets, like Earth, seem no show no pref­er­ence for turn­ing up near heavy-el­e­ment-rich stars.

Anoth­er con­cern is that a glob­u­lar clus­ter’s crowd­ed envi­ronment would threat­en any plan­ets that do form. A neigh­bor­ing star could wan­der too close and gravita­t­ionally dis­rupt a plan­etary sys­tem, fling­ing worlds in­to icy in­ter­stel­lar space.

How­ev­er, the au­thors count­er that a star’s hab­it­a­ble zone—the dis­tance at which a plan­et would be warm enough for liq­uid wa­ter—varies de­pend­ing on the star. Smaller stars tend to have smaller hab­it­a­ble zones, they ar­gue, so plan­ets float­ing there would be close enough to the par­ent star to be rel­a­tively safe from stel­lar in­ter­ac­tions.

“Once plan­ets form, they can sur­vive for long pe­ri­ods of time, even long­er than the cur­rent age of the uni­verse,” said Di Ste­fano.

So if hab­it­a­ble plan­ets can form in glob­u­lar clus­ters and sur­vive for bil­lions of years, what are the con­se­quenc­es for life should it evolve? Life would have am­ple time to be­come in­creas­ingly com­plex, and even po­ten­tially de­vel­op in­tel­li­gence.

Such a civ­il­iz­a­tion would en­joy a very dif­fer­ent envi­ronment than our own. The near­est star to our so­lar sys­tem is four light-years, or 24 tril­lion miles, away. In con­trast, the near­est star with­in a glob­u­lar clus­ter could be a mere tril­lion miles away. This would make in­ter­stel­lar commu­nica­t­ion and ex­plora­t­ion eas­i­er, with sig­nals tak­ing per­haps only a cou­ple of months to trav­el.

“Send­ing a broad­cast be­tween the stars would­n’t take any long­er than a let­ter from the U.S. to Eu­rope in the 18th cen­tu­ry,” said Di Ste­fano.

“Interstel­lar trav­el would take less time too. The Voy­ag­er probes are 100 bil­lion miles from Earth, or one-tenth as far as it would take to reach the clos­est star if we lived in a glob­u­lar clus­ter. That means send­ing an in­ter­stel­lar probe is some­thing a civ­il­iz­a­tion at our tech­no­log­i­cal lev­el could do in a glob­u­lar clus­ter,” she adds.

The clos­est glob­u­lar clus­ter to Earth is still sev­er­al thou­sand light-years away, mak­ing it hard to find plan­ets, par­tic­u­larly in a clus­ter’s crowd­ed co­re. But it could be pos­si­ble to de­tect tran­sit­ing plan­ets on the out­skirts of glob­u­lar clus­ters, Di Ste­fano and Ray said. As­tro­no­mers might even spot free-float­ing plan­ets through gravita­t­ional lens­ing, in which the plan­et’s gra­vity mag­ni­fies light from a back­ground star.

A more in­tri­guing idea might be to tar­get glob­u­lar clus­ters by look­ing for ra­di­o or la­ser broad­casts. The con­cept has a long his­to­ry: In 1974 as­tron­o­mer Frank Drake used the Are­ci­bo ra­di­o tel­e­scope to broad­cast the first de­lib­er­ate mes­sage from Earth to out­er space. It was di­rect­ed at the glob­u­lar clus­ter Mess­i­er 13.

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An ideal place to look for spacefaring civilizations may be within bunches of stars called globular clusters, a study claims—because they’re tightly packed with old stars. The idea is that the stars are within communicating distance of each other, and old enough to host planets that have evolved advanced life forms. “A globular cluster might be the first place in which intelligent life is identified in our galaxy,” said Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics, lead author of a report on the findings. She presented the work Jan. 6 at a press conference at a meeting of the American Astronomical Society. Our Milky Way galaxy hosts about 150 globular clusters, most of them orbiting in the galactic outskirts, Di Stefano said. They formed about 10 billion years ago on average. As a result, their stars contain fewer of the heavy elements needed to construct planets, since those elements (like iron and silicon) must be created in earlier generations of stars. Some scientists have argued that this makes globular cluster stars less likely to host planets. In fact, only one planet has been found in a globular cluster to date. But Di Stefano and her colleague Alak Ray of the Tata Institute of Fundamental Research in Mumbai argue that this view is too pessimistic. Planets have been identified around stars only one-tenth as metal-rich as our Sun. And smaller planets, like Earth, seem no show no preference for turning up near metal-rich stars. “It’s premature to say there are no planets in globular clusters,” said Ray. Another concern is that a globular cluster’s crowded environment would threaten any planets that do form. A neighboring star could wander too close and gravitationally disrupt a planetary system, flinging worlds into icy interstellar space. However, the authors counter that a star’s habitable zone—the distance at which a planet would be warm enough for liquid water—varies depending on the star. Smaller stars tend to have smaller habitable zones, they argue, so planets floating there would be close enough to the parent star to be relatively safe from stellar interactions. “Once planets form, they can survive for long periods of time, even longer than the current age of the universe,” said Di Stefano. So if habitable planets can form in globular clusters and survive for billions of years, what are the consequences for life should it evolve? Life would have ample time to become increasingly complex, and even potentially develop intelligence. Such a civilization would enjoy a very different environment than our own. The nearest star to our solar system is four light-years, or 24 trillion miles, away. In contrast, the nearest star within a globular cluster could be a mere trillion miles away. This would make interstellar communication and exploration easier, with signals taking perhaps only a couple of months to travel. “Sending a broadcast between the stars wouldn’t take any longer than a letter from the U.S. to Europe in the 18th century,” said Di Stefano. “Interstellar travel would take less time too. The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster,” she adds. The closest globular cluster to Earth is still several thousand light-years away, making it difficult to find planets, particularly in a cluster’s crowded core. But it could be possible to detect transiting planets on the outskirts of globular clusters, Di Stefano and Ray said. Astronomers might even spot free-floating planets through gravitational lensing, in which the planet’s gravity magnifies light from a background star. A more intriguing idea might be to target globular clusters with SETI search methods, looking for radio or laser broadcasts. The concept has a long history: In 1974 astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from Earth to outer space. It was directed at the globular cluster Messier 13.