"Long before it's in the papers"
October 21, 2015

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Most Earth-like worlds have yet to form, study says

Oct. 21, 2015
Courtesy of NASA/Goddard Space Flight Center
and World Science staff

Earth came early to the par­ty in the evolv­ing uni­verse, ac­cord­ing to a new the­o­ret­i­cal stu­dy. It con­cludes that only eight per­cent of po­ten­tially hab­it­a­ble plan­ets have even formed yet.

But many fu­ture civ­il­iz­a­tions may arise so late that by then, ev­i­dence for the Big Bang—the explosion-like event that gave birth to the uni­verse—will have dis­ap­peared.

An artist's impression of countless Earth-like planets that have yet to be born over the next trillion years. (Credit: NASA, ESA, and G. Bacon (STScI))


The con­clu­sion is based on an as­sess­ment of da­ta col­lect­ed by NASA’s Hub­ble Space Tel­e­scope and the pro­lif­ic plan­et-hunting Kep­ler space ob­serv­a­to­ry.

“Com­pared to all the plan­ets that will ev­er form in the uni­verse, the Earth is ac­tu­ally quite ear­ly,” said study au­thor Pe­ter Beh­roozi of the Space Tel­e­scope Sci­ence In­sti­tute in Bal­ti­more, Md. 

Look­ing far away and far back in time, Hub­ble has giv­en as­tro­no­mers a “family al­bum” of gal­axy ob­serva­t­ions that chron­i­cle the uni­verse’s star forma­t­ion his­to­ry as ga­lax­ies grew. 

The da­ta show, the re­search­ers said, that the uni­verse was mak­ing stars quickly 10 bil­lion years ago, but the frac­tion of the uni­verse’s hy­dro­gen and he­li­um gas that was in­volved to make those stars was very low. To­day, star birth is hap­pen­ing much more slow­ly, but there is so much left­o­ver gas avail­a­ble that the uni­verse will keep cook­ing up stars and plan­ets for a very long time.

“There is enough re­main­ing ma­te­ri­al to pro­duce even more plan­ets in the fu­ture, in the Milky Way and be­yond,” said co-invest­iga­tor Molly Peeples of the in­sti­tute.

Kep­ler’s plan­et sur­vey in­di­cates that Earth-sized plan­ets in a star’s hab­it­a­ble zone, the per­fect dis­tance that could al­low wa­ter to pool on the sur­face, exist all over our gal­axy. Sci­en­tists pre­dict there should be a bil­lion Earth-sized worlds in the Milky Way gal­axy now, a good por­tion of them pre­sumed to be rocky, like ours. That es­ti­mate sky­rock­ets when you in­clude the oth­er 100 bil­lion ga­lax­ies in the ob­serva­ble uni­verse.

This leaves plen­ty of op­por­tun­ity, they said, for un­told more Earth-sized plan­ets in the hab­it­a­ble zone to arise in the fu­ture. The last star is­n’t ex­pected to burn out un­til 100 tril­lion years from now. That’s plen­ty of time for an­y­thing to hap­pen on the plan­et land­scape. 

The re­search­ers say that fu­ture Earths are more likely to ap­pear in­side gi­ant gal­axy clus­ters and al­so in dwarf ga­lax­ies, which have yet to use up all their gas for build­ing stars and ac­com­pa­nying plan­etary sys­tems. By con­trast, our Milky Way gal­axy has used up much more of the gas avail­a­ble for fu­ture star forma­t­ion. 

A big ad­van­tage to our civ­il­iz­a­tion aris­ing early in the ev­o­lu­tion of the uni­verse is our be­ing able to use pow­er­ful tele­scopes like Hub­ble to trace our line­age from the Big Bang through the early ev­o­lu­tion of ga­lax­ies. The ob­serva­t­ional ev­i­dence for the Big Bang and cos­mic ev­o­lu­tion, en­cod­ed in light, will be all but erased away a tril­lion years from now due to the runa­way ex­pan­sion of space. Any far-fu­ture civ­il­iz­a­tions that might arise will be largely clue­less as to how or if the uni­verse be­gan and evolved.

The re­sults ap­pear in the Oct. 20 is­sue of the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.


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Earth came early to the party in the evolving universe, according to a new theoretical study. It concludes that only eight percent of potentially habitable planets have even formed yet. But some future civilizations may arise so late that by then all the evidence for the Big Bang—the explosion-like event that gave birth to the universe—will have disappeared. The conclusion is based on an assessment of data collected by NASA’s Hubble Space Telescope and the prolific planet-hunting Kepler space observatory. “Our main motivation was understanding the Earth’s place in the context of the rest of the universe,” said study author Peter Behroozi of the Space Telescope Science Institute in Baltimore, Maryland, “Compared to all the planets that will ever form in the universe, the Earth is actually quite early.” Looking far away and far back in time, Hubble has given astronomers a “family album” of galaxy observations that chronicle the universe’s star formation history as galaxies grew. The data show, the researchers said, that the universe was making stars quickly 10 billion years ago, but the fraction of the universe’s hydrogen and helium gas that was involved to make those stars was very low. Today, star birth is happening much more slowly, but there is so much leftover gas available that the universe will keep cooking up stars and planets for a very long time to come. “There is enough remaining material to produce even more planets in the future, in the Milky Way and beyond,” said co-investigator Molly Peeples of the institute. Kepler’s planet survey indicates that Earth-sized planets in a star’s habitable zone, the perfect distance that could allow water to pool on the surface, are everywhere in our galaxy. Based on the survey, scientists predict there should be a billion Earth-sized worlds in the Milky Way galaxy now, a good portion of them presumed to be rocky, like ours. That estimate skyrockets when you include the other 100 billion galaxies in the observable universe. This leaves plenty of opportunity, they said, for untold more Earth-sized planets in the habitable zone to arise in the future. The last star isn’t expected to burn out until 100 trillion years from now. That’s plenty of time for literally anything to happen on the planet landscape. The researchers say that future Earths are more likely to appear inside giant galaxy clusters and also in dwarf galaxies, which have yet to use up all their gas for building stars and accompanying planetary systems. By contrast, our Milky Way galaxy has used up much more of the gas available for future star formation. A big advantage to our civilization arising early in the evolution of the universe is our being able to use powerful telescopes like Hubble to trace our lineage from the big bang through the early evolution of galaxies. The observational evidence for the big bang and cosmic evolution, encoded in light, will be all but erased away a trillion years from now due to the runaway expansion of space. Any far-future civilizations that might arise will be largely clueless as to how or if the universe began and evolved. The results appear in the Oct. 20 issue of the journal Monthly Notices of the Royal Astronomical Society.