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June 03, 2013

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Earth-like planets could number “billions” in our galaxy

March 29, 2012
Courtesy of ESO
and World Science staff


The first di­rect es­ti­mate of the num­ber of small plan­ets around red dwarf stars sug­gests that bil­lions of those worlds, po­ten­tially with liq­uid wa­ter, ex­ist in our gal­axy alone.

Red dwarf stars are faint, rel­a­tively cool stars that live long be­cause they go through their fu­el slow­ly, and ac­count for an es­ti­mated 80 per­cent of all the stars in our gal­axy.

This artist's im­pres­sion shows a sun­set seen from the super-Earth Gliese 667 Cc. The bright­est star in the sky is the red dwarf Gliese 667 C, which is part of a tri­ple star sys­tem. The oth­er two more dis­tant stars, Gliese 667 A and B ap­pear in the sky al­so to the right. As­tro­no­mers have es­ti­mat­ed that there are tens of bil­lions of such rocky worlds or­bit­ing faint red dwarf stars in the Milky Way alone. (Cred­it: ESO/L. Cal­çada )


Sci­en­tists used ob­serva­t­ions from a light-analyzing in­stru­ment known as the HARPS spec­tro­graph at the Eu­ro­pe­an South­ern Ob­ser­va­to­ry’s La Silla Ob­serv­a­to­ry in Chil­e.

Xa­vi­er Bon­fils of the Sci­ences Ob­serv­a­to­ry at the Uni­vers­ity of Gre­no­ble, France, who led the re­search team, said the find­ings “mean that about 40 per­cent of all red dwarf stars have a super-Earth or­bit­ing in the hab­it­a­ble zone where liq­uid wa­ter can ex­ist on the sur­face of the plan­et.” A super-Earth is a plan­et ap­prox­i­mately like Earth in its make­up, but some­what larg­er. Plan­ets smaller than Earth weren’t a fo­cus of the study be­cause they are ex­tremely hard to de­tect.

“Be­cause red dwarfs are so com­mon — there are about 160 bil­lion of them in the Milky Way — this leads us to the as­ton­ish­ing re­sult that there are tens of bil­lions of these [super-Earths] in our gal­axy alone,” he added. Tem­per­a­tures would be suit­a­ble for liq­uid wa­ter in ar­eas rel­a­tively close to those red dwarfs, cre­at­ing a real pos­si­bil­ity of life, he added. On the oth­er hand, mem­bers of his team not­ed, red dwarfs are prone to stel­lar erup­tions that could af­fect their sur­round­ing plan­ets and make life dif­fi­cult, quite lit­er­al­ly.

The team sur­veyed what they de­scribed as a care­fully cho­sen sam­ple of 102 red dwarf stars in the south­ern skies over a six-year pe­ri­od. Nine super-Earths—plan­ets weigh­ing the equiv­a­lent of one to ten Earth­s—were found, in­clud­ing two in the “hab­it­a­ble zones” of the stars Gliese 581 and Gliese 667 C re­spec­tive­ly. Hab­it­a­ble zones are areas where temp­er­a­tures al­low for liquid water.

By com­bin­ing all the da­ta, in­clud­ing ob­serva­t­ions of stars that did not have plan­ets, and look­ing at the frac­tion of ex­isting plan­ets that could be disco­vered, the team worked out how com­mon dif­fer­ent sorts of plan­ets are around red dwarfs. On the oth­er hand, gi­ant gas­e­ous plan­ets like Ju­pi­ter and Sat­urn were found to be rare around red dwarfs.

As there are many red dwarf stars close to the Sun, the new es­ti­mate means that there are probably about 100 super-Earth plan­ets in the hab­it­a­ble zones of stars less than 30 light-years from the Sun, ac­cord­ing to Bon­fils’s group. A light-year is the dis­tance light trav­els in a year.

The new re­search is to ap­pear in a pa­per in the jour­nal As­tron­o­my & As­t­ro­phys­ics.


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The first direct estimate of the number of small planets around red dwarf stars suggests that billions of those worlds, potentially with liquid water, exist in our galaxy alone. Red dwarf stars are faint, relatively cool stars that live long because they go through their fuel slowly, and account for an estimated 80% of all the stars in our galaxy. Scientists used observations from a light-analyzing instrument known as the HARPS spectrograph at the European Southern Observatory’s La Silla Observatory in Chile. Xavier Bonfils of the Sciences Observatory at the University of Grenoble, France, who led the research team, said the findings “mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet.” A super-Earth is a planet approximately like Earth in its makeup, but somewhat larger. Planets smaller than Earth weren’t a focus of the study because they are extremely hard to detect. “Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these [super-Earths] in our galaxy alone,” he added. Temperatures would be suitable for liquid water in areas relatively close to those red dwarfs, creating a real possibility of life, he added. On the other hand, members of his team noted, red dwarfs are prone to stellar eruptions that could affect their surrounding planets. These could make life difficult, quite literally. The team surveyed what they described as a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths—planets weighing the equivalent of one to ten Earths—were found, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively. By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team worked out how common different sorts of planets are around red dwarfs. On the other hand, giant gaseous planets like Jupiter and Saturn were found to be rare around red dwarfs. As there are many red dwarf stars close to the Sun, the new estimate means that there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years, according to Bonfils’s group. A light-year is the distance light travels in a year. The new research is to appear in a paper in the journal Astronomy & Astrophysics. our galaxy