"Long before it's in the papers"
October 17, 2012

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An Earth-sized planet in our stellar backyard?

Oct. 17, 2012
Courtesy of the European 
Southern Observatory
and World Science staff

As­tro­no­mers have iden­ti­fied a plan­et about the same weight as Earth or­bit­ing a star in the Al­pha Cen­tau­ri sys­tem—the star sys­tem near­est to Earth.

It’s the light­est plan­et ev­er dis­cov­ered around a Sun-like star out­side our so­lar sys­tem, mak­ing it a huge chal­lenge to de­tect, the re­search­ers said. Though too hot to be hab­it­a­ble, they added, tech­niques used to find it could be ap­plied to re­veal plan­ets that are more like ours.

This artist’s im­pres­sion shows a new­found plan­et or­bit­ing the star Al­pha Cen­tau­ri B, a mem­ber of the tri­ple star sys­tem that is the clos­est to Earth. Al­pha Cen­tau­ri B is the most bril­liant ob­ject in the sky and the oth­er daz­zling ob­ject is Al­pha Cen­tau­ri A. Our own Sun is vis­i­ble to the up­per right. (Cred­it: ES­O/L. Calçada/N. Risinger (sky­survey.org))

“This re­sult rep­re­sents a ma­jor step to­wards the de­tection of a twin Earth in the im­me­di­ate vicin­ity of the Sun. We live in ex­cit­ing times!” said the lead au­thor of a re­port on the find­ings, Xa­vi­er Du­musque of the Ge­ne­va Ob­serv­a­to­ry and Cen­tro de As­tro­fi­si­ca da Uni­ver­si­dade do Porto in Por­tu­gal.

Al­pha Cen­tau­ri ap­pears to the eye as one of the bright­est stars in the south­ern skies. But is really a tri­ple star: a sys­tem con­sist­ing of two stars si­m­i­lar to the Sun or­bit­ing close to each oth­er, called Al­pha Cen­tau­ri A and B, and a more dis­tant and faint red com­po­nent known as Prox­i­ma Cen­tau­ri. 

The system lies only 4.3 light-years away, a light-year be­ing the dis­tance light trav­els in a year.

Since the 19th cen­tu­ry as­tro­no­mers have spec­u­lat­ed about worlds or­bit­ing these bod­ies, the clos­est pos­si­ble abodes for life be­yond the So­lar Sys­tem, but pre­vious searches of in­creas­ing pre­ci­sion had re­vealed noth­ing.

The new finding was made us­ing an in­stru­ment called HARPS on the 3.6-metre tel­e­scope at ES­O’s La Silla Ob­serv­a­to­ry in Chil­e. The in­stru­ment is a spec­trom­e­ter, which an­a­lyzes the col­ors of starlight—col­ors that change slightly de­pend­ing on a star’s mo­tion to­ward or away from us. The sci­en­tists said they de­tected the plan­et by pick­ing up ti­ny wob­bles in the mo­tion of the star Al­pha Cen­tau­ri B cre­at­ed by the plan­et’s gravita­t­ional pull.

The re­port is to to ap­pear on­line in the re­search jour­nal Na­ture on Oct. 17.

“Our ob­serva­t­ions ex­tend­ed over more than four years us­ing the HARPS in­stru­ment and have re­vealed a ti­ny, but real, sig­nal from a plan­et or­bit­ing Al­pha Cen­tau­ri B ev­ery 3.2 days,” said Du­musque. “It’s an ex­tra­or­di­nary disco­very and it has pushed our tech­nique to the lim­it!”

The wob­bles in Al­pha Cen­tau­ri B are ti­ny—the star moves back and forth by no more than 1.8 km (1.1 miles) per hour, about the speed of a ba­by crawl­ing. This is the high­est pre­ci­sion ev­er achieved us­ing this meth­od, the re­search­ers said.

Al­pha Cen­tau­ri B is like our Sun but slightly smaller and dim­mer. The plan­et, with a weight (or mass, more prope­rly speak­ing) meas­ured at a lit­tle more than that of the Earth, is or­bit­ing an es­ti­mat­ed six mil­lion km (3.7 mil­lion miles) away from the star. That’s much clos­er than Mer­cu­ry is to our sun. The oth­er bright com­po­nent of the dou­ble star, Al­pha Cen­tau­ri A, is hun­dreds of times fur­ther away, but would still shine very brightly in the plan­et’s skies.

The same re­search team found the first plan­et around a Sun-like star oth­er than our own sun in 1995, and since then there have been more than 800 con­firmed disco­veries, Du­musque and col­leagues said. But most are much big­ger than the Earth, and many are Jupiter-sized. The chal­lenge as­tro­no­mers now face, they added, is to de­tect a roughly Earth-sized world or­bit­ing in the “hab­it­a­ble zone” around anoth­er star, the ar­ea suitably warm enough for liq­uid wa­ter.

“This is the first plan­et with a mass si­m­i­lar to Earth ev­er found around a star like the Sun. Its or­bit is very close to its star and it must be much too hot for life as we know it,” said Stéphane Udry of Ge­ne­va Ob­serv­a­to­ry, a co-au­thor of the pape­r and mem­ber of the team. “It may well be just one plan­et in a sys­tem of sev­eral,” Udry added, as oth­er da­ta “show clearly that the ma­jor­ity of low-mass plan­ets are found in such sys­tems.”

* * *

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Astronomers have identified a planet about the same weight as Earth orbiting a star in the Alpha Centauri system—the star system nearest to Earth. It’s the lightest planet ever discovered around a Sun-like star outside our solar system, making it a huge challenge to detect, the researchers said. Though too hot to be habitable, they added, techniques used to find it could be applied to reveal planets that are more like ours. “This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the Sun. We live in exciting times!” said the lead author of a report on the findings, Xavier Dumusque of the Geneva Observatory and Centro de Astrofisica da Universidade do Porto in Portugal. Alpha Centauri appears as one of the brightest stars in the southern skies and is the nearest stellar system to our Solar System. It’s only 4.3 light-years away, a light-year being the distance light travels in a year. Alpha Centauri is really a triple star: a system consisting of two stars similar to the Sun orbiting close to each other, called Alpha Centauri A and B, and a more distant and faint red component known as Proxima Centauri. Since the 19th century astronomers have speculated about worlds orbiting these bodies, the closest possible abodes for life beyond the Solar System, but searches of increasing precision had revealed nothing. The planet was detected using an instrument called HARPS on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. The instrument is a spectrometer, which analyzes the colors of starlight—colors that change slightly depending on a star’s motion toward or away from us. The scientists said they detected the planet by picking up tiny wobbles in the motion of the star Alpha Centauri B created by the planet’s gravitational pull. The report is to to appear online in the research journal Nature on Oct. 17. “Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days,” said Dumusque. “It’s an extraordinary discovery and it has pushed our technique to the limit!” The wobbles in Alpha Centauri B are tiny—the star moves back and forth by no more than 1.8 km (1.1 miles) per hour, about the speed of a baby crawling. This is the highest precision ever achieved using this method, the researchers said. Alpha Centauri B is like our Sun but slightly smaller and dimmer. The planet, with a weight (or mass, more properly speaking) measured at a little more than that of the Earth, is orbiting an estimated six million km (3.7 million miles) away from the star. That’s much closer than Mercury is to our sun. The other bright component of the double star, Alpha Centauri A, is hundreds of times further away, but would still shine very brightly in the planet’s skies. The same research team found the first planet around a Sun-like star other than our own sun in 1995, and since then there have been more than 800 confirmed discoveries, Dumusque and colleagues said. But most are much bigger than the Earth, and many are Jupiter-sized. The challenge astronomers now face, they added, is to detect a roughly Earth-sized world orbiting in the “habitable zone” around another star, the area suitably warm enough for liquid water. “This is the first planet with a mass similar to Earth ever found around a star like the Sun. Its orbit is very close to its star and it must be much too hot for life as we know it,” said Stéphane Udry of Geneva Observatory, a co-author of the paper and member of the team. “It may well be just one planet in a system of several,” Udry added, as other data “show clearly that the majority of low-mass planets are found in such systems.”