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First “super-Earths” found orbiting sun-like stars

Dec. 15, 2009
Courtesy University of California, Santa Cruz
and World Science staff

As­tro­no­mers have re­ported find­ing as many as six plan­ets, not many times heav­i­er than Earth, or­bit­ing two near­by Sun-like stars.

The ob­jects, which in­clude two that are about 5 and 7.5 times as heavy as Earth, are rais­ing sci­en­tists’ hopes that it will be just a few years that plan­ets very much like ours turn up.

Image from a sci­en­tists' an­i­ma­tion of the 5-Earth-mass plan­et 61 Vir B, or­bit­ing the star 61 Vir­gi­nis. This plan­et moves in a tight, 4-day or­bit around its star. Half of the plan­et sur­face is much hot­ter than the oth­er half be­cause one side al­ways faces the star. (Cour­tesy U. Hert­ford­shire)


The re­search­ers, led by Ste­ven Vogt of the Un­ivers­ity of Cal­i­for­nia, San­ta Cruz, and Paul But­ler of the Car­ne­gie In­sti­tu­tion of Wash­ing­ton, said the two “super-Earths” are the first ones found around Sun-like stars.

“These de­tec­tions in­di­cate that low-mass plan­ets are quite com­mon around near­by stars. The dis­cov­ery of po­ten­tially hab­it­a­ble near­by worlds may be just a few years away,” said Vogt. As­tro­no­mers claim they’re over­com­ing past dif­fi­cul­ties in find­ing smaller plan­ets, which are more like ours in size and are con­sid­ered like­li­er to be hab­it­a­ble than large plan­ets.

The team found the new plan­et sys­tems by com­bin­ing da­ta gath­ered at the W. M. Keck Ob­serv­a­to­ry in Ha­waii and the An­glo-Aus­tral­ia Tel­e­scope in New South Wales, Aus­tral­ia. Two pa­pers de­scrib­ing the new plan­ets have been ac­cept­ed for pub­lica­t­ion in the As­t­ro­phys­i­cal Jour­nal.

Three of the new plan­ets or­bit the bright star 61 Vir­gi­nis, vis­i­ble with the na­ked eye un­der dark skies in the Spring con­stella­t­ion Vir­go. Space sci­en­tists have long been fas­ci­nat­ed with this star, which is a re­la­tive­ly close 28 light years away (a light year is the dis­tance light trav­els in a year). Among hun­dreds of our near­est stel­lar neigh­bors, 61 Vir stands out as be­ing the most nearly si­m­i­lar to the Sun in terms of age, mass, and oth­er es­sen­tial prop­er­ties. 

Vogt and col­leagues have found that 61 Vir hosts at least three plan­ets, weigh­ing in the range of about 5 to 25 Earths. All would be ex­treme­ly hot, though, as they are well with­in orbits equi­va­lent to that of Ve­nus. 

Re­cent­ly, a sep­a­rate team of as­tro­no­mers used NASA’s Spitzer Space Tel­e­scope to dis­cov­er that 61 Vir al­so con­tains a thick ring of dust at a dis­tance roughly twice as far from 61 Vir as Plu­to is from our Sun. The dust is ap­par­ently cre­at­ed by col­li­sions of comet-like bod­ies in the cold out­er reaches of the sys­tem.

“Spitzer’s de­tec­tion of cold dust or­bit­ing 61 Vir in­di­cates that there’s a real kin­ship be­tween the Sun and 61 Vir,” said Eu­ge­nio Ri­ve­ra of the Un­ivers­ity of Cal­i­for­nia, San­ta Cruz. Ri­ve­ra com­put­ed an ex­ten­sive set of sim­ula­t­ions to find that a hab­it­a­ble Earth-like world could easily ex­ist in the as-yet un­ex­plored re­gion be­tween the newly dis­cov­ered plan­ets and the out­er dust disk.

Ac­cord­ing to Vogt, the plan­etary sys­tem around 61 Vir is an ex­cel­lent can­di­date for study by the new Au­to­mat­ed Plan­et Find­er Tel­e­scope re­cently con­structed at Lick Ob­serv­a­to­ry on Mount Ham­il­ton near San Jose, Ca­lif. “Need­less to say, we’re very ex­cit­ed to con­tin­ue mon­i­tor­ing this sys­tem” us­ing that de­vice, said Vogt, who is the prin­ci­pal in­ves­ti­ga­tor for the tel­e­scope.

The sec­ond new sys­tem found by the team fea­tures a plan­et weigh­ing the equiv­a­lent of about 7.5 Earths and or­bit­ing the star HD 1461, anoth­er near per­fect twin of the Sun about 76 light-years away. The plan­et, des­ig­nat­ed HD 1461b, is about half­way be­tween Earth and Ura­nus in weight. The re­search­ers said they can­not tell yet if it’s a scaled-up ver­sion of Earth, com­posed largely of rock and iron, or wheth­er, like Ura­nus and Nep­tune, it is made mostly of wa­ter.

At least one and pos­sibly two ad­di­tion­al plan­ets al­so or­bit the star, the group said. Ly­ing in the con­stella­t­ion Ce­tus, HD 1461 can be seen with the na­ked eye in the early eve­ning un­der good dark-sky con­di­tions.

The Lick Car­ne­gie Exoplan­et Sur­vey Team led by Vogt and But­ler uses ve­locity mea­sure­ments from ground-based tel­e­scopes to de­tect the “wob­ble” in­duced in a star by the gravita­t­ional tug of an or­bit­ing plan­et. In the past year, im­prov­ing meth­ods have made it ev­i­dent that plan­ets or­bit­ing the Sun’s near­est neigh­bors are ex­tremely com­mon: cur­rent in­dica­t­ions are that fully half of near­by stars have a de­tectable plan­et with mass equal to or less than Nep­tune’s, But­ler said.

The Lick-Car­ne­gie Exoplan­et Sur­vey Team has de­vel­oped a pub­licly avail­a­ble tool, the Sys­tem­ic Con­sole, which en­ables mem­bers of the pub­lic to search for the sig­nals of ex­tra­so­lar plan­ets by ex­plor­ing real da­ta sets. This tool is avail­a­ble on­line at www.ok­lo.org.


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Astronomers have reported finding as many as six planets, not many times heavier than Earth, orbiting two nearby Sun-like stars. The objects, which include two that are about 5 and 7.5 times as heavy as Earth, are raising scientists’ hopes that it will be just a few years that planets very much like ours turn up. The researchers, led by Steven Vogt of the University of California, Santa Cruz, and Paul Butler of the Carnegie Institution of Washington, said the two “super-Earths” are the first ones found around Sun-like stars. “These detections indicate that low-mass planets are quite common around nearby stars. The discovery of potentially habitable nearby worlds may be just a few years away,” said Vogt. Astronomers claim they’re overcoming past difficulties in find smaller planets, which are more like ours in size and are considered likelier to be habitable than large planets. The team found the new planet systems by combining data gathered at the W. M. Keck Observatory in Hawaii and the Anglo-Australian Telescope in New South Wales, Australia. Two papers describing the new planets have been accepted for publication in the Astrophysical Journal. Three of the new planets orbit the bright star 61 Virginis, visible with the naked eye under dark skies in the Spring constellation Virgo. Astronomers and astrobiologists have long been fascinated with this star, which is only 28 light years away (a light year is the distance light travels in a year). Among hundreds of our nearest stellar neighbors, 61 Vir stands out as being the most nearly similar to the Sun in terms of age, mass, and other essential properties. Vogt and colleagues have found that 61 Vir hosts at least three planets, weighing in the range of about 5 to 25 Earths. Recently, a separate team of astronomers used NASA’s Spitzer Space Telescope to discover that 61 Vir also contains a thick ring of dust at a distance roughly twice as far from 61 Vir as Pluto is from our Sun. The dust is apparently created by collisions of comet-like bodies in the cold outer reaches of the system. “Spitzer’s detection of cold dust orbiting 61 Vir indicates that there’s a real kinship between the Sun and 61 Vir,” said Eugenio Rivera of the University of California, Santa Cruz. Rivera computed an extensive set of numerical simulations to find that a habitable Earth-like world could easily exist in the as-yet unexplored region between the newly discovered planets and the outer dust disk. According to Vogt, the planetary system around 61 Vir is an excellent candidate for study by the new Automated Planet Finder Telescope recently constructed at Lick Observatory on Mount Hamilton near San Jose, Calif. “Needless to say, we’re very excited to continue monitoring this system” using that device, said Vogt, who is the principal investigator for the telescope. The second new system found by the team features a planet weighing the equivalent of about 7.5 Earths and orbiting the star HD 1461, another near perfect twin of the Sun about 76 light-years away. The planet, designated HD 1461b, is about halfway between Earth and Uranus in weight. The researchers said they cannot tell yet if it’s a scaled-up version of Earth, composed largely of rock and iron, or whether, like Uranus and Neptune, it is made mostly of water. At least one and possibly two additional planets also orbit the star, the group said. Lying in the constellation Cetus, HD 1461 can be seen with the naked eye in the early evening under good dark-sky conditions. The Lick Carnegie Exoplanet Survey Team led by Vogt and Butler uses velocity measurements from ground-based telescopes to detect the “wobble” induced in a star by the gravitational tug of an orbiting planet. In the past year, improving methods have made it evident that planets orbiting the Sun’s nearest neighbors are extremely common: current indications are that fully half of nearby stars have a detectable planet with mass equal to or less than Neptune’s, Butler said. The Lick-Carnegie Exoplanet Survey Team has developed a publicly available tool, the Systemic Console, which enables members of the public to search for the signals of extrasolar planets by exploring real data sets. This tool is available online at www.oklo.org.