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Possibly “habitable” planet called smallest yet found

Dec. 19, 2012
Courtesy of the University of Hertfordshire
and World Science staff

As­tro­no­mers have found that Tau Ce­ti, one of the clos­est and most Sun-like stars, may host five plan­ets—with one in the star’s “hab­it­a­ble zone.”

Vis­i­ble with the na­ked eye in the eve­ning sky, Tau Ce­ti is the clos­est sin­gle star that has the same spec­tral clas­sifica­t­ion as our Sun, mean­ing its light is sim­i­lar, sci­en­tists say.

The im­age shows the lo­ca­tion of the star Tau Ce­ti with­in the con­stel­la­tion Ce­tus or The Whale, which is vis­i­ble from the South­ern Hem­i­sphere.


Its five plan­ets are es­ti­mat­ed to weigh be­tween two and six “Earths”—mak­ing it the light­est plan­e­tary sys­tem known, ac­cord­ing to the as­tro­no­mers who an­nounced the new find­ings.

One of those worlds is be­lieved to lie in the star’s hab­it­a­ble zone, where tem­per­a­tures could al­low for liq­uid wa­ter. It’s es­ti­mat­ed to weigh around five times Earth, mak­ing it the small­est plan­et in the hab­it­a­ble zone of any Sun-like star, the sci­en­tists added. The star is 12 light-years away, mean­ing it lies at a dis­tance where light takes 12 year to trav­el be­tween us and it.

The as­tro­no­mers com­bined more than 6,000 ob­serva­t­ions from three in­stru­ments. Us­ing new tech­niques, they said, they de­vel­oped a way to de­tect sig­nals half the size pre­vi­ously thought pos­si­ble. That would greatly im­prove the sen­si­ti­vity of searches for small plan­ets, which tend to re­sist de­tection ow­ing to the ex­tremely faint im­print they leave on tel­e­scope da­ta.

“We pi­o­neered new da­ta mod­el­ing tech­niques by adding ar­ti­fi­cial sig­nals to the da­ta and test­ing our re­cov­ery of the sig­nals with a va­ri­e­ty of dif­fer­ent ap­proach­es,” ex­plained Mikko Tuomi of the Uni­vers­ity of Hert­ford­shire, one of the as­tro­no­mers. This “in­creased our abil­ity to find low-mass plan­ets.” 

“We chose Tau Ce­ti for this noise mod­el­ing study be­cause we had thought it con­tained no sig­nals. And as it is so bright and si­m­i­lar to our Sun it is an ide­al bench­mark sys­tem to test out our meth­ods for the de­tection of small plan­ets,” added Hugh Jones of the Uni­vers­ity of Hert­ford­shire, which led the pro­ject.

“Tau Ce­ti is one of our near­est cos­mic neigh­bors and so bright that we may be able to study the at­mo­spheres of these plan­ets in the not too dis­tant fu­ture. Plan­e­tary sys­tems found around near­by stars close to our Sun in­di­cate that these sys­tems are com­mon in our Milky Way gal­axy,” said James Jen­kins of the Uni­ver­si­dad de Chil­e, a vis­it­ing fel­low at the Uni­vers­ity of Hert­ford­shire and a mem­ber of the re­search team.

Over 800 plan­ets have been iden­ti­fied or­bit­ing oth­er worlds. 

“This dis­cov­ery is in keep­ing with our emerg­ing view that vir­tu­ally eve­ry star has plan­ets, and that the gal­axy must have many such po­ten­tially hab­it­a­ble Earth-sized plan­ets,” said co-re­searcher Steve Vogt from Uni­vers­ity of Cal­i­for­nia San­ta Cruz

“They are eve­rywhere, even right next door! We are now be­gin­ning to un­der­stand that Na­ture seems to over­whelm­ingly pre­fer sys­tems that have a mul­ti­ple plan­ets with or­bits of less than one hun­dred days. This is quite un­like our own so­lar sys­tem where there is noth­ing with an or­bit in­side that of Mer­cu­ry. So our so­lar sys­tem is, in some sense, a bit of a freak and not the most typ­i­cal kind of sys­tem that Na­ture cooks up.”

“As we stare the night sky, it is worth con­tem­plat­ing that there may well be more plan­ets out there than there are stars,” said anoth­er mem­ber of the re­search group, Chris Tin­ney of the Uni­vers­ity of New South Wales in Aus­tral­ia. “Some frac­tion of which,” he went on, “may well be hab­it­a­ble.”


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Astronomers have found that Tau Ceti, one of the closest and most Sun-like stars, may host five planets—with one in the star’s “habitable zone.” Visible with the naked eye in the evening sky, Tau Ceti is the closest single star that has the same spectral classification as our Sun, meaning its light is similar, scientists say. Its five planets are estimated to weigh between two and six “Earths”—making it the lightest planetary system known, according to the astronomers who announced the new findings, led by the University of Hertfordshire in the U.K. One of those worlds is believed to lie in the star’s habitable zone, where temperatures could allow for liquid water. It’s estimated to weigh around five times Earth, making it the smallest planet in the habitable zone of any Sun-like star, the scientists added. The star is 12 light-years away, meaning it lies at a distance where light takes 12 year to travel between us and it. The astronomers combined more than 6,000 observations from three different instruments. Using new techniques, they developed a way to detect signals half the size previously thought possible. That would greatly improve the sensitivity of searches for small planets, which tend to resist detection owing to the extremely faint imprint they leave on telescope data. “We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches,” explained Mikko Tuomi of the University of Hertfordshire, one of the astronomers. This “increased our ability to find low-mass planets.” “We chose Tau Ceti for this noise modeling study because we had thought it contained no signals. And as it is so bright and similar to our Sun it is an ideal benchmark system to test out our methods for the detection of small planets,” added Hugh Jones from the University of Hertfordshire. “Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not too distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy,” said James Jenkins of the Universidad de Chile, a visiting fellow at the University of Hertfordshire and a member of the research team. Over 800 planets have been identified orbiting other worlds. “This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets,” said co-researcher Steve Vogt from University of California Santa Cruz “They are everywhere, even right next door! We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have a multiple planets with orbits of less than one hundred days. This is quite unlike our own solar system where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.” “As we stare the night sky, it is worth contemplating that there may well be more planets out there than there are stars … some fraction of which may well be habitable,” said another member of the research group, Chris Tinney of the University of New South Wales in Australi