"Long before it's in the papers"
March 31, 2016

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Distant planet said to be half-melted

March 31, 2016
Courtesy of the University of Cambridge
and World Science staff

As­tro­no­mers have ob­tained what they call the most de­tailed “fin­ger­print” of a rocky plan­et out­side our so­lar sys­tem to date, and con­clud­ed that half of that world is al­most com­pletely mol­ten.

Sci­en­tists al­so say the plan­et is even hot­ter than radia­t­ion from its star alone would ex­plain, so there must be an un­known heat source.

An il­lus­tra­tion of 55 Can­cri e. (Cred­it: NA­SA/JPL-Caltech )


The re­search­ers, led by the Un­ivers­ity of Cam­bridge in the U.K., say scorch­ing temp­era­tures on the plan­et’s hot­ter side may have made the at­mos­phere evap­o­rate. That could have led the two sides to have radic­ally dif­fer­ent tem­per­a­tures by pre­venting some heat from spread­ing around.

Tem­per­a­tures on the hot side can reach 2,500 de­grees C (4,500 F), while tem­per­a­tures on the cool side are around 1,100 de­grees C (2,000 de­grees F), ac­cord­ing to the find­ings, re­ported in the jour­nal Na­ture.

Us­ing da­ta from NASA’s Spitzer Space Tel­e­scope, the re­search­ers ex­am­ined a plan­et known as 55 Can­cri e, which or­bits a sun-like star lo­cat­ed 40 light years away in the Can­cer con­stella­t­ion. They have mapped how con­di­tions on the plan­et change through­out a com­plete or­bit, the first time this has been done for such a small plan­et.

55 Can­cri e is clas­si­fied as a “su­per Earth,” a rocky world about twice the size and eight times the weight of Earth, and or­bits its par­ent star so closely that a year lasts just 18 hours. The plan­et is al­so tid­ally locked, mean­ing that it al­ways shows the same face to its par­ent star, si­m­i­lar to the Moon, so there is a per­ma­nent “day” side and a “night” side. 

It’s among the near­est known su­per Earths whose make­up can be stud­ied.

Un­cov­er­ing the char­ac­ter­is­tics of su­per Earths is hard, since they’re so small com­pared to the par­ent star and their con­trast rel­a­tive to the star is ti­ny com­pared to gas gi­ant plan­ets, so-called “hot Jup­iters.”

“We haven’t yet found any oth­er plan­et that is this small and or­bits so close to its par­ent star, and is rel­a­tively close to us, so 55 Can­cri e of­fers lots of pos­si­bil­i­ties,” said Brice-Olivier De­mory of the uni­vers­ity’s Cav­en­dish Lab­o­r­a­to­ry, the pa­per’s lead au­thor. “We still don’t know ex­actly what this plan­et is made of—it’s still a rid­dle. These re­sults are like adding anoth­er brick to the wall, but the ex­act na­ture of this plan­et is still not com­pletely un­der­stood.”

55 Can­cri e has been ex­ten­sively stud­ied since its 2011 dis­cov­ery. Based on read­ings tak­en at dif­fer­ent times, it was thought to be a wa­ter world, or even made of dia­mond, but re­search­ers now think it’s al­most com­pletely lava-covered.

“We have en­tered a new era of at­mos­pher­ic re­mote sens­ing of rocky exo­plan­ets,” said study co-au­thor Nikku Mad­husud­han, from Cam­bridge’s In­sti­tute of As­tron­o­my. An exo­plan­et is a plan­et out­side our so­lar sys­tem. “It is in­cred­i­ble that we are now able to meas­ure the large scale tempe­rature dis­tri­bu­tion on the sur­face of a rocky exo­plan­et.”

One thing that’s un­clear is that there seems to be an un­known heat source that makes the plan­et hot­ter than ex­pected solely from its star, the astron­omers said. They added that they may have to wait un­til the next gen­era­t­ion of space tele­scopes are launched to find out.


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Astronomers have obtained what they call the most detailed “fingerprint” of a rocky planet outside our solar system to date, and concluded that half of that world is almost completely molten. Scientists also say the planet is even hotter than radiation from its star alone would explain, so there must be an unknown heat source. The researchers, led by the University of Cambridge in the U.K., say conditions on the planet’s hot side are so extreme that it may have caused the atmosphere to evaporate. That could have led the two sides to have radically different temperatures by preventing some heat from spreading around. Temperatures on the hot side can reach 2,500 degrees C (4,500 F), while temperatures on the cool side are around 1,100 degrees C (2,000 degrees F), according to the findings, reported in the journal Nature. Using data from NASA’s Spitzer Space Telescope, the researchers examined a planet known as 55 Cancri e, which orbits a sun-like star located 40 light years away in the Cancer constellation. They have mapped how conditions on the planet change throughout a complete orbit, the first time this has been done for such a small planet. 55 Cancri e is classified as a “super Earth,” a rocky world about twice the size and eight times the weight of Earth, and orbits its parent star so closely that a year lasts just 18 hours. The planet is also tidally locked, meaning that it always shows the same face to its parent star, similar to the Moon, so there is a permanent “day” side and a “night” side. It’s among the nearest known super Earths whose makeup can be studied. Uncovering the characteristics of super Earths is hard, since they’re so small compared to the parent star and their contrast relative to the star is tiny compared to gas giant planets, so-called “hot Jupiters.” “We haven’t yet found any other planet that is this small and orbits so close to its parent star, and is relatively close to us, so 55 Cancri e offers lots of possibilities,” said Brice-Olivier Demory of the university’s Cavendish Laboratory, the paper’s lead author. “We still don’t know exactly what this planet is made of—it’s still a riddle. These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood.” 55 Cancri e has been extensively studied since its 2011 discovery. Based on readings taken at different times, it was thought to be a water world, or even made of diamond, but researchers now think it’s almost completely lava-covered. “We have entered a new era of atmospheric remote sensing of rocky exoplanets,” said study co-author Nikku Madhusudhan, from Cambridge’s Institute of Astronomy. An exoplanet is a planet outside our solar system. “It is incredible that we are now able to measure the large scale temperature distribution on the surface of a rocky exoplanet.” One thing is unclear is that there seems to be an unknown heat source that makes the planet hotter than expected solely from its star. The researchers said they may have to wait until the next generation of space telescopes are launched to find out.