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Mystery world baffles astronomers

Oct. 30, 2013
Courtesy of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics
and World Science staff

Kep­ler-78b is a plan­et that should­n’t ex­ist: this scorch­ing la­va world, sci­en­tists say, cir­cles its star eve­ry 8½ hours at a dis­tance of less than a mil­lion miles—one of the tight­est known or­bits. 

Ac­cord­ing to cur­rent the­o­ries of plan­et forma­t­ion, it could­n’t have formed so close to its star, nor could it have moved there.

Kepler-78b in an artist's im­pres­sion, with its par­ent star in the back­ground. (Cour­tesy CfA)


“This plan­et is a com­plete mys­tery,” said as­tron­o­mer Da­vid Latham of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. “We don’t know how it formed or how it got to where it is to­day. What we do know is that it’s not go­ing to last for­ev­er.”

“Kep­ler-78b is go­ing to end up in the star very soon, as­tro­nom­ic­ally speak­ing,” ex­plained as­tron­o­mer Dim­i­tar Sas­selov of the cen­ter.

Not only is it a mys­tery world, as­tron­o­mers are call­ing it the first known Earth-sized plan­et with an Earth-like dens­ity, or com­pact­ness. It’s about 15 pe­r­cent wid­er than the Earth, ac­cord­ing to cal­cula­t­ions, and weighs al­most twice as much, sug­gest­ing an Earth-like dens­ity con­sist­ent with an iron-and-rock make­up.

When this plan­etary sys­tem was form­ing, the young star was larg­er than it is now, so the plan­et’s cur­rent or­bit would have been in­side the swol­len star, ac­cord­ing to as­tron­o­mers.

“It could­n’t have formed in place be­cause you can’t form a plan­et in­side a star. It could­n’t have formed fur­ther out and mi­grat­ed in­ward, be­cause it would have mi­grat­ed all the way in­to the star. This plan­et is an enig­ma,” said Sas­selov.

Ac­cord­ing to Latham, Kep­ler-78b is a mem­ber of a new class of plan­ets re­cently iden­ti­fied in da­ta from NASA’s Kep­ler space­craft. These new­found worlds all or­bit their stars with less than 12 hours per or­bit. They’re al­so small, about the size of Earth. Kep­ler-78b is the first plan­et in the new class to have its mass (or “weight”) meas­ured.

“Kep­ler-78b is the post­er child for this new class of plan­ets,” notes Latham. The team stud­ied it us­ing spec­tro­graphs, which break up light in­to its col­ors.

It’s a doomed world be­cause gravita­t­ional tides will draw it even clos­er to its star, they added. Even­tu­ally it will move so close that the star’s gra­vity will rip the world apart. The­o­rists pre­dict that Kep­ler-78b will van­ish with­in three bil­lion years. In­ter­est­ing­, they add, our so­lar sys­tem could have held a plan­et like Kep­ler-78b. If it had, the plan­et would have been de­stroyed long ago leav­ing no signs for as­tron­o­mers to­day.

Kep­ler-78b or­bits a Sun-like G-type star lo­cat­ed an es­ti­mat­ed 400 light-years from Earth in the con­stella­t­ion Cyg­nus. A light-year is the dis­tance light trav­els in a year. The new find­ings are pub­lished in the Oct. 31 is­sue of the jour­nal Na­ture.


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Kepler-78b is a planet that shouldn’t exist: this scorching lava world, scientists say, circles its star every 8½ hours at a distance of less than a million miles—one of the tightest known orbits. According to current theories of planet formation, it couldn’t have formed so close to its star, nor could it have moved there. “This planet is a complete mystery,” said astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We don’t know how it formed or how it got to where it is today. What we do know is that it’s not going to last forever.” “Kepler-78b is going to end up in the star very soon, astronomically speaking,” explained astronomer Dimitar Sasselov of the center. Not only is it a mystery world, astronomers are calling it the first known Earth-sized planet with an Earth-like density, or compactness. It’s about 15 percent wider than the Earth, according to calculations, and weighs almost twice as much, suggesting an Earth-like density consistent with an iron-and-rock makeup. When this planetary system was forming, the young star was larger than it is now, so the planet’s current orbit would have been inside the swollen star, according to astronomers. “It couldn’t have formed in place because you can’t form a planet inside a star. It couldn’t have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma,” said Sasselov. According to Latham, Kepler-78b is a member of a new class of planets recently identified in data from NASA’s Kepler spacecraft. These newfound worlds all orbit their stars with less than 12 hours per orbit. They’re also small, about the size of Earth. Kepler-78b is the first planet in the new class to have its mass (or “weight”), measured. “Kepler-78b is the poster child for this new class of planets,” notes Latham. The team studied it using spectrographs, which break up light into its colors. It’s a doomed world because gravitational tides will draw it even closer to its star, they added. Eventually it will move so close that the star’s gravity will rip the world apart. Theorists predict that Kepler-78b will vanish within three billion years. Interestingly, they add, our solar system could have held a planet like Kepler-78b. If it had, the planet would have been destroyed long ago leaving no signs for astronomers today. Kepler-78b orbits a Sun-like G-type star located an estimated 400 light-years from Earth in the constellation Cygnus. A light-year is the distance light travels in a year.