A ball of rock and iron comparable with the Earth in its size and make-up but scorching hot and orbiting a distant star appears to defy conventional theories of planet formation, astronomers said on Wednesday. The planet, Kepler 78b, is less than two million kilometres from its parent star and completes an orbit every eight-and-a-half hours from a distance so close to the star its dayside surface temperatures exceed 2000 degrees Celsius, two international planet-hunting teams said.
“This planet is a complete mystery,” said David Latham, an astronomer and team member at the Harvard Smithsonian Centre for Astrophysics in the US. “We don’t know how it formed or how it got where it is today.” The planet’s host star, Kepler 78, is about 700 light years from the solar system, in the constellation Cygnus. The astronomers have described the planet in papers to be published on Thursday in the journal Nature.
Under existing theories, planets are believed to form early during a star's history when the star is much bigger. “For the planet to be where it has been seen, it would have had to form inside the star,” Latham told The Telegraph over the telephone. The discovery, he said, will compel astronomers to tweak theories to explain formation and transfer into its currently observed orbit. The planet’s orbit will eventually decay and it will be pulled into its star.
The astronomers used telescopes to study subtle changes in the star’s light and its movements to infer the presence of Kepler 78b. The observations suggest that it weighs about 1.8 times the Earth’s mass and is 20 per cent larger than the Earth and is made up mainly of iron and rock.
llustrations: David Aguilar, Harvard Smithsonian Centre for Astrophysics