A small hot planet orbiting a star 700 light years away has the same size and rocky composition as the Earth, say scientists.
Kepler 78b is the smallest exoplanet whose mass and size are known. It is just 1.2 times bigger than Earth, and 1.7 times more massive, and has an almost identical density.
Scientists believe that, like the Earth, Kepler 78b mostly consists of rock and iron. There the similarities end, however. The planet hugs its parent star so closely that nothing could live on its scorching surface.
Kepler 78b’s year is just 8.5 hours long, the length of time which it takes to complete one orbit round its star.
“It’s Earth-like in the sense that it’s about the same size and mass, but of course it’s extremely unlike the Earth in that it’s at least 2,000 degrees hotter,” said Dr Josh Winn, from the Massachusetts Institute of Technology in the United States.
“It’s a step along the way of studying truly Earth-like planets.”
Dr Chris Watson from Queen’s University in Belfast, whose team also studied the planet, said: “Kepler 78 is a scorching lava world that, put simply, shouldn’t exist. Its close proximity to its star, and how it got there, is still a mystery. What we do know is that it won’t exist forever. Gravitational tides will slowly disrupt Kepler-78, drawing it closer to its star and eventually ripping it apart.”
Kepler 78b’s orbital period and size was previously determined by analysing the amount of light blocked as the planet passed in front of its star.
Because the planet orbits so close to its star, it was also possible to measure gravitational effects that gave away its mass.
American and European teams of astronomers examined data from observatories in Hawaii and the Canary Islands to analyse the star’s “wobble” caused by the gravitational tug of Kepler 78b.
The planet was first identified by the American space agency Nasa’s Kepler space telescope.
“When you have both the size and the mass of an object, you can calculate its density, and thereby determine its composition,” said Dr Andrew Howard, from the University of Hawaii.
Dr Watson said: “This discovery is a fine example of the great diversity of the exoplanets that we are uncovering, thanks to the tremendous progress in astronomy technology and techniques.
“Nature seems to like conjuring up planets in the least expected places. Just five years ago this work would have been impossible.”
“As we probe deeper and deeper we are finding that science fact is stranger than science fiction Kepler-78b certainly fits that bill,” added Dr Watson.
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