Distant planet Kepler-78b a lot like Earth but much hotter

Scientists have found a planet far out in the cosmos that's close in size and content to Earth -- an astronomical first.

But this rocky world is so close to its sun that it's almost certainly too hot for life. Surface temperatures are believed to top 3,500 degrees Fahrenheit, hot enough to melt rock.

The newly discovered exoplanet is called Kepler-78b. It was first detected last year by NASA's Kepler Space Telescope, then measured and studied by astronomers at observatories here on Earth.

Astrophysicists reported Wednesday in the journal Nature that Kepler-78b appears to be made of rock and iron, just like our own planet.

They measured the planet's mass to determine its density and content. It's actually a little bigger than Earth and nearly double its mass, or weight.

Kepler-78b is located in the Cygnus constellation, about 400 light-years from Earth -- a distance of some six trillion miles.

It orbits a sun-like star from a very close distance -- about 900,000 miles -- and makes a complete orbit every eight and a half hours. By comparison, Earth is almost 93 million miles from the sun and each orbit takes 365 days.

"This planet is a complete mystery," astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA) said in a press release. "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."

Scientists agree the planet will be sucked up by the sun in a few billion years, so its time remaining, astronomically speaking, is short.

More than 1,000 exoplanets -- worlds outside our solar system -- have been confirmed so far.

NASA's Kepler Space Telescope has identified 3,500 more potential candidates. The telescope lost its precise pointing ability earlier this year, and NASA has given up trying to fix it.

Scientific teams in the United States and Switzerland used ground observatories to measure Kepler-78b.