Scientists have found the most similar planet to Earth ever discovered - except it is 2,000C degrees hotter.
The small hot planet orbiting a star 700 light years away has the same size and rocky composition as the Earth.
Kepler 78b is the smallest exoplanet whose mass and size are known. It is just 1.2 times bigger than Earth, 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 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.
"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.
"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.
A handful of exoplanets the size or mass of the Earth have been discovered, but Kepler 78b is the first for which both values have been measured.
The planet was first identified by the American space agency Nasa's Kepler space telescope, which looked for planets crossing in front of 150,000 stars.