NASA scientists have, for the first time, seen the light from a planet outside our solar system that's a similar size to the Earth.
The planet, called 55 Cancri e, is about twice as big and eight times as massive as Earth, although it's not habitable. It orbits its star, 55 Cancri, in just 18 hours.
"Spitzer has amazed us yet again," said Bill Danchi, Spitzer program scientist. "The spacecraft is pioneering the study of atmospheres of distant planets and paving the way for NASA's upcoming James Webb Space Telescope to apply a similar technique on potentially habitable planets."
Until now, the Spitzer Space Telescope has been able to study the planet only by analyzing how the light from 55 Cancri changes as the planet transits.
In the new study, however, Spitzer has measured how much infrared light comes from the planet itself. The results reveal that the tidally-locked planet is likely dark, with its sun-facing side reaching more than 2,000 Kelvin (3,140 degrees Fahrenheit) - hot enough to melt metal.
This fits with the theory that 55 Cancri e is a water world: a rocky core surrounded by a layer of water in a 'supercritical' state where it is both liquid and gas, and topped by a blanket of steam.
"It could be very similar to Neptune, if you pulled Neptune in toward our sun and watched its atmosphere boil away," says Michaël Gillon of Université de Liège in Belgium.
The 55 Cancri system is 41 light-years away and has five planets, of which 55 Cancri e is the closest to the star. Spitzer hasdiscovered that the sun-facing side is extremely hot, indicating the planet probably does not have much of an atmosphere to carry the sun's heat to the unlit side.
"When we conceived of Spitzer more than 40 years ago, exoplanets hadn't even been discovered," says Michael Werner, Spitzer project scientist at NASA's Jet Propulsion Laboratory.
"Because Spitzer was built very well, it's been able to adapt to this new field and make historic advances such as this."