Astronomers have, for the first time, mapped a nearby "super-Earth" exoplanet to find that one hemisphere is almost completely molten rock, while the other half is almost completely solid.

Key points: 55 Cancri e is a small rocky world eight times the mass of Earth

55 Cancri e is a small rocky world eight times the mass of Earth Mapping reveals startling temperature differences between the 'day' and 'night' side

Mapping reveals startling temperature differences between the 'day' and 'night' side The alien planet does not spin so the side facing its sun is constantly cooked

The study used NASA's Spitzer Space Telescope to produce a precision temperature (exo-)map of 55 Cancri e, which is approximately 40 light-years from Earth in the constellation of Cancer.

This is the first time such a map has been produced of such a small rocky world around eight times the mass of Earth.

As 55 Cancri e orbits so close to its parent star, completing one orbit every 18 hours, it has become tidally locked — one hemisphere of the world is constantly facing the star, whereas the opposite side is trapped in eternal night.

This has a dramatic effect on the exoplanet's surface; the star-facing side has a temperature that soars close to 2,500 degrees Celsius, whereas the dark side is a less savage 1,027 degrees Celsius.

Though the far side isn't exactly cold, it's the temperature difference that is the most startling and Spitzer was able to record the steep temperature gradient throughout the planet's orbit.

"We haven't yet found any other planet that is this small and orbits so close to its parent star, and is relatively close to us, so 55 Cancri e offers lots of possibilities," said Dr Brice-Olivier Demory of the University of Cambridge's Cavendish Laboratory.

"We still don't know exactly what this planet is made of — it's still a riddle. These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood," said Dr Demory, who is the lead author of the study published in the journal Nature.

One hemisphere constantly cooked

On Earth, extreme temperature gradients from the day side to the night side are prevented by our 24-hour rotation and efficient atmospheric circulation — warm air is quickly distributed around the globe.

55 Cancri e doesn't have this luxury; it doesn't spin, so one hemisphere is constantly cooked and it appears to have little to no global atmosphere to transport heat away from the star-facing side to the night.

"We think that there could still be an atmosphere on the night side, but temperatures on the day side are so extreme that the atmosphere may have evaporated completely, meaning that heat is not being efficiently transferred, or transferred at all from the day side to the night side," said Dr Demory.

There is a mystery, however. The researchers have realised that the day side of 55 Cancri e is a lot hotter than would be predicted from stellar heating alone — there seems to be "extra heat" that, for now, will remain a puzzle until more advanced space telescopes can be launched.

Perhaps NASA's James Webb Space Telescope will take a closer look in the hope of revealing more about this very alien world.

This story was originally published on DiscoveryNews.com