And they found it suffers extreme temperatures, being as hot as fire on one side and as cold as ice on the other.
Scientists turned a heat-seeking space telescope called Spitzer onto the remote world which orbits a star in the system Upsilon Andromedae.
The planet is a giant ball of gas, like our own companion world Jupiter, but it orbits very close to its own sun, circling it once every 4.6 days.
Experts believe tidal forces have left the planet always keeping the same face pointed towards its parent star, just as our Moon always presents the same side to the Earth.
According to the astronomers, the difference in temperature between the two sides of the planet, called Upsilon Andromedae b, is an incredible 1,400 degrees Celsius – 2,550 degrees Fahrenheit.
Dr Brad Hansen of the University of California, Los Angeles, said: “If you were moving across the planet from the night side to the day side, the temperature jump would be equivalent to leaping into a volcano.”
The scientists’ findings are the first ever made of the suirface of an extrasolar planet. The Spitzer telescope observes the universe with infrared eyes, detecting heat rather than normal light.
Dr Michael Werner, project scientist for Spitzer at Nasa’s Jet Propulsion Laboratory, in California, said: “This is a spectacular result. When we designed Spitzer years ago, we did not anticipate that it would be revolutionising extrasolar-planet science.”
Upsilon Andromedae b was discovered in 1996 and lies only 40 light-years away from us.