For the first time in history, an alien planet outside our solar system is proven to have water in its atmosphere. Previous theories said water vapor should be present in the atmospheres of nearly all the known extrasolar planets.

Travis Barman, an astronomer at Lowell Observatory in Flagstaff, Arizona, said water vapor has been found in the atmosphere of a large, Jupiter-like gaseous planet located 150 light years from Earth in the constellation Pegasus. The planet is known as HD 209458b, and it has been dubbed Orion.

Well known among astronomers and planet hunters, Orion was the first planet to be directly observed around a normal star outside our solar system and the first confirmed to have oxygen and carbon in its atmosphere.

It's about 100 times closer to its sun than Jupiter to ours and it's also made up of gas, only hotter, that's why it has been nicknamed "hot Jupiter". It orbits so close to the sun that its atmosphere reaches 1100 C (2012 F) and is losing about 10 000 tons of material every second as vented gas.

The detection of the presence of water vapor was possible because this planet, from the vantage point of the Earth, orbits directly in front of its star every 3-1/2 days, allowing crucial measurements to be made. It is what is known as a transiting planet.

"It's definitely good news because water has been predicted to be present in the atmosphere of this planet and many of the other ones for some time. Water actually survives over a broad range of temperatures," Barman explained. "It would need to get quite a bit hotter to completely break the water molecules apart."

However, it's unlikely that this gas giant could also host alien lifeforms, since, as far as we know, no organisms could survive in those harsh conditions.

So, scientists searching for signs of life beyond Earth are keen to learn about the presence of water on other planets - both in and beyond our solar system - because water is thought to be fundamental to the existence of life.