Water seen on planet outside solar system

news.yahoo.com...

Evidence of water has been detected for the first time in a planet outside our solar system, an astronomer said on Tuesday, a tantalizing find for scientists eager to know whether life exists beyond Earth.

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.

This is truly amazing, if their findings are correct then it will show that there is possibilities for life outside of Earth.

But my question is: How do they exactly detect water vapours? How do they know it is water and not something else?

ThiThere is already a current thread on this topic.

It can be found here.
Water Identified In Extrasolar Planets Atmosphere