British scientists are to lead a mission searching for signs of life on planets orbiting nearby stars.
Astronomers will use a new 1.2-metre space telescope to look for biomarkers in the atmospheres of exoplanets.
Molecules of chemicals such as ozone, carbon dioxide and methane may indicate the presence of life.
Scientists at University College London (UCL) are leading the £400 million Exoplanet Characterisation Observatory (EChO) mission, which is backed by the European Space Agency (ESA).
Planetary scientist Dr Giovanna Tinetti, who heads the UCL team, said: "This is tremendously exciting news.
"One of the key aims of our mission is to see if we can detect molecules such as ozone and carbon dioxide in the atmospheres of planets not much bigger than Earth.
"These molecules are key biomarkers - signs that life might be, or might have been, present."
EChO will focus partly on Earth-sized planets in the "habitable zone" of Sun-like stars.
Also known as the "Goldilocks zone", this is the orbital region where conditions are not too hot and not too cold but "just right" to allow the existence of liquid surface water.
Most experts agree that liquid water is a pre-requisite for life as we know it.
Hotter stars will have a habitable zone further out than the one in which the Earth orbits the Sun. Planets around cooler stars have to be closer in to occupy the habitable zone.
The EChO space telescope is expected to be launched between 2020 and 2022.
It was one of nearly 50 mission proposals pitched to ESA, Dr Tinetti said.
"We had to overcome really tough competition to get selected for further study and possible launch," she added.
Dr Tinetti led the team that made the first discovery of water in an exoplanet atmosphere.
ESA's decision to back the mission follows new discoveries made by the American space agency Nasa's Kepler space telescope.
Earlier this month, Kepler scientists announced a new tally of 1,235 planet "candidates" orbiting stars beyond the Sun. They included 54 potential "habitable zone" planets, five of which were Earth-sized.
Kepler has also discovered evidence of 170 solar systems containing families of planets. One, Kepler-11, which is some 2,000 light years from Earth, has six confirmed planets all with orbits smaller than that of Venus.
Like Kepler, EChO will look for tiny changes in luminosity caused when planets cross the face of, or "transit", their stars.
Sensitive analysis of light wavelengths can reveal information about the chemical make-up of planet atmospheres.