Astronomers call the most promising of these places "Goldilocks planets, " because they orbit in a temperate zone not too hot and not too cold to support liquid water essential for carbon-based life as we know it.

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"What's particularly interesting is four new planets - less than twice the size of Earth - that are potentially in the habitable zone, the location around a star where it could potentially have liquid water to sustain life, " Dr Burke told BBC News.

BBC: Kepler telescope: Earth-sized planets 'number 17bn'

The astronomers who will use these telescopes are particularly interested in finding any Earth-like objects in the so-called habitable zone around a star, where temperatures permit water to exist in a liquid state.

ECONOMIST: Planetary science: When a world is born | The

"The role of liquid water on Mars is of great importance for its habitability and this study using Mars Express describes a very large zone where groundwater was present for a long time, " said Olivier Witasse, a project scientist with ESA's Mars Express.

MSN: New proof that water existed underground on ancient Mars