Three potentially habitable Earth-sized planets in another solar system are likely to contain substantial amounts of water, say astronomers.

The discovery by the Hubble Space Telescope increases the chances of life evolving on planets orbiting Trappist-1, a dwarf star 40 light years from the sun.

Each of the worlds orbits in the star's "habitable zone", the narrow corridor where temperatures are mild enough to permit bodies of surface water such as lakes and oceans.

A total of seven planets similar in size to Earth are believed to circle the cool star.

The new research suggests that the outer planets in the system still harbour large quantities of water, despite powerful radiation from the star leading to enormous levels of water loss.

Dr Amaury Triaud, a member of the international team from the University of Birmingham, said: "Hubble's observations are of great significance, since they inform us on the irradiative environment of the Trappist-1 planets, notably on whether they can remain habitable for billions of years, like Earth has.

"However, some of our conclusions about the habitability of Trappist-1's seven are somewhat dampened by our fuzzy knowledge about the masses of the planets. Crucial observations, able to refine the planetary masses, are being obtained as we write."

Ultraviolet radiation from a star can cause planets to dry out through a process called disassociation which causes water molecules to break up.

The astronomers studied levels of UV radiation emitted by Trappist-1 which suggest its inner planets could have lost 20 times more water in the last eight billion years than all the Earth's oceans combined.

But the system's outer planets - including the three in the habitable zone - may have lost less than three Earth-oceans worth of water. This means the planets could have retained "substantial" amounts of water on their surfaces, said the scientists.

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The research is published in the Astronomical Journal.