New research published in Nature Astronomy has poured, er, cold water on hopes that it may be possible to detect life on Earth-sized planets in the TRAPPIST-1 system.

The planets might be just too wet.

There was considerable excitement a year ago, when NASA's veteran Spitzer Space Telescope spotted seven Earth-sized planets orbiting a single star.

Astroboffins noted at the time that three of the planets were firmly in the habitable zone and so might have both liquid water and life. With a crushing lack of romance, scientists named the planets b, c, d, e, f, g and h.

TRAPPIST-1 itself is a cool red-dwarf star, with the proportions of the planetary system closer to Jupiter and its moons than our Solar System.

The team took estimates of the mass and diameter of the planets, and ran simulations to determine the most likely composition, focusing on the four planets for which they had most information (b, c, f and g).

The results don't make happy reading for alien hunters.

The density of the planets indicates that they all contain far more water than Earth. Since none of the planets are massive enough to hold on to much of an atmosphere, that water is likely to be either liquid or ice.

And goodness, there is rather a lot of it.

Up to 15 per cent of planets b and c may be water while over 50 per cent of planets f and g (both in the notional habitable zone) is composed of H 2 O.

To put that into context, if you were to wring out the Earth, you'd find that less than 0.7 per cent of our world is water.

Uncertainties in mass measurement means there is still some hope for the remaining planet in the habitable zone, planet e, but with the rest of the planets being so water-rich, things don't look good.

TRAPPIST-1f Phase Diagram. Click to enlarge

Don't expect all the water to be sloshing around on the planets either. The huge depths of the oceans on planet f mean that after 200km, explorers will find over 1,000km of ice formed by the immense pressure.

The research concludes on a final gloomy note.

While M-dwarf systems such as TRAPPIST-1 may be the most common host for planets in the habitable zone, an excess of water may mean that the crust is unlikely to reach above the water layer.

This means that while the planets may be habitable, distinguishing signs of life from simple geological sources will be tricky, making M-dwarf planets the toughest on which to detect life. ®