A frozen planet in a nearby solar system may have been warm and habitable for hundreds of millions of years, scientists believe.

Astronomers made the discovery after studying the orbits of seven planets circling Trappist-1, a cool dwarf star 40 light years away in the constellation Aquarius.

The outermost planet, Trappist-1h, is currently an ice ball with an average surface temperature of minus 100C - but this may not always have been the case, the research suggests.

The scientists found that the star's planets were gravitationally linked in a complex dance known as "orbital resonance", which was probably forged early in the life of the system.

Tracing back the history of Trappist-1h, they showed that the planet was likely to have spent several hundred million years in a much warmer state when its host star was younger and brighter.

Lead researcher Rodrigo Luger, a doctoral student at the University of Washington in the US, said: "We could therefore be looking at a planet that was once habitable and has since frozen over, which is amazing to contemplate and great for follow-up studies."

The study, published in the journal Nature Astronomy, is based on data gathered by Nasa's Kepler Space Telescope.

Three more of Trappist-1's planets appear to lie within the star's habitable zone, the orbital band within which temperatures are mild enough to permit liquid surface water and possibly life.