The search for habitable alien planets will be harder because tidal forces could remove water from planets to leave them dry worlds.

Tidal heating could affect searches for habitable exoplanet, according to researchers from the University of Washington. Strong tidal forces could render planets in the habitable zone unlivable. Tidal heating occurs as orbital and rotational energy is dissipated as heat on the crust of a planet or a moon.

"This has fundamentally changed the concept of a habitable zone," said researcher Rory Barnes, a planetary scientist and astrobiologist at the University of Washington.

"We figured out you can actually limit a planet's habitability with an energy source other than starlight."

A planet could experience tidal heating if it is close to its parent star. Planets that orbit dimmer stars could move into the habitable zone- the zone that is just right for water to exist in its liquid form. However the tidal forces could be enough for the planet to lose all of its water leaving nothing but a dead planet.

This fact is important as scientists search for habitable exoplanets. Astronomers could be fooled into thinking that a planet in the habitable zone still has water when tidal heating has sterilized the planet.

"As candidates for habitable worlds are found, tidal effects need careful attention," Barnes said. "You don't want to waste time on desiccated planets."

Barnes noted that Jupiter's moon Io experiences the largest amount of tidal heating in the solar system. Io experiences tidal heating 2 watts per square meter on its surface. Astronomers should take tidal heating into account in the search for planets in the habitable zone. Barnes also noted that astronomers should consider how multi-planet systems affect the results of tidal heating.

"We've looked at just a single star and a single planet evolving together, but when you have additional planets, you introduce gravitational perturbations, and how will that affect orbits and tidal heating and habitability? They could very well increase the threat of catastrophic tidal heating."

The team's findings were presented last January 11 at the annual meeting of the American Astronomical Society in Austin, Texas.