Many 'habitable' exoplanets aren't, say astronomers

Posted by Emma Woollacott

The so-called 'habitable zone' around low-mass stars is often nothing of the sort, according to astronomers, because of the effect of tides.

New exoplanets have been discovered at a fantastic rate over the last couple of years, and astronomers have theorized that a habitable one could be found as early as this year.

But predictions such as this rest on the assumption that a planet is potentially habitable if it orbits at a distance at which water is liquid. In other words, the two determining factors are distance from the star and atmospheric composition.

Stars with a mass lower than our sun's have been seen as potential hosts. But by studying the tides caused by low-mass stars on potential Earthlike companions, René Heller of the Astrophysical Institute Potsdam and his colleagues have concluded that, around such stars, tidal effects would make even planets in this zone uninhabitable.

"I think that the chances for life existing on exoplanets in the traditional habitable zone around low-mass stars are pretty bleak, when considering tidal effects," says Heller. "If you want to find a second Earth, it seems that you need to look for a second sun."

Heller based his conclusion on three different effects. Firstly, tides can cause the axis of a planet`s rotation to become perpendicular to its orbit in just a few million years.

This means there would be no seasonal variation on planets in the habitable zone of low-mass stars. Their poles would be perpetually frozen, and their equators very hot, which in the long run would evaporate any atmosphere.

The second effect of tides would be to heat up the exoplanet. And, finally, tides could cause the rotational period of the planet - its day - to synchronize with the orbital period, or year. Thus, one half of the exoplanet would receive extreme radiation from the star while the other half froze in eternal darkness.

Heller and his colleagues have applied their theory to GI581g: an exoplanet candidate that has recently been claimed to be habitable. They find that it wouldn't experience any seasons and that its day is synchronized with its year. There probably wouldn't be any water on the surface, and it really wouldn't make a great place to live.