A study by astrophysicists at the University of Toronto suggests that exoplanets - planets that are outside our solar system - are more likely to have liquid water, and therefore may be more hospitable to life than researchers originally thought.
Scientists originally believed that exoplanets behave in a manner very different from the Earth. They have long thought that these planets always show their same side to their star. If so, these planets would rotate in sync with their star, meaning there is always one hemisphere facing it while the other remained in perpetual darkness and cold.
"Planets with potential oceans could have a climate that is much more similar to Earth's than previously expected," lead author of the study and postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, Jeremy Leconte says.
The study published this week in the journal Science Express suggests that these exoplanets rotate around their stars and spin at such a speed as to behave much like the Earth, with a full daily cycle of days and nights.
"If we are correct, there is no permanent, cold night side on exoplanets causing water to remain trapped in a gigantic ice sheet" Leconte says. "Whether this new understanding of exoplanets' climate increases the ability of these planets to develop life remains an open question."
Leconte and his team reached their conclusions using a three-dimensional climate model they developed to predict the effect of a given planet's atmosphere on the speed of its rotation.
"Atmosphere is a key factor affecting a planet's spin, the impact of which can be of enough significance to overcome synchronous rotation and put a planet in a day-night cycle," Leconte says.
In the case of Earth, the atmosphere is so thin that most of the light from the Sun reaches the surface. This maximizes the effect of heating throughout the atmosphere producing a more moderate climate. These temperature differences and solar heating drive winds that redistribute the mass of the atmosphere. The impact is so significant in fact that it overcomes the effect of tidal friction exerted by a star or whatever satellite is orbiting it, much like the Earth does on the Moon.
"The Moon always shows us the same side, because the tides raised by Earth create a friction that alters its spin," Leconte says. "The Moon is in synchronous rotation with Earth because the time it takes to spin once on its axis equals the time it takes for it to orbit around Earth. That is why there is a dark side of the moon. The tidal theory, however, neglects the effects of an atmosphere."
Researchers now believe that a large number of these exoplanets are not in a state of synchronous rotation, as initially believed. However, while these models do show these planets have a day and night cycle, the duration of these days could last weeks or even months.
