Exo-Planets are more habitable than previous predictions

A study by astronomers at the University of Toronto, Canada proposes that exoplanets - planets outside our solar system - possibly have liquid water and be more inhabitable than we believed. "Planets with possible oceans could have a environment that is much more like Earth's than formerly thought," said Jérémy Leconte, a postdoctoral scholar at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, and lead writer of a study printed in Science Express. Researchers have believed that exoplanets behave in a manner different to that of Earth - that is they always display their same side to their star. If so, exoplanets would orbit in sync with their star so that there is always one hemisphere towards it while the other hemisphere is in continuous cold darkness. Leconte's study proposes, however, that as exoplanets orbits about their stars, they spin at such a speed as to show a day-night sequence similar to Earth.

"If we are right, there is no perpetual, cold night side on exoplanets forcing water to remain confined in a huge ice sheet. Whether this new understanding of exoplanets' climate raises the ability of these planets to mature life remains an open question."

Leconte and his team reached their decisions via a three-dimensional climate model they made to forecast the effect of a given planet's atmosphere on the speed of its revolution, which outcomes in changes to its climate," said Leconte. "Atmosphere is a key aspect affecting a planet's spin, the influence of which can be of sufficient to overcome synchronous revolution and put a planet in a day-night cycle."

Though astrophysicists are still awaiting observational proof, theoretical arguments propose that many exoplanets should be able to sustain an atmosphere as huge that of Earth. In Earth's case - with its comparatively thin atmosphere - most of the light from the Sun touches the surface of the planet, maximizing the effect of heating all over the atmosphere and creating a more modest climate across the planet. By making temperature differences at the surface, between day and night and among equator and poles, the solar heating drives winds that reallocate the mass of the atmosphere.

The impact is so substantial that it overwhelms the effect of tidal friction exerted by a star on whatever satellite is circling it, much like Earth does on the Moon.

"The Moon always shows us the same side, for the reason that the tides elevated by Earth create a friction that alters its spin," said Leconte. "The Moon is in synchronous revolution with Earth because the time it takes to spin once on its axis equals the time it takes for it to circle about Earth. That is why there is a dark side of the moon. The tidal theory, however, neglects the effects of an atmosphere."

The scientists say that a large number of known terrestrial exoplanets should not be in a state of synchronous rotation, as originally believed. While their models show that they would have a day-night cycle making them much more similar to Earth, the duration of their days could last between a few weeks and few months.

The discoveries are reported in the paper "Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars" published today in Science Express. The work was supported by grants from the Natural Sciences and Engineering Research Council of Canada.



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