Exoplanets probably more habitable than we thought, says new study

Exoplanets – planets outside our solar system – are probably more hospitable to life than most people think, says a team of astrophysicists from the University of Toronto. Many of them probably have liquid water.

Lead author Jérémy Leconte, a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, said:

“Planets with potential oceans could have a climate that is much more similar to Earth’s than previously expected.”

The study was published in the journal Science.

Astronomers have believed that exoplanets orbit their stars with one hemisphere facing their sun, while the other side exists in permanent darkness. If this is the case, exoplanets would rotate in sync with their stars.

Jérémy Leconte

Mr. Leconte says exoplanets’ atmospheres could be making them spin faster. (Image: Leconte’s homepage)

Leconte’s study suggests that exoplanets spin very much like our Earth does, and exhibit day-night cycles.

Leconte said:

“If we are correct, there is no permanent, cold night side on exoplanets causing water to remain trapped in a gigantic ice sheet. Whether this new understanding of exoplanets’ climate increases the ability of these planets to develop life remains an open question.”

The scientists reached their conclusions through a 3-dimensional climate model they developed to determine the effect of a given planet’s atmosphere on its rotation speed, which would lead to changes in its climate.

Leconte explained:

“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.”

Many astronomers believe that several exoplanets should be able to maintain an atmosphere as plentiful as that of Earth, even though we have yet to find observational evidence.

In the case of our planet, with its relatively thin atmosphere, most Sunlight reaches the surface of Earth, maximizing the effect of heating throughout the atmosphere, which leads to a more moderate climate across the whole planet.

The day-night cycle as well as the difference between the equator and the poles, both of which create differences in the temperature at the surface, drive winds that redistribute the mass of the atmosphere.

The impact is such that it overcomes the effect of tidal friction exerted by a sun (star) on whatever satellite is orbiting it, much like our planet does on the Moon.

Leconte said:

“The Moon always shows us the same side, because the tides raised by Earth create a friction that alters its spin.”

“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.”

The team in Toronto say that many known terrestrial exoplanets should not be in a state of synchronous rotation, as initially believed.

While their models show that these exoplanets would have day-night cycles, much like Earth does, each day/night might last from a few weeks to several months.

Venus, the planet in our Solar System with an atmosphere closes to the Sun, does not spin synchronously. The Earth doesn’t either, but we are too far away for the tidal friction to be significant. Venus, in fact, rotates backward – the Sun sets in the East and rises in the West. Tidal friction, however, does affect Venus’ spin (it rotates once every 243 days).

On the CITA website the authors wrote:

“Planets with potential oceans could thus have a climate that is much more similar to the Earth’s than previously expected. When a diurnal cycle is present, there is no permanent, cold night side where water can remain trapped in a gigantic ice sheet. Does this increase the ability of these planets to develop life as we know it? This is still an open question.”

The study was supported by grants from the Natural Sciences and Engineering Research Council of Canada.

Citation: Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars,” Jérémy Leconte, Hanbo Wu, Kristen Menou, and Norman Murray. Science 1258686. Published online 15 January, 2015. [DOI:10.1126/science.1258686].

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