Planets outside our solar system more hospitable to life than thought

January 15, 2015
This artist's concept depicts a planetary system. Credit: NASA/JPL-Caltech

A study by astrophysicists at the University of Toronto suggests that exoplanets - planets outside our solar system - are more likely to have liquid water and be more habitable than we thought.

"Planets with potential oceans could have a climate that is much more similar to Earth's than previously expected," said Jérémy Leconte, a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, and lead author of a study published today in Science Express.

Scientists have thought that behave in a manner contrary to that of Earth - that is they always show their same side to their star. If so, exoplanets would rotate in sync with their star so that there is always one hemisphere facing it while the other hemisphere is in perpetual cold darkness.

Leconte's study suggests, however, that as exoplanets rotate around their stars, they spin at such a speed as to exhibit a day-night cycle similar to Earth.

"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."

Leconte and his team reached their conclusions via a three-dimensional climate model they developed to predict the effect of a given planet's on the speed of its rotation, which results in changes to its climate," said Leconte. "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."

Though astronomers are still awaiting observational evidence, theoretical arguments suggest that many exoplanets should be able to maintain an atmosphere as massive that of Earth. In Earth's case - with its relatively thin atmosphere - most of the light from the Sun reaches the surface of the planet, maximizing the effect of heating throughout the atmosphere and producing a more moderate climate across the planet. By creating temperature differences at the surface, between day and night and between equator and poles, the solar heating drives winds that redistribute the mass of the atmosphere.

The impact is so significant that it overcomes the effect of tidal friction exerted by a star on whatever satellite is orbiting it, much like 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," said Leconte. "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 researchers say that a large number of known terrestrial exoplanets should not be in a state of synchronous rotation, as initially 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.

Explore further: Researchers find tidally distorted exoplanets may have unique signatures

More information: The findings are reported in the paper "Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars" published today in Science Express: www.sciencemag.org/lookup/doi/ … 1126/science.1258686

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Tangent2
4.2 / 5 (5) Jan 15, 2015
Scientists have thought that exoplanets behave in a manner contrary to that of Earth - that is they always show their same side to their star.


Why such a counter-intuitive assumption?? Applying what we know on Earth to other systems has been the foundation of science, and yet it is ignored in this respect?
gculpex
2.7 / 5 (3) Jan 15, 2015
Did they eliminate all other possible causes for rotation?
Doesn't sound like they did.
Veneficus
5 / 5 (5) Jan 16, 2015
Same idea here: why on earth would you think that *all* exoplanets are in synchronous rotation? Are we back to the middle ages where the earth was thought to be "special"?
nkalanaga
5 / 5 (4) Jan 16, 2015
There's an even simpler reason to think the heat would be redistributed around the planet, and it's been shown in papers going back many years. Even if the planet is tidally locked, the atmosphere isn't, and circulation driven by stellar heating would keep the dark side warmer. If the atmosphere was thick enough, the dark side would stay above freezing, and there wouldn't be any "freezeout". Earth's atmosphere is well above the lower limit, so Earth would remain habitable even if it was tidally locked. Current species, adapted to 24-hour days, might not be comfortable, but life would do fine.

Veneficus: Most of the exoplanets we've found are so close to their stars that they should be locked, mathematically. Planets farther out wouldn't be, but they're harder to find. It's discovery bias, nothing special about Earth.
Twirlip
4.4 / 5 (5) Jan 16, 2015
This is a terrible article. It completely fails to point out that the study is specifically about planets in the habitable zone of low mass stars like M dwarfs. Those planets would therefore in close orbits, and consequently it was thought they would all be tide locked.

Obviously *all* exoplanets were not previously thought to be tide-locked.
Torbjorn_Larsson_OM
4.3 / 5 (6) Jan 16, 2015
Just to clarify, the article is correct. It says "a large number", which is the case. ~ 70 % of stars are red dwarfs (as depicted in the image) and they have more terrestrial planets and more planets in their habitable zone than other types of stars. So they are the majority of stars.

Since these stars are relatively cold and dim, their habitable zone is so close to the star that a planet can come into tidal lock. But there are mechanisms that act against that, so without an atmosphere about half the planets comes into a resonance as Mercury and have a month long rotation. And apparently a thin atmosphere push even more planets out of lock.

Moreover, because the stars are dim and the planets orbits in weeks or months (so we see relatively many transits), habitable planets around red (or white) dwarfs are the planets that we can first characterize atmospheres of. That is why there is a huge interest to know more about those specifically.
Surly
5 / 5 (4) Jan 16, 2015
Scientists have thought that exoplanets behave in a manner contrary to that of Earth - that is they always show their same side to their star.


Why such a counter-intuitive assumption?? Applying what we know on Earth to other systems has been the foundation of science, and yet it is ignored in this respect?

Because the author's summarizing a bit sloppily. Scientists have long thought low-mass stars like red dwarves would have tidally locked planets, not that *all* non-Earth stars with exoplanets would have tidally locked planets.
thingumbobesquire
not rated yet Jan 18, 2015
The problem is that this field of inquiry has no idea of what is both sufficient and necessary for life on other planets. http://thingumbob...ogy.html

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