New definition could further limit habitable zones around distant suns

June 10, 2009

As astronomers gaze toward nearby planetary systems in search of life, they are focusing their attention on each system's habitable zone, where heat radiated from the star is just right to keep a planet's water in liquid form.

A number of have been discovered orbiting red dwarf stars, which make up about three-quarters of the stars close to our solar system. Potentially habitable planets must orbit close to those stars - perhaps one-fiftieth the distance of Earth to the sun - since those stars are smaller and generate less heat than our sun.

But new calculations indicate that, with planets so close, tidal forces exerted on planets by the parent star's gravity could limit what is regarded as a star's habitable zone and change the criteria for planets where life could potentially take root.

Scientists believe liquid water is essential for life. But a planet also must have plate tectonics to pull excess carbon from its atmosphere and confine it in rocks to prevent runaway greenhouse warming. Tectonics, or the movement of the plates that make up a planet's surface, typically is driven by radioactive decay in the planet's core, but a star's gravity can cause tides in the planet, which creates more energy to drive plate tectonics.

"If you have plate tectonics, then you can have long-term climate stability, which we think is a prerequisite for life," said Rory Barnes, a University of Washington postdoctoral researcher in astronomy.

However, tectonic forces cannot be so severe that geologic events quickly repave a planet's surface and destroy life that might have gotten a foothold, he said. The planet must be at a distance where tugging from the star's gravitational field generates tectonics without setting off extreme volcanic activity that resurfaces the planet in too short a time for life to prosper.

Barnes is lead author of a paper to be published by The Astrophysical Journal Letters that uses new calculations from computer modeling to define a "tidal habitable zone." Co-authors are Brian Jackson and Richard Greenberg from the University of Arizona and Sean Raymond from the University of Colorado. The research was funded by NASA.

"Overall, the effect of this work is to reduce the number of habitable environments in the universe, or at least what we have thought of as habitable environments," Barnes said. "The best places to look for habitability are where this new definition and the old definition overlap."

The new calculations have implications for planets previously considered too small for habitability. An example is Mars, which used to experience tectonics but that activity ceased as heat from the planet's decaying inner core dissipated.

But as planets get closer to their suns, the gravitational pull gets stronger, tidal forces increase and more energy is released. If Mars were to move closer to the sun, the sun's tidal tugs could possibly restart the tectonics, releasing gases from the core to provide more atmosphere. If Mars harbors liquid water, at that point it could be habitable for life as we know it.

Various moons of Jupiter have long been considered as potentially harboring life. But one of them, Io, has so much volcanic activity, the result of tidal forces from Jupiter, that it is not regarded as a good candidate. Tectonic activity remakes Io's surface in less than 1 million years.

"If that were to happen on Earth, it would be hard to imagine how life would develop," Barnes said.

A potential Earth-like planet, but eight times more massive, called Gliese 581d was discovered in 2007 about 20 light years away in the constellation Libra. At first it was thought the planet was too far from its sun, Gliese 581, to have liquid water, but recent observations have determined the orbit is within the habitable zone for liquid water. However, the planet is outside the for its sun's tidal forces, which the authors believe drastically limits the possibility of life.

"Our model predicts that tides may contribute only one-quarter of the heating required to make the planet habitable, so a lot of heat from decay of radioactive isotopes may be required to make up the difference," Jackson said.

Barnes added, "The bottom line is that tidal forcing is an important factor that we are going to have to consider when looking for habitable planets."

Source: University of Washington (news : web)

Explore further: Researchers Say Tides Can Cut Life Short On Planets Orbiting Smaller Stars

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not rated yet Jun 10, 2009
I think we should expand rather than limit the habitable zones. As we see, liquid water exists on Mars, Encelaedus and other places we would not normally consider "habitable". In other words. life could be anywhere but if we limit ourselves to where we think it should be then we will miss it!
1 / 5 (2) Jun 10, 2009
One wonders if the authors considered the effect and potential of a Moon and the added energy to plate tectonics. Or perhaps they did but since a moon in orbit around a planet, itself in orbit around another star would be undetectable by current technologies they did not include it in thier calculations.

Wonder if a double planet system ( orbitting a central point of gravity ) would provide enough energy to propel plate tectonics.
not rated yet Jun 10, 2009
Possibly. The problem is that if there are only two bodies, planet-planet or planet-star, the orbit will probably be circular. In a circular orbit, the tidal forces are constant, once the rotation is tidally locked, so there isn't any heating. You need a third body to perturb the orbit to produce tidal heating. In Io's case, the other large moons keep the orbit slightly eccentric, which means that Io "rocks" slightly as it's orbital speed changes. This is what produces the heating. The Moon helps heat the Earth, and the Earth also produces heating in the Moon, but the effect on the Moon isn't enough to produce visible activity.

A single small planet, orbiting close to a red dwarf, would have circular orbit, and no tidal heating. If there was a second planet, large enough and/or close enough, that could perturb the orbit and produce heating, from a small amount for small eccentricities to Io-like for larger perturbation.

A pair of planets, locked to each other, and orbiting a star, would be similar to the Earth-Moon system, and both would be heated. In that case, it would be a combination of solar and joint tidal forces, with the star perturbing the mutual orbit, and the mutual tidal lock preventing either from being locked to the star.
not rated yet Jun 11, 2009
While the news item may be bad news, it is just what we can expect, considering Fermi's paradox.
If the very numerous red dwarf star systems were suitable places for life to develop, the Milky Way would already be overrun by aliens with eyes that use red and infra-red, and who consider rapidly rotating planets bizarre quirks of nature.
Using the Copernican principle, we are probably average among intelligent species, with a G- or K-class star of intermediate metallicity, no "hot Jupiters", and a home world with a big satellite. Unfortunately, this makes our nearest neighbours likely to be very far away.
not rated yet Jun 11, 2009
It's also possible that the red dwarfs are home to numerous civilizations, who look at our star the way we would an A or hotter type: Too active and short lived for intelligent life. If they have interstellar travel, they're probably looking for "life as they know it", orbiting their favorite type of star.

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