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article imageTidal heating may strip exoplanets in habitable zone of all water

By JohnThomas Didymus     Feb 11, 2012 in Science
Austin - We are familiar with the phenomenon of tides caused by the gravitational pull of the Moon and Sun on bodies of water on the Earth. Scientists say that on some alien planets tidal forces may be strong enough to strip the planet entirely of water.
The findings by Rory Barnes and his colleagues were presented at the January 11annual meeting of the American Astronomical Society in Austin, Texas.
The researchers believe their findings may impact significantly on the search for habitable explanets because some planets located in the habitable zone of their star's planetary system may be exposed to such powerful gravitational tidal forces as to make them dry worlds incapable of supporting life.
According to Space.com, the gravitational pull of Jupiter on Europa, for instance, creates tidal forces on the surface of Europa 1,000 times stronger than that which the Moon exerts on our Earth. The powerful tidal forces, according to scientists, flexes and heats Europa.
The theoretical concept of the habitable zone of a star in defined primarily by whether its surface can retain liquid water. Heat is a major factor in this context. A planet that is too far from its star is too cold and cannot sustain liquid water because low temperatures freeze all surface water. A planet too close to its star gets heated up and liquid water on its surface vaporizes. This explains why the planet Venus and Mercury are not considered prime candidates for life-bearing planets. Conditions on Venus, for instance, has interested scientists because it is thought the planet experienced the runaway greenhouse effect that some claim the Earth may also experience with the increase in concentration of greenhouse gasses in the atmosphere.
But now scientists have realized that radiation from a star is not the only thing that can lead to a planet experiencing runaway greenhouse effect. Scientists now realize that tidal forces may heat a planet and that the resultant tidal heating can also trigger run away greenhouse effect in a planet. Planets stripped of all water because of tidal heating are called "tidal Venuses."
Universe Today reports that according to Rory Barnes, planetary scientist and astrobiologist at the University of Washington, "This (realization) has fundamentally changed the concept of a habitable zone. We figured out you can actually limit a planet's habitability with an energy source other than starlight."
Scientists say tidal Venuses cannot occur around a star like our Sun because tidal forces weaken very rapidly with distance. Before a planet can come close enough to our Sun to experience heating from tidal forces, solar radiation would have heated up the planet sufficiently to make in inhabitable.
The best candidate planetary systems for tidal Venuses are those with dim stars with relatively low mass. Main sequence stars (that is stars in the prime of their natural life cycles) less than a third of the mass of the Sun, or stars known as brown dwarfs and white dwarfs may have tidal Venuses in their planetary systems. Such planetary systems have their defined habitable zones much closer than in other planetary systems and planets can come close enough to their stars to experience tidal heating effects.
Astronomers have found that planets very close to dim stars sometimes eclipse their stars making them more easy to detect than planets farther away. It is, therefore, no coincidence that some of the first planets astronomers discovered in the "habitable zone" of their stars were planets orbiting dim stars such as brown dwarfs. Astrobiologists have therefore taken interest in dim low-mass stars in their search for habitable worlds.
According to Physorg.com, a tidal Venus that has lost all its water to tidal heating could have its orbit around its star altered so that it no longer experiences tidal heating forces. Such a star would not look at casual observation like a tidal Venus and could lead astronomers to the mistaken impression that it is a habitable world with conditions favorable for life, while in actual fact it is a dry and sterile world.
Scientists now realize that they have to factor these new findings in their theoretical understanding of the concept of habitable zone and in their search for habitable worlds. Barnes, according to Space.com, says, "As candidates for habitable worlds are found, tidal effects need careful attention. You don't want to waste time on desiccated planets."
Barnes says more work needs to be done to understand tidal Venuses. He explains: "In our solar system, the largest amount of tidal heating is with Jupiter's moon Io, which experiences 2 watts per square meter on its surface. We're trying to see if tidal heating can generate 300 watts per square meter on a planet's surface, and it's still unclear if planets will actually behave this way — maybe there's a saturation point where tidal heating can't reach tidal Venus levels. Planets are complicated beasts, and it's not always obvious how they will act."
Universe Today reports that planetary scientist Norman Sleep, at Stanford University, said: "We'll have to be careful when assessing objects that are very near dim stars, where the tides are much stronger than we feel on present-day Earth. Even Venus now is not substantially heated by tides, and neither is Mercury."
According to Sleep: "The only good example of this we might have had like this in the solar system is Earth early in its history soon after the moon-forming impact, where tidal heating from the moon was significant for 10 million years or so, enough for a brief runaway greenhouse. Eventually the moon moved far enough away for tidal heating to decrease."
Sleep, however, explains that tidal heating may not be sufficient to trigger a runaway greenhouse effect but rather warm a planet sufficiently for liquid water to exist on its surface. According to Universe Today, tidal forces exerted by Jupiter on its moon Europa, are thought to create enough heat to allow a liquid water ocean to exist beneath its outer ice crust. This may also apply to Saturn’s moon Enceladus.
Barnes says scientists still have to explain how tidal effects operate in multi-planet systems. He said: "We've looked at just a single star and a single planet evolving together, but when you have additional planets, you introduce gravitational perturbations, and how will that affect orbits and tidal heating and habitability? They could very well increase the threat of catastrophic tidal heating."
More about Gravity, Water, tidal forces, tidal heating, tidal venuses
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