Tidal heating may leave some alien planets waterless
Washington: Some exoplanets may experience powerful tidal forces, which could strip them of water completely, thus leaving behind hot, dry worlds like Venus, a new study has revealed.
These findings might considerably affect searches for habitable exoplanets, scientists explained.
The tides that are experienced on Earth are caused due to gravitational pull of the moon and sun. But these tides are nothing compared to what is seen elsewhere in the solar system — the gravitational pull Europa experiences from Jupiter leads to tidal forces roughly 1,000 times stronger than what Earth feels from our moon, flexing and heating Europa, the Discovery News reported.
Heat is a major factor in determining how capable a planet might be of supporting life. What scientists consider as the habitable zone of a star is defined by whether liquid water can survive on its surface, given that life exists virtually wherever there is liquid water on Earth.
Being extremely far from a star, and the lack of light makes a planet too cold, freezing all its water while being too close to a star, and all that blazing heat makes a world too hot, boiling all of its water off in what is known as a runaway greenhouse effect.
However, now the scientists have found that stellar heat is not the only thing that can prompt a runaway greenhouse climate catastrophe. Tidal heating can too, for what they call “tidal Venuses.”
“This has fundamentally changed the concept of a habitable zone,” said researcher Rory Barnes, a planetary scientist and astrobiologist at the University of Washington.
“We figured out you can actually limit a planet’s habitability with an energy source other than starlight.”
Tidal Venuses could not take place around stars like our sun because the effects of tides fall off quickly with distance, Barnes noted.
For a planet to experience tidal heating from a star like our sun, it would have to be so close in that heat from its light would leave it uninhabitable even without any tidal heating.
However, tidal Venuses could occur close to dimmer and much less massive bodies — main-sequence stars less than a third the mass of our sun, for instance, or failed stars called brown dwarfs, or dead stars such as white dwarfs.
These bodies have been of interest to astrobiologists because their dim nature means their habitable zones are theoretically very close.
Planets near their stars eclipse them more often, making them easier to detect than planets that are farther away — as such, researchers had thought dim, low-mass stars could be ideal places to find habitable worlds.
After a tidal Venus its water entirely and becomes uninhabitable, the tides could alter its orbit so that it no longer experiences tidal heating.
As such, it might no longer resemble a tidal Venus, but look just like any other world in its star’s habitable zone, tricking researchers into thinking it is potentially friendly for life, even though it has basically been sterilized.
As terrestrial worlds are found around dim bodies, factoring these findings into searches for habitable exoplanets could result in scientists wasting less time on dry worlds.
“As candidates for habitable worlds are found, tidal effects need careful attention,” Barnes said.
“You don’t want to waste time on desiccated planets.”
Barnes also insisted that more work is required to be done for analysing how the effects of tidal heating might actually manifest themselves.
“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," Barnes said.
“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,” Barnes added.
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