July 10, 2014
Distant Earths May Need Balanced Friction Tides To Support Life
Gerard LeBlond for redOrbit.com - Your Universe Online
Other star systems commonly house Earth-sized planets. To some, friction heat could be destructive, but given the correct amount of heat, it could be helpful in creating conditions to support life.
“We found some unexpected good news for planets in vulnerable orbits. It turns out these planets will often experience just enough friction to move them out of harm’s way and into safer, more-circular orbits more quickly than previously predicted,” said lead author of the study Wade Henning, from the University of Maryland and working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
In simulations, giant planets of young star systems often disrupt the orbits of smaller planets. The smaller planet can be left in an unstable elliptical orbit with the chance of colliding with another planet, being absorbed into the host star, or be thrown out of the system altogether.
The amount of tidal stress imposed on a planet with an elliptical orbit is immense when the planet moves closer to the star then moves away. The gravitational force when the planet is near the star is so powerful it could deform the planet, and while it is in a distant orbit it could slowly regain its shape. This produces friction heat and in extreme cases, the tidal stress could generate enough heat to liquefy the planet.
The new study was published online July 1 in an issue of the Astrophysical Journal. Henning, along with his colleague Terry Hurford, also from Goddard, studied the effects of tidal stress on planets with multiple layers of rock, mantle or iron core.
The study revealed that some planets could move into a safe orbit much faster than was previously expected -- in a few hundred thousand years instead of several million. These planets would be on the verge of melting or at least have a nearly melted layer, like the one just below our Earth’s crust. The interior temperature of these planets could range from slightly warmer than Earth’s to having magma oceans.
The planet’s transitions into a circular orbit would be a speedy one because the interior layer would flex easily creating friction heat. As the heat was released from the planet it would lose energy at a rapid rate and settle into a circular orbit. Eventually the tidal heat would diminish and the surface could become safe to walk on.
If a planet had a completely melted layer it would produce little friction, is what was expected prior to this study. A cold planet tends to resist tidal stress and will release its energy slowly.
The study revealed that many of the planets will generate less friction than once thought. If the planet is not crowded by other bodies, they could be stable for a long time, even in an eccentric orbit.
“In this case, the longer, non-circular orbits could increase the ‘habitable zone,’ because the tidal stress will remain an energy source for longer periods of time. This is great for dim stars or ice worlds with subsurface oceans,” said Hurford.
A planet that is covered in a very thick ice shell can also achieve high amounts of heating. Ice is a low friction surface, but a layer of ice thousands of miles thick would be springy. A shell of this type would need to have the correct properties to respond to tidal stress and generate a lot of heat. These planets have high pressure inside them that could prevent all but the upper layer from liquefying.
Even though a planet has a relatively thin layer of ice, a few hundred miles thick, it could still dominate the global behavior.
The model included Earth-sized planets and those up to two-and-a-half times larger. The larger planets would most likely experience stronger tidal stress and could benefit from the friction heating. The next step is to investigate how layers of melted material flow and change over a period of time.
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