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Definition of a [Habitable] Planet?

Ever since Pluto got voted off the island, most astronomers have defined a planet as a body orbiting a star—dead or alive—that is a) massive enough to be rounded by its own gravity, b) not massive enough to ignite itself into starhood, and c) domineering enough to have swept its neighborhood clean of smaller planetary seedlings.

Phew, what a mouthful!

But as we know from our own solar system, not all planets are created equal, and things get really interesting when we try to define the types of planets that might support life.

Traditionally when we think of a habitable world, we think of Earth. Makes sense: To date it’s our only frame of reference for a planet that supports plants, animals, even microbes. So it’s as good a model as any in terms of what we’d want habitable exoplanets to look like.


A 3-D view of Mount Kilimanjaro in Tanzania, compiled from satellite data

—Image courtesy NASA/JPL/NIMA

Hence the huge emphasis among planet hunters on the so-called Goldilocks Zone, where it’s not too hot and not too cold. A planet inside this zone would be just right for liquid water and life-giving sunshine.

In recent years that hypothetical zone has been getting bigger, it seems, especially as expeditions to the deep ocean and volcanic peaks have expanded the conditions in which we thought life could exist.

Enter Rory Barnes, a University of Washington postdoctoral researcher who’s here to rain on that parade.

In addition to the right amounts of heat and water, planets that could support life need just the right kind of plate tectonics, Barnes argues in a paper soon to appear in the Astrophysical Journal Letters.

Plate tectonics on Earth plays a role in climate, Barnes says, by contributing to the global carbon cycle that keeps the atmospheric greenhouse effect well balanced [human emissions excluded, of course].

“If you have plate tectonics, then you can have long-term climate stability, which we think is a prerequisite for life,” Barnes said in a statement.

No tectonics and you get a dead world, a la Mars.

On the other hand, too much tectonics and a planet’s surface gets reshuffled too fast and furious. It’d be like a dandelion trying to grow in a crack on a well-maintained freeway—things would get repaved too quickly for life to keep its grip.

Take Jupiter’s moon Io, for example. Although far from the sun, the moon is jostled between Jupiter and two larger moons, which creates what’s known as tidal heating—heat from friction as the intense and irregular pull of gravity moves the moon’s crust.


—Image courtesy NASA/JPL/University of Arizona

All this motion makes Io volcanically active, but so very active that scientists don’t think the moon is a candidate for life.

Barnes therefore proposes establishing a “tidal habitable zone,” where a planet is close enough to its star for stellar gravity to drive plate tectonics, but not so close that tectonics goes on overdrive and the surface is constantly changing.

This, apparently, would squelsh fans of Gliese 581d, an exoplanet championed by some as the most likely “other Earth” yet found.

Recent calculations placed the rocky world within its star’s habitable zone. But Barnes’ theory says poor Gliese would be outside the zone for ideal tidal forces.

“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.”


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