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Earth Barely Big Enough for Life, Study Says

Richard A. Lovett
for National Geographic News
January 10, 2008
 
Astronomers searching for habitable worlds might do best to look for rocky planets several times larger than Earth.

That's because, according to a new study, our planet is at the lower end of the size range needed for plate tectonics—which scientists believe are vital for stabilizing temperatures enough for life.

Tectonics—the continent-shifting forces that build mountains and fuel volcanoes—recycle Earth's crust by drawing it underground, where it melts and later re-emerges as magma, pointed out Diana Valencia of Harvard University.

That helps keep carbon dioxide levels in the atmosphere more or less stable, as excess gas is removed from the atmosphere by reacting with fresh rocks in a process called "weathering."

The carbon dioxide is later returned to the atmosphere via volcanic gases.

"Plate tectonics is important for the carbon cycle to operate," Valencia said. "Carbon cycling is the Earth's climate thermostat. Over time, it's kept the Earth's temperature around [that of] liquid water, allowing life to emerge."

Valencia, who presented her study yesterday at a meeting of the American Astronomical Society in Austin, Texas, and her team looked at factors that determine whether rocky planets experience tectonics.

Rocky worlds significantly larger than Earth—commonly known as "super-Earths"—are more likely to have plate tectonics, the researchers concluded.

"It gives us hope" for finding habitable worlds, Valencia said. "Finding an Earth analog is going to be hard. But finding super-Earths is easier."

(Related: "First Habitable Earthlike Planet Found, Experts Say" [April 24, 2007].)

Shaky Findings

Plate tectonics occur because of convection currents in a planet's mantle, which cause the surface to crack into plates that slide past one another.

Valencia found that the presence of water, which forms hydrated minerals, can lubricate the movement process.

That, Valencia said, might be why waterless Venus, which is otherwise quite similar to Earth, has no plate tectonics.

But the size of a planet turned out to be just as important.

Bigger rocky planet have more heat trapped in their interiors, producing more rapid convection currents, Valencia said.

"The bigger planets would not need water to have plate tectonics," she said.

Since our solar system's other rocky planets—Mercury, Venus, and Mars—do not have tectonics, the work suggests Earth is at the small end of the size range of habitable planets, Valencia added.

Earthlike Planets

The scientists believe that super-Earths look very much like our world—especially if they contain water. (Related: "'Super-Earth' Discovered Orbiting Distant Star" [March 13, 2006].)

The main difference would be higher surface gravity.

"It would be much more similar to Earth than something like Mercury or Mars or even Venus," Valencia said.

There might also be more earthquakes on a super-Earth, since the continental plates would be grinding past each other at a rate as much as ten times faster than Earth's.

The rate of motion might be on the order of a foot (30 centimeters) a year.

"[That] is still pretty slow, but over geological time scales, it's fast, so you would have mountains forming a lot faster, " Valencia said.

"Humans living on such a planet might not be aware day-to-day, but ... it would be more active than Earth."

Late Bloomers

Super-Earth's weren't the only planets drawing attention at the meeting.

Another study, presented by University of California, Los Angeles, graduate student Carl Melis, found that planets can form much later in a star's evolution than was previously believed.

Melis was looking through decades-old infrared photographs when he spotted odd stars in the constellation Pisces and Ursa Major. The stars showed signs that planets were forming.

Planets are conventionally believed to form in sync with the stars they circle, but both of these stars were hundreds of millions to billions of years old.

"It just didn't add up," said Melis's advisor, physics and astronomy professor Benjamin Zuckerman. "It was a surprise."

The pair concluded that the old stars were forming new planets long after their first generations of planets had matured.

The most likely reason for the phenomenon is that some other object, perhaps a small star or large planet, had recently been drawn into each stars' atmosphere, he added.

"It may still be there, burrowing through the outer reaches of the atmosphere," Zuckerman said.

"The gravitational interaction is enough to toss material out into this gaseous ring where this second generation of planets might form. People have been writing theoretical papers about the formation of such gaseous rings for a long time. This may be the first observation."

It's unlikely that such late-forming planets would support life. But "it would be interesting to see what kind of planets form in such an environment, and what their characteristics would be," Zuckerman said.

Harvard's Valencia said it's an interesting find.

"If it's the case that you have [that] second wave," she said, "you would have even more interesting and different planets."

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