waterworlds.jpg An artist’s rendering of an Earth-like planet at sunset; note the ‘Hot Jupiter’ just above the horizon.  Credit: Nahks Tr’Enhl

Since 1991 more than 200 planetary systems other than our own have been discovered, all of them completely inhospitable to Earth-like planets, or so astronomers thought until now.

In light of new computer simulations, a team of scientists say more than a third of known planetary systems—made up of a central star, like our Sun, surrounded by gaseous planets and rocky debris—could include planets similar to Earth.

These other “Earths” would be much like our home planet, except that they would be completely covered by water.

Previous efforts to find habitable planets orbiting other stars were stymied by the presence of “Hot Jupiters” in these planetary systems. Hot Jupiters are gaseous planets as large as Jupiter, and are situated much closer to their central stars. As these planetary systems form, Hot Jupiters spiral inward toward their parent stars. During this violent migration, researchers believed, these Hot Jupiters vacuumup the swarms of gas and rock debris that would have been able to form terrestrial planets.

“The general thinking was that if you made a Hot Jupiter, you’d destroy any other terrestrial planets,” said Penn State astronomer Steinn Sigurdsson, one of the new study’s authors.

In 2002, Sigurdsson and grad student Avi Mandell ran a “crude simulation” of the formation of planetary systems that include Hot Jupiters in order to check this assumption.

“We found that a quarter of the time, ” Sigurdsson said, “the terrestrial planet actually survived.”

This winter, Sigurdsson and Mandell teamed up with Sean Raymond at the University of Colorado for a more thorough simulation.

“Previous simulations were only for a million years or less,” Raymond said, “so you really couldn’t tell if the [rocky] material was going to accumulate.”

Harnessing the power of a dozen computers, the team ran its simulation for eight months, the equivalent of 200 million years. 

The longer time frame paid off.

The team found that as Hot Jupiters spiral in toward the central star, their gravity flings rocky debris outward, where it can coalesce into planets. Meanwhile turbulent forces in the gas at the outer reaches of the planetary disc slow down icy bodies, causing them to spiral inward and collide with a proto-Earth-like planet. This planet could then settle into a “habitable zone” where the ice melts but the temperature is not so high that the water evaporates completely, said Sigurdsson.

These planets could look quite similar to Earth, but would contain up to 100 times more water.

“They’re ‘waterworld’ planets,” Raymond says, “probably covered in global oceans.

“On Earth, storms are big at sea and fizzle up when they get to land,” he continued. “So without land, we’re thinking the storms would be really big.”

This work, which will appear in the Sept. 8 issue of Science, estimates that over one-third of known planetary systems would have the right conditions to form such a planet. (The other two-thirds, according to Sigurdsson, have several Hot Jupiters, or Hot Jupiters that are “in eccentric orbits, bouncing around all over the system, so nothing would survive.”) The 54 planetary systems that might have Earth-like planets constitute a good “target list” for those looking for extraterrestrial life, Sigurdsson said.

Penn State atmospheric and planetary scientist James Kasting, who was not involved in this study, contends that other planetary systems are probably better bets for habitable planets—especially those orbiting the 93 percent of stars that don’t host Hot Jupiters.

“Those would still be our prime targets because they’re the ones that are most likely to be close analogs of our own solar system,” said Kasting.

Neil deGrasse Tyson, astrophysicist and director of New York’s Hayden Planetarium, “applauds” this study and any others that look for life in unexpected places.

“The more we learn about what kinds of solar systems there are, the wider is the net that we can cast in the search for life,” he said. “That’s quite encouraging, if you’re worried about whether we’re alone in the universe.”

Originally published September 7, 2006

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