"Hot Jupiters" Could Give Rise to Earthlike Worlds, Study Says

Anne Minard
for National Geographic News
September 7, 2006
It's more likely than ever that we are not alone in the universe, new research suggests.

The latest computer models are telling scientists that more than a third of the star systems containing Jupiterlike gas giants may also harbor Earthlike planets.

These so-called habitable exoplanets could be awash in oceans of liquid water, which means they might support life (related news: "Are Neighborhood Aliens Listening to Earth Radio?" [September 7, 2006]).

The latest work focuses on a type of star system that contains gas giants known as hot Jupiters.

Unlike gas giants in our solar system, hot Jupiters have orbits that swing tightly around their stars, says Sean Raymond, study co-author and astrophysicist at the University of Colorado in Boulder.

Scientists believe that hot Jupiters initially form far from their host stars. Over time the gas giants migrate inward due to the irregular twisting motions of the gaseous disks in which they formed.

As they move into their near-star orbits, hot Jupiters could be playing violent games of planetary billiards that produce Earthlike planets, he says.

Big Bullies

In general, massive gas giants have a reputation for slinging things around in space.

Our Jupiter (Hubble image) is capable of hurling asteroids out of the solar system or into the sun and other planets by the sheer force of its gravity.

"These gas giants cause quite a ruckus," Raymond said.

Ten years ago, when scientists detected the first hot Jupiter, they assumed that as the giant exoplanets plowed through debris during their inward migrations, any surrounding material would be similarly ejected.

"The new models indicate these early ideas were … completely wrong," Raymond said.

He and his colleagues now think that such planets possibly only shake up debris in the habitable zone—the region at the right distance from a star where liquid water, necessary to life as we know it, could exist.

This disrupted debris could coalesce into Earthlike planets (related news: "Many 'Earths' Are Out There, Study Says" [April 2005]).

At the same time, small icy bodies from farther out in the star system also could have spiraled inward, delivering water to the fledgling planets.

"We now think there is a new class of ocean-covered—and possibly habitable—planets in solar systems unlike our own," Raymond said.

The simulations also showed that rocky planets known as hot Earths may often form when hot Jupiters push material forward during their inward treks.

But hot Earths, which can be up to five times bigger than our Earth, orbit closer to their stars and are not likely to support life.

Even if water does contribute to their formation, most hot Earths probably end up dry, study co-author Raymond says.

"We don't think that they're really good places to harbor life, if you need liquid water on the surface [to support life]."

Raymond and colleagues at Pennsylvania State University in University Park present their research in tomorrow's issue of the journal Science.

Telltale Wobble

For now, inhospitable gas giants appear to make up the majority of known planets outside our solar system.

Astronomers have detected about 200 exoplanets, and 40 percent of them are hot Jupiters with orbits tighter than Mercury's.

But the sample is biased, says study co-author Avi Mandell, an astrophysicist at Penn State and a researcher with the NASA Goddard Space Flight Center in Greenbelt, Maryland.

That's because astronomers can't actually see the distant planets. They're mostly relying on the so-called Doppler wobble, the gravitational tug of the unseen planets on their host stars.

The wobble decreases—and becomes harder to see—when it's caused by small planets that orbit farther away.

As space scientists build ever bigger and more powerful telescopes, Mandell thinks higher percentages of small and distant planets will be found.

The new findings will help narrow the search for life-supporting planets, the researchers write—just in time for NASA's upcoming Terrestrial Planet Finder mission and the European Space Agency's Darwin mission, which are dedicated to that effort.

But other researchers are more cautious.

Nick Woolf, an astronomer at the University of Arizona in Tucson, points out that the only certain planetary system we've actually seen is our own.

"Our understanding of the processes that occur in the formation of a planetary system," he said, "is rudimentary."

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