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Hot Jupiters do not rule out alien Earths

Habitable, Earth-like planets can form even after giant planets have barrelled through their birthplace on epic migrations towards their host stars, new computer simulations suggest. The finding contradicts early ideas of how planets behave and suggests future space missions should search for terrestrial planets near known "hot Jupiters".

Many of the 160 or so known extrasolar planets are hot Jupiters - massive planets that are closer to their stars than Mercury is to our Sun. But the planets probably did not form in these scorching regions because there would not have been enough gas and dust there to amass such giant worlds.

That suggests the planets formed farther out and migrated towards their host stars as they lost energy due to friction with gas in their natal discs. But that migration was thought to prevent the formation of terrestrial planets. That is because the roving giants would hit or fling their rocky building blocks out of the planetary system, says Avi Mandell of Pennsylvania State University in University Park and NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.

But recent computer simulations have shown that up to half of the rocky bodies could survive in the planetary system during the 100,000 years it takes for the giant planet to migrate in to the star. Now, Mandell and colleagues have performed computer simulations to see how those rocky objects evolve over 200 million years. He presented the initial results this week at an astrobiology meeting in Washington DC, US.

Crazy orbits

Initially, the migration of the giant planets throws the objects out of the plane of the planetary system and moves them from circular to elliptical orbits. But then gas in the planet-forming disc "forms a cushion, damping the effect of these crazy, chaotic orbits," he told New Scientist.

The objects then move back into flatter, more circular orbits and form terrestrial planets. Some of these planets orbit within the "habitable zone" - a region around the star where liquid water can exist and which is therefore a potential haven for life.

Without that damping effect, the planets' elongated orbits would send them close to and then far from the Sun, and the resulting extremes of temperature would make the formation of life difficult, Mandell says. The migration and damping also provide other aids to the development of life by mixing up different parts of the disc, delivering water and other volatiles to the terrestrial planets.

"It's not as favourable as having no migration," says Mandell. "But with the right conditions, you can get giant planets in the interior and habitable terrestrial planets on the exterior" of the planetary system.

Stopping migration

"It's very promising for the formation of habitable planets," agrees Nader Haghighipour of the University of Hawaii in Manoa, US. But he says the work is still preliminary and cautions that scientists do not yet fully understand the planetary migration process. "We still don't know what is a realistic mechanism for stopping the migration," he points out.

Mandell agrees and plans to vary where the giant planet stops in future simulations. He will also test out discs with different ratios of gas to dust, since the amount of gas is crucial to the damping effect.

But he says the preliminary work "shows there's a strong probability" that terrestrial planets formed in systems with migrating planets and suggests future space missions search for them around hot Jupiters.


Hot Jupiters are huge planets that orbit nearer their stars than our closest planet, Mercury, orbits the Sun (Artist's impression: NASA)


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