Artist's impression of a habitable planetary system with a Hot Jupiter, compared with our Solar System (upper planets).
Credit: University of Colorado/NASA
SYDNEY, 8 September 2006: More than one-third of the giant planet systems recently detected outside our Solar System may harbour Earth-like planets - many covered in deep oceans with the potential to support life, a U.S. study suggests.
The study, published today in the U.S. journal, Science, focuses on a type of planetary system that contains gas giants known as 'Hot Jupiters' orbiting extremely close to their parent stars - even closer than Mercury to our Sun.
According to the study, these gas giants migrated inward toward their parent stars as the planetary systems formed, disrupting the space environment and triggering the formation of ocean-covered, Earth-like planets in a habitable zone conducive to the evolution of life.
"These gas giants cause quite a ruckus," said Sean Raymond of the University of Colorado in Boulder, one of the authors of the study. "We now think there is a new class of ocean-covered, and possibly habitable, planets in solar systems unlike our own."
The study suggests that Hot Jupiters push and pull proto-planetary disk material during their journeys, flinging rocky debris outward where it is likely to coalesce into Earth-like planets. At the same time, turbulent forces from the dense surrounding gas slow down the orbits of small, icy bodies in the outer reaches of the disk, causing them to spiral inward and deliver water to the fledgling planets. Such planets may eventually host oceans several kilometres deep.
Scientists had previously assumed that as Hot Jupiters ploughed through proto-planetary material on their inward migrations toward parent stars, all the surrounding material would be 'vacuumed up' or ejected from the system. "The new models indicate these early ideas were probably wrong," said Raymond.
The research team ran exhaustive computer simulations of the formation and behaviour of Earth-sized planets, starting with proto-planetary discs containing more than 1,000 Moon-sized, rocky and icy bodies. The initial conditions were based on current theories of how planets form in our own Solar System, and each simulation modelled about 200 million years of planetary evolution.
The team concluded that about one out of every three known planetary systems could have evolved as-yet-undetected Earth-like planets in habitable zones similar to the Earth's. A whopping 40 per cent of the 200 or so known planets around other stars are Hot Jupiters, although the percentage will probably decrease as more distant planets are discovered, said Raymond.
In addition to Earth-like planets that form in habitable zones outside Hot Jupiters, the simulations showed some rocky planets - known as hot Earths - often form inside the orbits of Hot Jupiters. In fact one of these 'Hot Earths', with a radius twice that of our own Earth, was discovered in 2005 in a nearby star system, orbiting just 3.2 million km from its parent star.
These new simulations showed that both Hot Earths and Earth-like planets in habitable zones formed with large amounts of water - up to 100 times the water present on Earth today. However the models also indicate such water-rich planets would probably contain a lower percentage of iron than Earth - which may pose a barrier to the evolution and oxygenation of atmospheres, said Raymond.
According to the team's simulations, Hot Earths can form astoundingly fast, in just 100,000 years or so. Earth-like planets in habitable zones form much more slowly, taking up to 200 million years, said Raymond. Geologists believe Earth took about 30 million years to 50 million years to fully form.
The new research effort may allow planet hunters to narrow their search for habitable planets, according to Raymond. "I think there are definitely habitable planets out there," he said. "But any life on these planets could be very different from ours. There are a lot of evolutionary steps in between the formation of such planets in other systems and the presence of life forms looking back at us."
With the University of Colorado
