Jupiter as seen by the Cassini spacecraft. Photo: NASA/JPL/University of Arizona

If you're looking for solar systems with Earth-like planets that could harbor life, one scientist believes you should first look for planets with the mass and orbit of Jupiter that could nurture smaller worlds.

In a paper published in a special astrobiology issue of Proceedings of the National Academy of Science, Jonathan Lunine of the University of Arizona argues that Jupiter's powerful gravity may have flung massive protoplanets towards Earth early in the solar systemıs history, delivering the water required for life to form.

That conclusion is based on measurements of the ratio of deuterium, a heavy isotope of hydrogen, to regular hydrogen. The "D-to-H" ratio of the water in Earth's oceans is six times larger than the interstellar medium, but is three times smaller than the ratio measured in comets like Halley and Hale-Bopp, which had been considered a likely source of the Earthıs water.

The D-to-H ratio on Earth, however, closely matches ratios measured in meteorites. Lunine suggests that in the early history of the solar system, Jupiter's powerful gravity perturbed the orbit of asteroids, causing them to accrete into massive protoplanets the size of Mars. Close approaches by these bodies to Jupiter flung them into orbits that took them into the inner solar system and raised the possibility of an impact with the still-forming Earth.

Those impacts, Lunine argues, delivered the water that exists on Earth today. It also meshes with existing models for the formation of the Moon, which call for the impact of a Mars-sized planetesimal with the proto-Earth.

"If deuterium abundances in the asteroid belt are correctly reflected by the meteorites, planetary embryos sent careening by Jupiter into the Earth are by far and away the biggest contribution to Earth's water," he said.

Some scientists had argued that Jupiter was essential to the formation of life on Earth because its gravity helped clear out the debris left over from the formation of the solar system and deflect asteroids and comets that could pose a threat to the Earth in modern times. "The bottom line is, the asteroid belt certainly had much more material when the solar system was forming than it does today, and Jupiter was responsible for clearing most of that material out," he said.

What's true for this solar system could apply for the dozens of other solar systems discovered by astronomers over the last several years. However, the location of massive gas giants is also critical, according to Lunine, which could spell trouble for those solar systems with massive planets orbiting far closer to their stars than Jupiter, which orbits over 5 astronomical units (AUs) from the Sun.

"If giant planets existed closer to a star than 5 AU -- say, at 3 AU -- there would still be terrestrial planets in stable orbits," he said. "But they could well be dry because the giant planet would have tossed water-bearing material away from the habitable zone."

On the other hand, gas giants much farther from the star than Jupiter is from the Sun -- or a lack of gas giants altogether -- could also spell trouble for habitable Earth-like worlds. "In that case, you might end up with a big but icy terrestrial planet at 4 or 5 AU: too cold to support life as we know it," he said.

While missions like NASA's Terrestrial Planet Finder, a space-based telescope proposed for launch in 2012, could directly detect Earth-like extrasolar Earths, near-term efforts using ground- and space-based telescopes to directly detect other Jupiters could also prove useful. "If you really want to discover another Earth," said Lunine, "you've got to understand where the Jupiters are and what they've done to their solar systems over time."