The search for Earth-like life on other worlds should focus on solar systems with Jupiter-like planets, a University of Arizona scientist reports today in the Jan. 30th issue of the Proceedings of the National Academy of Sciences.
Jupiter-like planets flinging Mars-sized objects toward their sun-like stars would deliver the water needed for carbon-based terrestrial life, said Professor Jonathan I. Lunine of the Lunar and Planetary Laboratory, chair of the UA Theoretical Astrophysics Program.
That, evidence says, is what happened in our solar system, Lunine concludes.
"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.
As the solar system formed, Jupiter's powerful gravity perturbed asteroids to accrete into larger and larger objects - terrestrial "embyros" as big as Mars or bigger - then tossed them into very unstable elliptical orbits. Those that hit Earth when flung toward the inner solar system delivered the water that now fills Earth's oceans. That happened when Earth was about half its present size.
Lunine and Italian and French colleagues published in the November 2000 Meteoritics and Planetary Science their model of how planetary embryos supplied most of the Earth's ocean water. Authors on the article are Alessandro Morbidelli and Jean Petit of the Observatory de la Cote d'Azur, John Chambers of NASA Ames, Lunine of the UA, Francois Robert of the Paris Museum of Natural History, Giovanni Valsecchi of the Institute for Space Astrophysics (Rome), and Kim Cyr of NASA Johnson Space Center.
A solar system with water-bearing asteroids but no giant planets might not evolve habitable worlds with oceans, they conclude.
The deuterium-to-hydrogen ratio in Earth's seawater is the key clue as to the source of the oceans. Seawater contains 150 ppm deuterium, or heavy hydrogen. That's about five or six times the deuterium-to-hydrogen ratio found in the sun and in the solar nebula gas, known from measurements made at Jupiter. But it's only about a third of the deuterium-to-hydrogen ratio measured in comets Halley, Hyakutake, and Hale-Bopp,. The findings contradict the popular idea that comets supplied the Earth with oceans.
"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,'" Lunine said.
That Mars meteorites are richer in deuterium than Earth's seawater is consistent with the model. Lunine said. So is the scenario that Earth's moon was created when a Mars-sized object slammed into proto-Earth, an idea developed by UA planetary sciences Professor Jay Melosh and others, Lunine noted.
Astronomers in the past half decade have discovered that there are more planets outside our solar system than in it. They have found what may be giant gas planets at least as massive as Jupiter in orbit around 50 nearby stars. All of the newly found gas giants are closer to their stars than Jupiter is to the sun - some as close to their parent stars as Mercury is to the sun.
That giant gas planets exist in the inner solar system "has enormous implications for the frequency of habitable Earth-like planets in the galaxy," Lunine said.
The radial velocity observing technique used in the discoveries reveals planets by the Dopper effect of starlight. But the technique is blind to planets that may be farther out in their solar systems. Lunine has found in research he did with David Trilling of the University of Pennsylvania and Willy Benz, University of Bern, Switzerland, that for every giant planet detected close to a parent star, two or three giant planets orbit farther out, waiting to be discovered.
With no plausible theory of how objects more massive than Jupiter can form so close to their parent stars, theorists like Lunine have modeled the complicated story of how Jupiter-like planets might form far out in the solar system and migrate inward. The gist of the story is that some planets migrate all the way in and transfer all their mass to the sun and disappear. Others migrate only partway in before the gaseous disk disappears, at which time inward migration stops and terrestrial planets form from leftover rocky debris.
Jupiter, at about 5 astronomical units (AU) from the sun, is well beyond the "habitable zone," the region where liquid water is stable. (Earth is one astronomical unit 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," Lunine said. "But they could well be dry because the giant planet would have tossed water-bearing material away from the habitable zone."
Or, if the giant gas planet were very distant in the outer solar system, it likely would fling water bound in planetary embryos to a region too cold for life. And it would send too few water-bearing embryos in toward terrestrial planets at 1 AU, Lunine added.
"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.
Lunine is a member of a key project for a future space astrometry mission called SIM.
Astrometry, a technique that measures the motions of stars with extreme precision, will do a better job in finding Jupiter-like planets that are moderately distant from their parent stars than does the radial velocity technique. Astrometry will also give actual rather than minimum planet masses, unlike the radial velocity method.
Direct imaging is the ultimate technique for planet searches, however, because the spectra, or colors of light, from a planet reveal planetary atmospheres and history.
The UA-led Large Binocular Telescope consortium, the California-led Keck Telescope consortium, and Europe's impressive national giant telescopes are developing adaptive optics for the direct detection of extra-solar planets. Future space-based, very long baseline interferometers called Terrestrial Planet Finder and Darwin promise to be more powerful tools in planet searches.
"If you really want to discover another Earth, you've got to understand where the Jupiters are and what they've done to their solar systems over time," Lunine said. "You might find water vapor in the atmosphere of that second Earth, but you don't know if that water vapor is supported by an ocean that is a kilometer, 10 kilometers or 5 meters deep."
Lunine recently argued the case at a workshop for participants in the Terrestrial Planet Finder (TPF) project. UA collaborates with Lockheed Martin to develop a winning design for TPF, a space observatory that NASA plans to launch in 2012 as part of Origins Program.
Lunine and other UA scientists working on TPF, including Nick Woolf and Roger Angel of Steward Observatory, propose a precursor project to TPF for direct mapping of Jupiter-like planets.
Contact: Jonathan I. Lunine 520-621-2789 jlunine@lpl.arizona.edu
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From Lori Stiles, UA News Services
