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Daily University Science News
The search
for Earth-like life on other worlds should focus on solar systems
with Jupiter-like planets, a University of Arizona scientist reports
in today's 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 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 Doppler 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 such as 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
atmosphere 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 extrasolar
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 the 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. - By Lori Stiles
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