[meteorite-list] Only Solar Systems With Jupiters May Harbor Life, UA Scientist Says

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:06 2004
Message-ID: <200101292224.OAA17167_at_zagami.jpl.nasa.gov>

>From Lori Stiles, UA News Services
January 29, 2001

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

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

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

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

"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.

Jonathan I. Lunine
Received on Mon 29 Jan 2001 05:24:02 PM PST

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