Are Solar Systems Like Ours Rare?
- Source: SpaceViews
January 30, 2000
Are Solar Systems Like Ours Rare?
While astronomers continue to discover possible new extrasolar
planets, the lack of discoveries from another planet search project
may mean that solar systems like our own may be relatively rare in the
Earlier this month, NASA astronomers reported the possible discovery
of a gas giant planet orbiting far from the star HD 163296, a star
twice as massive as the Sun and located 400 light-years away.
The existence of the planet is inferred from the discovery of a gap in
the dust disk surrounding the star, as seen in images from the Hubble
Space Telescope. Astronomers Carol Grady and David Devine of NASA's
Goddard Space Flight Center say the gap in the disk could be explained
by a single planet 1.3 times the mass of Saturn forming there.
The discovery is unusual because the gap is located about 200
astronomical units (30 billion km, 18.6 billion mi.) from the star. By
comparison, Neptune, the most distant gas giant in our solar system,
is nearly seven times closer to the Sun.
"The potential planet is much further away from its star than any
known bodies in the plane of our solar system, and it is forming much
faster than most models predict," said Grady.
While the astronomers could not see the inner portion of the disk,
jets of material being ejected by the star include clumps that are
being emitted at five-year intervals -- suggestive of a planet forming
closer to the star in a five-year orbit.
If a large planet is forming in such an orbit, it might be the
exception rather than rule for extrasolar planetary systems. Two
astronomers at Ohio State University, using data from another planet
search project, think that solar systems like our own -- with gas
giants at moderate distances from their parent stars -- may be rare.
B. Scott Gaudi and Andrew Gould attempted to look for gas giants
around other stars by monitoring microlensing events, when a
gravitational lens -- a dark, massive object like a dim star -- passes
between the Earth and a distant star. The lens makes the star appear
to brighten briefly.
If a large planet orbits that star, the light it reflects from its
star will also be magnified, and will appear as a blip in the
otherwise smooth lightcurve of the star. This allows astronomers the
possibility to detect gas giant planets at greater distances from
their stars than possible currently with the radial velocity
technique, which has discovered over two dozen large planets orbiting
near their parent stars.
To date, however, Gaudi and Gould have yet to detect such a blip. They
monitored 23 microlensing events in 1998 and 1999 and found no blips,
which led Gaudi to calculate that fewer than 30 percent of stars have
Jupiter-sized planets at moderate distances from their stars.
"Radial velocity surveys are finding that about 10 percent of stars
have planets close in," said Gaudi. "We can say that most don't have
Jupiter-mass planets in the distance between Earth's orbit and
Jupiter's. So more often than not there aren't Jupiter-mass planets
around these stars at those distances."
Data from a microlensing event, MACHO-97-BLG-41, has been used to
claim the discovery of a planet three times the mass of Jupiter
orbiting a binary star system. But the Ohio State astronomers say even
this discovery is suspect.
"MACHO 97-BLG-41 is an ordinary binary without a planet," said Gould.
The unusual lightcurve of that event, which was explained by one team
of astronomers as caused by a planet, can also be explained by the
rotation of the binary star system without a planet.
A lack of "Jupiters" could have implications on the number of star
systems with planets capable of supporting life. Past research has
suggested that the formation of gas giants like Jupiter may be
important to for the formation of life on Earth, since Jupiter's
massive gravity can help keep the inner solar system clear of comets
and asteroids that would otherwise strike the Earth.
"What Scott [Gaudi] has shown is that when you look for systems we
Earthlings would feel comfortable in, they are not that easy to find,"
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