Planet search results suggest our Solar System may be uncommon
Andrew Yee
Contact:
B. Scott Gaudi, (614)292-2076; Ga...@astronomy.ohio-state.edu
Andrew Gould, (614)292-1892; Go...@astronomy.ohio-state.edu
Written by Pam Frost, (614)292-9475; Fros...@osu.edu
January 12, 2000
PLANET SEARCH RESULTS SUGGEST OUR SOLAR SYSTEM MAY BE UNCOMMON
COLUMBUS, Ohio -- An Ohio State University astronomer is helping to
answer a question mankind has asked since Copernicus first proclaimed
that Earth orbited the sun: could many other stars have planetary systems
like ours?
The answer may not please those who believe such planetary systems are
required for the existence of extraterrestrial intelligence.
"Our results seem to indicate that whatever happened when our solar
system formed was not the norm," elsewhere in the galaxy, said B. Scott
Gaudi, a graduate student in astronomy at Ohio State.
Gaudi has developed a new method for zeroing in on the likelihood that
extra-solar planets exist. He and his colleagues at the Probing Lensing
Anomalies NETwork (PLANET)
[http://www.astro.rug.nl/~psackett/NVWS/MicroPLANET.html]
collaboration have calculated that less than 45 percent of stars could
harbor planets in a configuration similar to our solar system.
"While a couple of dozen extra-solar planets have been detected by other
collaborations, these systems are very unlike our own, and probably not
capable of sustaining advanced life forms," said Andrew Gould, associate
professor of astronomy and Gaudi's advisor. "What Scott has shown is
that when you look for systems we Earthlings would feel comfortable
in, they are not that easy to find."
At the biannual meeting of the American Astronomical Society in Atlanta,
Gaudi presented an analysis of two years' worth of PLANET data.
The eight institutions that make up the PLANET collaboration, including
Ohio State, watch the skies for signs of gravitational lensing -- what
happens when a massive dark object in space such as a dim star crosses
in front of a luminous source star in the background. The dark object
bends light rays emanating from the source, like a lens or magnifying
glass.
Here on Earth, we see the star get brighter as the lens crosses in
front of it, and then fade as the lens gets farther away. This is what
astronomers call a "lensing event."
Gaudi explained that the collaboration's data closely matches
astronomers' theories of what lensing events should look like. Given
that, planets orbiting stars should reveal themselves as a warp in the
gravitational lens -- a "blip" of extra brightness during a lensing event.
Other planet hunters use different techniques that can only detect
planets as close to a star as Mercury is to ours, whereas gravitational
lensing enables PLANET to see Jupiter-like planets at Jupiter-like
distances from stars.
They haven't seen any blips yet. In its five years of operation, PLANET
hasn't detected any planets.
"If you look really hard to find something, and you don't find it, there
are two possible reasons why: either you're not looking hard enough, or
it's not there," Gaudi said.
"The difference between these two possibilities depends on the
efficiency with which gravitational lensing should reveal the existence
of planets given their size, distance, and angle of alignment with a star
as viewed from earth," he continued.
Without any detections, PLANET cannot point to an exact number of
planets that do exist, but can put an upper limit on the number of such
planets that can exist. Based on 23 lensing events that occurred in
1998 and 1999, Gaudi calculated that less than 30 percent of stars
could have a Jupiter-like planet at distances between Earth's orbit
and Jupiter's orbit.
"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."
With the current data, PLANET cannot put limits on how many identical
twins of Jupiter exist. But the fact that PLANET hasn't found any
planets three times the size of Jupiter or bigger at Jupiter-like
distances suggests that Jupiter-like planets in general are not very
common, Gaudi said.
"You might expect that if we have a Jupiter, then other solar systems
probably have Jupiters three or even five times bigger," he said. "But
the fact that we're not detecting any planets bigger than Jupiter
probably indicates that there aren't many Jupiters out there."
"We're going to continue to push that limit down farther," Gaudi said.
"Either we'll push it down to 10 percent, or we'll find something."
Other PLANET member institutions include: the Kapteyn Institute,
Rijksuniversiteit Groningen, The Netherlands; Space Telescope
Science Institute, Baltimore, Maryland; South African Astronomical
Observatory, Capetown, South Africa; University of Canterbury,
Christchurch, New Zealand; Perth Observatory, Bickley, Western
Australia; and Canopus Observatory, Hobart, Tasmania, Australia.
PLANET operates telescopes at many different longitudes around the
world, so members can watch the sky at all times, weather permitting.
"We like to say the sun never rises on our collaboration," Gaudi smiled.
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Andrew Yee
ay...@nova.astro.utoronto.ca