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Original Date: Wed, 3 Jul 2002 14:46:40 +0200 (CEST)
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Monday, 1 July, 2002, 14:55 GMT 15:55 UK
Scientists estimate 30 billion Earths
There could be billions of Earths out there
By Dr David Whitehouse
BBC News Online science editor
Astronomers say there could be billions of Earths in our galaxy, the
Their assessment comes after the discovery of the 100th exoplanet - a
planet that circles a star other than our own.
The latest find is a gas giant, just like all the other exoplanets so
far detected, and orbits a Sun-like star 293 light-years away.
Scientists say they are now in a position to try to estimate how many
planets may exist in the galaxy and speculate on just how many could
be like the Earth. The answer in both cases is billions.
Virtually all the stars out to about 100 light-years distant have been
surveyed. Of these 1,000 or so stars, about 10% have been found to
possess planetary systems.
So, with about 300 billion stars in our galaxy, there could be about
30 billion planetary systems in the Milky Way alone; and a great many
of these systems are very likely to include Earth-like worlds, say
The 100th new planet circles the star HD 2039. It was found by
astronomers using the Anglo-Australian Telescope as part of the
Carnegie Institution Planet Search Program.
The Jupiter-sized world circles its star every 1,210 days at a
distance of about 320 million kilometres (200 million miles).
Astronomer Dr Jean Schneider, who compiles the Extrasolar Planets
Catalogue, told BBC News Online: "The 100th planet is symbolic and
"The first discoveries concentrated on short orbital periods because
of the limited timebase of observations. Now, we are learning more
about the statistics of long orbital periods and know to what extent
our own Jupiter is exceptional or not."
With the new world, astronomers say that they have just about finished
surveying all the Sun-like stars out to a distance of 100 light-years
Current planet detection technology - based on the "wobble" induced in
the parent star by the gravitational pull of the orbiting planet - can
only detect worlds about the mass of Saturn or larger. Earth-sized
worlds are too small to be seen.
But even in this "biased" survey of giants, the smaller worlds
predominate - which makes astronomers think that Earth-like worlds do
exist. They may even be as common as Jupiter-sized exoplanets.
And if stellar statistics gathered in our local region of space are
applied to our galaxy of 300 billion stars, then there may be 30
billion Jupiter-like worlds and perhaps as many Earth-like worlds as
Astronomers will have to wait for a new generation of space-based
telescopes incorporating advanced detectors before they can detect
Earth-sized worlds orbiting other stars.
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