Looking for Earth-like planets

By Robert Adler and Henry Bortman

New Scientist

09.23.99

Our first glimpses of Earthly worlds beyond the Solar System
might just be looming into focus, say two teams of astronomers.
Independently they have found two promising candidates for
low-mass planets that could play host to life.

Over the past few years, astronomers have found more than a
dozen extrasolar planets by looking for stars that "wobble" due to
the gravity of planets that orbit them. But this technique only
picks up very massive planets -- gas giants like Jupiter. These
are unlikely to be hospitable to life. "We live on a low-mass
planet," says David Bennett, an astrophysicist at the University of
Notre Dame in Indiana. "Those are the ones you need to look
for."

In their quest for low-mass planets, Bennett and his colleagues
have been watching the way the gravity of stars magnifies the
light from objects behind them, an effect called gravitational
lensing. When two stars line up with the Earth, the gravity of the
one nearest us can focus and brighten the one behind for weeks.
If the nearer star has an orbiting planet, it could add a short extra
brightness blip as it passes across our line of sight.

That's what Bennett's team thinks it saw on 4 July last year. In a
paper submitted to The Astrophysical Journal, they conclude
that a planet caused a 2.5-hour-long blip in the brightness of a
star near the centre of our Galaxy. From the size and timing of
the blip, they calculate that the planet may be as light as a few
Earths, and orbits its star in the inner zone where rocky planets
are likely to form.

"The method is fantastic," says Keith Horne, an astrophysicist at
the University of St Andrews. But he cautions that Bennett's
finding must be corroborated by other observations of the event:
"It's an intriguing finding but I wasn't quite convinced."

Another way to search for Earth-sized planets is to look for tiny
changes in the brightness of a star as a planet passes in front of
it (New Scientist, 18 September, p 32). Since 1994, a team led
by Laurance Doyle of the SETI Institute in Mountain View,
California, and Hans-Jörg Deeg of the Institute of Astrophysics in
the Canary Islands, has been attempting to catch this via a
network of telescopes round the world. The team has created
computer simulations of the way a faint double star system 55
light years away called CM Draconis should brighten and fade
when planets with various sizes and orbits pass in front of it.

Observed brightness changes in CM Draconis matched one of
these patterns, suggesting the system may host a planet 2.5
times the size of Earth. "It would receive the equivalent energy
that the Earth receives from the Sun," says Doyle. "It's smack in
the middle of the habitable zone." The astronomers say the
brightness of the system should change in a predictable way in
early October. Then they will be able to say for certain whether
the discovery stands up.

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