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CX2SA > SCIENC 17.11.05 06:47l 86 Lines 4636 Bytes #999 (0) @ WW
BID : 61165_CX2SA
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Subj: Polarised light may reveal..
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Sent: 051117/0536Z @:CX2SA.LAV.URY.SA #:61165 [Minas] FBB7.00e $:61165_CX2SA
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To : SCIENC@WW
Polarised light may reveal hidden exoplanets
============================================
Scattered starlight may soon reveal the presence of extrasolar planets that
cannot be detected by any other means, according to a pair of scientists in
India. But some other experts say the method is best suited to studying the
properties of known exoplanets - not turning up new discoveries.
Astronomers have already discovered about 155 extrasolar planets by watching
how they make their host stars wobble or dim as they circle around them. But
these methods are best suited to detecting so-called "hot Jupiters" - giant
planets that orbit close to their stars, leaving any smaller or more distant
planets unseen.
Now, Sujan Sengupta and Malay Maiti of the Indian Institute of Astrophysics in
Bangalore say astronomers may soon be able to detect these elusive worlds too -
providing they have atmospheres. Light bouncing off particles in their
atmospheres should become linearly polarised, with its electric field aligned
in one plane, they say.
Other researchers have modelled the polarisation signal expected from large,
spherical planets travelling in tight circles around their host stars. But
Sengupta and Maiti have now expanded these calculations to include planets that
are slightly squashed at their poles because of their own spin - like Jupiter -
as well as planets that are travelling in elliptical orbits around their host
stars, which are less likely to make their stars wobble or blink by passing in
front of them.
Size isn't important
--------------------
They found that non-spherical planets should actually produce a stronger signal
because the polarisation averaged over the surface of a perfectly round object
tends to cancel itself out.
Because the polarisation signal depends strongly on the angle between the star
and planet, the researchers also argue that detecting any periodic change in
polarisation implies the presence of a planet.
"It's not the amount of polarisation but the systematic time variation that
will be important in detecting exoplanets," Sengupta told New Scientist. He
says the method should work regardless of the planet's size, mass and distance
from the star - and even with relatively imprecise polarisation measurements.
"According to my proposal, polarisation can detect exoplanets even if they
cannot be detected by any other method," he says, adding that it should even
pick up signs from Earth-like planets.
Doubts over detection
---------------------
But other researchers say the method may not be as robust as Sengupta believes.
"I think there's a good chance to make a detection, but it's only the hot
Jupiters that are going to be detected easily," says Sara Seager of the
Carnegie Institution of Washington in Washington DC, US. She says the nearness
of these large planets to their host stars means they will scatter more photons
of starlight.
She also cautions starlight is likely to scatter multiple times inside a
planet's atmosphere, potentially "washing out" the polarisation signal. "But
beyond making a detection, polarisation is a powerful method for determining
what is in a planet's atmosphere," she says, adding that such measurements of
Venus from Earth revealed it to have sulphuric acid clouds.
Jim Hough, an astronomer at the University of Hertfordshire, UK, agrees. "I
think the method is very important not for finding planets but for determining
the inclination of their orbit, radius, surface reflectivity and properties of
their atmospheres," he told New Scientist.
He has been searching for the polarisation of a known hot Jupiter around the
star Tau Bootis but has so far not been able to find it. The planet is probably
so massive and so close to the star that it is "stirring up the stellar surface
and producing additional polarisation," he says.
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