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Three planets have been found around
distant stars by scientists from the Anglo-Australian Observatory
and nine institutions in the UK and USA, using a new high-precision system
on the 4-m Anglo-Australian Telescope (AAT) near Coonabarabran, NSW. They
are the first planets to be discovered from Australia.
Forty-six other extrasolar planets
have been found since 1995. None are believed to be capable of supporting
life.
Most planet searches have been able
to find only planets more massive than Jupiter, the largest planet in our
Solar System. “As a result searches are picking up all the weird giant
planets first,” says team leader Dr Chris Tinney of the Anglo-Australian
Observatory.
The new planets were found around nearby
stars within 150 light-years of Earth.
The
smallest is a kind planet hunters call a `hot Jupiter’. It has a mass at
least 84% that of Jupiter’s but lies scorchingly close to its parent star,
far closer than Mercury does to the Sun. Its `year’ is a mere three Earth
days.
The middle-weight planet lies in an
Earth-like orbit inside the `habitable zone’ where liquid water could exist.
But the planet itself is not Earth-like: weighing at least 1.26 Jupiter
masses it is almost certainly a Jupiter-like gas giant. It takes a leisurely
426 days to complete the voyage around its star, epsilon Reticulum in the
constellation of the Net.
The third planet is another gas giant,
of at least 1.86 Jupiter masses. Its orbit extends just a bit further from
its star than Mars does from the Sun and it takes 743 days to crawl around
its star, mu Ara, in the constellation of the Altar.
Since 1998 the AAT search has looked
at 200 nearby stars in the southern sky. There are probably more planets
in the pipeline, says Dr Tinney.
“In three years you can catch only
the short-period planets,” he explains. “To pick up ones with longer orbits
you have observe for a few more years.”
The AAT search complements searches
of the northern sky being done by veteran planet hunters Drs Geoffrey Marcy,
Paul Butler and Michel Mayor.
Both these and the AAT search use the
`Doppler wobble’ technique. As an unseen planets orbits a distant star
it tugs on it, causing the star to move back and forth in space. That wobble
can be detected by the `Doppler shift’ it causes in the star’s light.
“The AAT search is the most sensitive
search in the Southern Hemisphere,” says team member Dr Alan Penny of Rutherford
Appleton Laboratory in the UK. “It can detect planets moving at only 10
ms-1 – the speed of a world-class sprinter.”
The precision comes from simple glass
tube containing specks of iodine, and “a bunch of clever software” written
by Dr Paul Butler, says Dr Tinney.
Heating the glass cell turns the iodine
to a purple gas. Starlight passing through the gas has its spectrum modified.
This `reference’ spectrum is then compared with unmodified starlight. “This
helps us get much of the junk out of the spectrum,” Dr Butler explains.
Seeing wobbling stars directly is the
next step in planet hunting. That job will fall first off to the Very Large
Telescope Inteferometer (VLTI) now being built in Chile and NASA’s Space
Interferometry Mission (SIM), due to launch in 2009. SIM will spend five
years probing nearby stars for Earth-sized planets. “The AAT will provide
target lists for the VLTI and SIM,” says Dr Tinney.
Is it worth finding more planets? Absolutely,
says Dr Butler. “It will be at least five years before we find enough planets
to even begin making sensible guesses about the whole population out there.”
But the planets found to date are so
different from those in the Solar System that theories of planet formation
have been “turned on their head,” he adds.
The members of the AAT planet search
team are: from Australia, Dr Chris Tinney (Anglo-Australian Observatory);
from the UK, Drs Hugh R. A. Jones (Liverpool John Moores University), Alan
J. Penny (Rutherford Appleton Laboratory) and Mr Kevin Apps (University
of Sussex); and from the US, Drs R. Paul Butler (Carnegie Institution of
Washington), Geoffrey W. Marcy (University of California Berkeley), Steven
S. Vogt, (University of Colorado and University of California Santa Cruz)
and Gregory W. Henry (Tennessee State University).
The Anglo-Australian Observatory is
funded by the UK’s Particle Physics and Astronomy Research Council, in
the UK, and by the Australian Government.
more information
12 Dec. (Please contact by e-mail therafter to arrange interview).
The AAT searchers also found a single
‘brown dwarf’ – a small ‘failed’ star – in orbit around HD 164427, one
of their target stars.
The results for HD 179949 and HD 164427
have been accepted for publication by the Astrophysical Journal and are
available at http://arXiv.org/archive/astro-ph
. Results for mu Ara and eps Reticulum are being prepared for publication
(see images below).
Details of new objects
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(in Sagittarius) |
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mu Ara |
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epsilon Reticulum |
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(in Telescopium) |
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Images
General images of the AAT can be found at http://www.aao.gov.au/images/general/aat.html.
