Astronomers find embryonic planet (Forwarded)

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Anita Heward
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CONTACTS

Dr Jane Greaves
School of Physics and Astronomy
University of St Andrews
North Haugh
St Andrews
Fife KY16 9SS
Scotland
Tel: +44 (0)1334 463199

Dr Anita Richards
Jodrell Bank Centre for Astrophysics
Alan Turing Building
University of Manchester
Manchester M13 9PL
U.K.
Tel: +44 (0)161 275 4124

Dr Tom Muxlow
MERLIN/VLBI National Facility
Jodrell Bank Observatory
Cheshire SK11 9DL
U.K.
Tel: +44 (0)1477 571321 (switchboard) 572607 (direct line)

Dr Ken Rice
Institute for Astronomy
University of Edinburgh
Royal Observatory
Blackford Hill
Edinburgh
Scotland EH9 3HJ
Tel : +44 (0)131 668 8384

EMBARGOED UNTIL 0001 BST, 2 April 2008

Ref.: PN 08/28 (NAM 19)

Astronomers find embryonic planet

Using radio observatories in the UK and US and computer simulations, a team
of astronomers have identified the youngest forming planet yet seen. Team
leader Dr Jane Greaves of the University of St Andrews will discuss the
'protoplanet' in her talk at the RAS National Astronomy Meeting in Belfast
on Wednesday 2 April.

Taking advantage of a rare opportunity to use the Very Large Array (VLA) of
radio telescopes in the US with the special addition of an extra telescope
50 km away, the team studied the disk of gas and rocky particles around the
star HL Tau. This star is thought to be less than 100000 years old (by
comparison the Sun is 4600 million years old) and lies in the direction of
the constellation of Taurus at a distance of 520 light years. The disk
around HL Tau is unusually massive and bright, which makes it an excellent
place to search for signs of forming planets.

The VLA gives very sharp images of HL Tau and its surroundings. The team
studied the system using radio emission at a wavelength of 1.3 cm,
specifically chosen to search for the emission from super-large rocky
particles about the size of pebbles. The presence of these pebbles is a clue
that rocky material is beginning to clump together to form planets.

In the UK, scientists used the MERLIN array of radio telescopes centred on
Jodrell Bank in Cheshire, to study the same system at longer wavelengths.
This allowed the astronomers to confirm that the emission is from rocks and
not from other sources such as hot gas. Jodrell Bank scientists Dr Anita
Richards and Dr Tom Muxlow analysed the data.

The big surprise was that, as well as detecting super-large dust in the disk
around HL Tau, an extra bright 'clump' was seen in the image. It confirms
tentative 'nebulosity' reported a few years earlier at around the same
position, by a team lead by Dr Jack Welch of the Berkeley-Illinois-Maryland
Array. The new image shows the same system in much greater detail.

Dr Greaves comments, "We see a distinct orbiting ball of gas and dust, which
is exactly how a very young protoplanet should look. In the future, we would
expect this to condense out into a gas giant planet like a massive version
of Jupiter. The protoplanet is about 14 times as massive as Jupiter and is
about twice as far from HL Tau as Neptune is from our Sun."

Dr Richards adds, "The new object, designated HL Tau b, is the youngest
planetary object ever seen and is just 1 percent as old as the young planet
found in orbit around the star TW Hydrae that made the news last year. HL
Tau b gives a unique view of how planets take shape, because the VLA image
also shows the parent disk material from which it formed."

Team member Dr Ken Rice of the University of Edinburgh ran a computer
simulation to find out how such a massive protoplanet could form. His
animation shows a very similar body condensing out of a disk with similar
properties to that actually observed around HL Tau. The planet forms because
of gravitational instability in the disk, which is about half as massive as
the star itself. This allows small regions to separate out and cool down
into self-contained structures. This instability mechanism has been
controversial, but the simulated and real data are such a good match that it
seems the mechanism really does operate in nature.

Dr Rice comments, "The simulations were as realistic as we could make them
and we were delighted that the results compare so well with the
observations."

One intriguing property is that XZ Tau, another young star in the same
region, may have passed near HL Tau about 1600 years ago. Although not
required for planet formation, it is possible that this flyby 'tweaked' the
disk and helped it become unstable. This would be a very recent event in
astronomical terms. Whether the proto-planet formed in only the last few
hundred years, or sometime in the 100000 years since the birth of HL Tau,
the images provide a unique view of planet formation in action, and the
first picture of a protoplanet still embedded in its birth material.

IMAGES AND MOVIE:

Images of HL Tau and still and movie from simulation:
http://www.roe.ac.uk/~wkmr/HLTau/HLTau.html

CAPTIONS

Figure 1: The false colour image is a map of the radio emission (at a
wavelength of 1.3 cm) emitted from the region around the star HL Tau. The
candidate protoplanet is marked 'b'. The bar at top left (marked 50 AU)
indicates 50 times the Earth-Sun distance on the same scale, or about the
size of the orbit of Pluto. HL Tau is located in the centre of the image.
The star is surrounded by a dusty disc tilted to the line of sight; only the
inner part is visible here but its extent is indicated by the white ellipse.
The arrows show the direction of the jets of hot gas emitted as 'overspill'
from the star growth process. Image: J. Greaves (St Andrews) / T. Muxlow /
A. Richards (Jodrell Bank), Ken Rice (Royal Observatory Edinburgh), VLA and
Pie Town antenna

Figure 2: This is an image from the computer simulation of HL Tau and its
surrounding disk. In the model the dense clump (seen here at top right)
forms with a mass of about 8 times that of Jupiter at a distance from the
star about 75 times that from the Earth to the Sun. Image: Ken Rice (Royal
Observatory Edinburgh), J. Greaves (St Andrews), A. Richards / T. Muxlow
(Jodrell Bank)

Movie: Animation from the simulation showing the growth of the protoplanet
in the disc around HL Tau. The total time is about 3000 years and shows the
protoplanet forming at the edge of the disc, and then spiralling inwards to
a distance about 75 times further from the central star than the Earth is
from the Sun. Movie credit : Ken Rice (Royal Observatory Edinburgh)

FURTHER INFORMATION

* Jodrell Bank Centre for Astrophysics
http://www.jb.man.ac.uk
* University of St Andrews Astronomy Group
http://star-www.st-and.ac.uk/index.php/
* Royal Observatory Edinburgh
http://www.roe.ac.uk
* Science and Technology Facilities Council
http://www.stfc.ac.uk

NOTES FOR EDITORS

The VLA is operated by the National Radio Astronomy Observatory (USA) on
behalf of United Universities, Inc.

MERLIN is operated by the University of Manchester, Jodrell Bank
Observatory, on behalf of the Science and Technology Facilities Council
(STFC).

The simulation ran on a supercomputer funded by the Scottish Universities
Physics Alliance (SUPA).

The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen's
University Belfast. It is principally sponsored by the RAS and the Science
and Technology Facilities Council (STFC). NAM 2008 is being held together
with the UK Solar Physics (UKSP) and Magnetosphere, Ionosphere and
Solar-Terrestrial (MIST) spring meetings.