Download P38.Planet Sedna

On 15 March 2004, astronomers
Mike Brown (California Institute of
Technology), Chad Trujillo (Gemini
Observatory), and David Rabinowitz
(Yale University), announced their
discovery of Sedna (2003 VB12),
the coldest, most distant object
known to orbit the Sun. How did
they search and what did they find?
Artist’s impression of Sedna: R. Hurt – SSC-Caltech, JPL-Caltech, NASA
inding Sedna: Since autumn 2001,
F
AU: astronomical unit, the
mean distance from the
Earth to the Sun, about 150
million kilometres.
Oort Cloud: a hypothetical
shell of icy proto-comets that
orbit around the Sun,
extending almost halfway to
the nearest star. Sometimes
passing stars cause slight
changes in the proto-comets’
orbits, sending them into the
inner solar system where we
see them as comets.
Kuiper Belt: a band of
comets or icy asteroids that
lies between about 35 and
1 000 AU, in the same plane
as the planets.
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astronomers have been surveying the
outer solar system using the Palomar
QUEST (Quasar Equatorial Survey Team) camera
and the Samuel Oschin Telescope at Palomar
Observatory in southern California. To find
objects, they take three pictures of a small
region of the night sky over three hours and
look for something that moves. (Satellites,
planets, asteroids, and comets seem to move;
stars and galaxies appear stationary.)
“Vast areas of the sky have to be searched
before something this unusual is found,” says
Brown. The team was lucky. After looking at
only 15% of the sky, they found Sedna in
November 2003. Improvements in technology –
more sensitive digital cameras and faster
computers to analyse data – allow astronomers
to find more objects, further away, than they
could even five years ago.
Name: Sedna, the Inuit (Eskimo) goddess of
the sea from which all sea creatures were
created, is the name the astronomers propose
for this icy body. Its temporary ‘scientific’ name
is 2003 VB12. The International Astronomical
Union (IAU) Committee on Small Body
Nomenclature, responsible for solar system
names, needs to approve the name ‘Sedna’.
Position: Sedna is twice as far from the Sun as
any other object in our solar system – three times
further than Pluto, 90 times further than the Earth,
and beyond the reach of almost all amateur
astronomers. Situated in the Oort cloud, its current
position is 90 AU (see box). “Standing on the
surface of Sedna, you can block the entire Sun
with the head of a pin held at arm’s length,”
explains Brown.
Orbit: Sedna’s orbit is extremely elliptical,
unlike that of closer planets, and could take it
out more than ten times further away from
Earth than it is now. Its unusual orbit may be
caused by passing stars or perhaps by a larger
body (not yet found), about the size of Mars or
bigger, existing around 70 AU.
Sedna will become closer (76 AU at its
‘perihelion’ or ‘closest distance to the Sun’) and
brighter over the next 72 years before starting
its 10 500-year trip to the far reaches of the
solar system and back again. “The last time
Sedna was this close to the Sun, Earth was just
coming out of the last ice age; the next time it
comes back, the world might again be a
completely different place,” observes Brown.
Rotation: Sedna rotates slowly – probably once
every 40 days or so (only Mercury and Venus are
known to rotate more slowly). This could be the
Discovering Planet(oid?) SEDNA
effect of a moon (to be confirmed, astronomers
hope, by Hubble Space Telescope observations).
Size: Astronomers cannot measure the size of
Sedna directly from the point of sunlight they see
on images reflected back to Earth from Sedna’s
surface: a small icy object and a large dark object
would reflect the same amount of light. They do
know that Sedna is about 1 800 km in diameter
because, if it were any bigger, they could have
seen it with the 30-metre IRAM telescope or the
Spitzer Space Telescope.
Classification: The discoverers of Sedna
believe it is not a planet but a ‘planetoid’, that
is, a round object in the solar system that is not
big enough to be considered a planet. Brown
defines a planet as “any body in the solar
system that is more massive than the total mass
of all of the other bodies in a similar orbit. For
example, many asteroids cross the orbit of the
Earth. Yet the Earth is more massive than all of
those put together. Thus the Earth is a planet.”
By this definition, Pluto is also not a planet.
Found in the Kuiper Belt, it is no more massive
than the total of the other Kuiper Belt objects, but
is simply the largest object in its class, and may be
an escaped moon or an unusual, large asteroid.
Some astronomers define a planet as anything
that is made round by its own gravity, orbits a
star, shines only by reflected light, and does not
have enough mass to become a star itself.
According to this definition, our solar system then
has hundreds of planets, including asteroids, the
Moon, and round Kuiper Belt objects.
Composition: We don’t know what Sedna is
made of. Some astronomers think it may be part
water ice or methane ice (because it is relatively
bright) and part rock. They do know that Sedna
is one of the reddest objects in the solar system,
almost as red as Mars, but have no idea why.
Temperature: Its surface temperature of
about -240˚C makes Sedna the coldest known
place in our solar system. ■
For more information consult John Gribbin, Companion to the Cosmos (London: Phoenix, 1997)
and visit the following web sites:
Michael Brown, California Institute of Technology – www.gps.caltech.edu/~mbrown/sedna/
Robert Roy Britt – www.space.com/scienceastronomy/new_object_040315.html
BBC – http://news.bbc.co.uk/1/hi/sci/tech/3511678.stm
Yale University – http://hepwww.physics.yale.edu/quest/palomar.html
Whitney Clavin, NASA’s Jet Propulsion Laboratory –
www.nasa.gov/vision/universe/solarsystem/planet_like_body.html
International Astronomical Union – www.iau.org/IAU/FAQ/sedna.html
California Institute of Technology – spitzer.caltech.edu
Star gazers at SAAO
As South Africa prepares to launch its new large telescope, training has begun
for a new generation of astronomers, explains Patricia Whitelock
of the South African Astronomical Observatory (SAAO).
Engineers from the Southern African Large
Telescope (SALT) project team are putting the
finishing touches to the largest single optical
telescope in the southern hemisphere. Meanwhile,
astronomers have been busy training a new
generation of African astronomers.
The task of constructing and perfecting this
giant instrument at Sutherland in the Northern
Cape has gone on for the past four years. SALT
should be ready to begin its working life in early
2005, and tension mounts as astronomers prepare
to use it to start exploring the early universe.
While engineers were building the telescope,
the astronomers were building a corps of
astronomers to collaborate internationally with
other astronomers and space physicists.
New career paths
Summer School participants in front
of SALT at Sutherland
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Based at the University of Cape Town (UCT),
the National Astrophysics and Space Science
Programme (NASSP) was designed to create an
African network of astronomers.
It offers postgraduate training for honours and
master’s students. Professor Peter Dunsby acts
as coordinator, and lecturers come from the
entire South African astronomy and space
science community. Graduates in physics are
accepted – and in engineering, mathematics,
and computer science if they have done enough
physics.
Generous sponsorship from the Ford
Foundation, Canon-Collins Foundation, UCT
Vice-Chancellor’s strategic fund, and the
National Research Foundation (NRF) has made
NASSP possible by providing student bursaries.
Many students qualifying from NASSP will go
on to do doctorates and further research in
optical, radio or gamma-ray astronomy or in the
related fields of space physics. Others will move
into industry or commerce – taking with them
practical skills in problem solving, data analysis,
computer programming, and science
communication.
In 2003, the first year of NASSP, 11 students
qualified with honours degrees. In 2004, 14 new
students started the honours course and 14
started the M.Sc. The South Africans come from
universities around the country: KwaZulu-Natal,
Pretoria, Venda, Rhodes, the North, and the
North West; others come from as far afield as
Ethiopia, Uganda, and Zambia.
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