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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. 38 Quest 1(1) 2 0 0 4 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 ▲ ▲ 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. Quest 1(1) 2 0 0 4 39