WHAT is Sedna? Is it a planet? A planetoid? Or even a comet? The most distant object so far spotted in our solar system and the largest since Pluto made its debut in 1930 has reopened the controversy over what should and should not be named a planet.
Tentatively named after an Inuit goddess of the ocean, Sedna is nearly as big as Pluto and takes a massive 12,000 years to orbit the sun. Mike Brown, the Caltech planetary scientist whose group discovered it, thinks it comes from the Oort cloud of comets that extends halfway to the nearest star. 鈥淭his is one of the most primordial objects we know,鈥� Brown says, and it may illuminate the birth of the solar system.
Sedna is in the Oort cloud and Pluto is in the Kuiper belt, a collection of icy bodies beyond Neptune, and Brown argues that neither is a planet, as they are little larger than their neighbours. He prefers calling them 鈥減lanetoids,鈥� and says he expects to find more like them.
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If Pluto is defined as a planet, 鈥済et ready for a 20-planet solar system,鈥� says Dave Jewitt of the University of Hawaii. And if Sedna were to come into the inner solar system, 鈥渨e would classify it as a comet, and it would be the most spectacular comet anyone has ever seen,鈥� says Brown. Its size makes it the queen of the comets, and the largest object found in the solar system since Pluto in 1930.
Brown鈥檚 group spotted Sedna on 13 November while cataloguing large, faint, slow-moving objects in the Kuiper belt. When they measured Sedna鈥檚 motion, Brown told a press conference this week, 鈥渨e were shocked to discover that our new object is at almost three times the [current] distance of Pluto鈥�. That puts it beyond the outer edge of the Kuiper belt at the outside of Pluto鈥檚 eccentric orbit (see Graphic).
While some Kuiper belt objects do wander that far afield, Sedna鈥檚 orbit is very different. When it reaches the innermost point of its orbit in 2076, it will still be 2.5 times as far from the sun as Pluto鈥檚 closest approach. At its farthest, 6000 years later, it will be more than 10 times as far away as Pluto. That is a surprise, because astronomers had thought this region was a void between the Kuiper belt and the Oort cloud. They had assumed that the inner edge of the cloud is extremely far out, perhaps 300 times the distance of Pluto, but Sedna suggests that it may actually begin much closer in.
Long-period comets are known to come in from far beyond Sedna鈥檚 orbit. Astronomers believe they collected there in the solar system鈥檚 first 10 to 100 million years, as the outer planets threw small icy bodies out of the solar system. Most escaped into space, but nearby stars deflected some of them back towards the young sun.
Their final orbits depended on how far away the objects were from the nearest stars at the time. At the distances between the stars today, the Oort cloud should be far from the sun. But for comets to wind up in Sedna鈥檚 orbit, many stars must have been much closer than they are now. Brown suggests that the sun formed in a cluster of stars, and that its young siblings deflected escaping objects so they became trapped in Sedna-like orbits around the sun. As that early cluster dispersed, it left the orbits of those primordial objects too stable to be disrupted unless a star passes exceptionally close.
More puzzles await. Sedna is redder than any other body in the solar system except Mars. And because it seems to be surprisingly cold, it must be reflecting 20 to 25 per cent of the sunlight falling on it. That makes it unusually shiny, and means that despite its brightness Sedna must be smaller than 1800 kilometres across, but probably larger than the 1250-kilometre Quaoar, which Brown, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University discovered in the Kuiper belt in 2002. Slow periodic variations in Sedna鈥檚 brightness suggest it may have a satellite, so Brown wants to use the Hubble Space Telescope to search for this companion.
Other planetary scientists are intrigued too. 鈥淚t鈥檚 a super-neat object,鈥� Jewitt told 快猫短视频. 鈥淚t cannot have formed where we see it now.鈥� He says it must have formed closer to the sun only to be ejected later.