Sedna swirls in

The discovery of Sedna, the coldest and the most distant object known to orbit the sun, sparks off a debate on whether it is a planet or a planetoid.

SEDNA is the proud goddess who Eskimos believe populated the oceans with fish and other creatures. Sedna is also the latest in the string of icy objects, approaching the size of a planet, discovered in the outer reaches of the solar system.

Of late, the universe seems to have got itself into the press. Beagle 2, the abandoning of the Hubble Space Telescope, the Mars expedition, the launch of the comet-chaser Rosetta, United States President George Bush's new space programme, the discovery of Sedna which is likely to be designated the 10th planet in our solar system - space is asking us to stay tuned.

Sedna, which made headlines recently, had a typically understated scientific entree. `Discovery of a candidate inner Oort cloud planetoid', was the title of the paper by Professor Michael Brown, an astronomer of the California Institute of Technology (Caltech), that heralded the find, which has remained hidden in an icy cometary dust cloud.

Brown said, "It really has been just sitting out there at 400 below zero for the past 4.5 billion years." He and his team, Chadwick Trujillo from Gemini Observatory and David Rabinowitz of Yale University, report in their paper that since November 2001 they had been systematically surveying the sky in search of distant slow-moving objects. Their survey was to be spread over five years from the Caltech's Palomar Observatory and, when finished, was expected to be the largest one done since 1961. The goal was to find rare large objects in the Kuiper belt that escape smaller but deeper surveys. And in the course of the survey, a new and interesting object was detected on November 14, 2003.

On March 15, four months after the object was spotted, astronomers announced the discovery of the coldest, most distant object known to orbit the sun, and the largest one to be detected since the discovery of Pluto in 1930. Seen from Sedna, the sun would be no more than a point of bright light in a swarm of solar system objects. It is at a distance of 90 astronomical units (A.U.), that is, 90 times as far from the sun as the earth is, making that a respectable 13 billion kilometres. At its most distant, it is supposed to be 130 billion km from the sun.

Sedna is the second icy planetoid to be discovered by the team of astronomers led by Brown. Two years ago, the astronomers found a smaller object that they called Quaoar, thus far the largest known object in the Kuiper belt.

The discovery of Sedna, though widely welcomed, quickly raised several questions. First, is it a planet? Is it in the Oort cloud at all? Why is its orbit so extremely elliptical? Does it have a moon? What is it made of?

While early speculation fuelled talk that a 10th planet had been discovered, Professor Brown admitted that Sedna, about three-quarters the size of Pluto, was not a true planet but more of a planetoid, somewhere between a planet and an asteroid.

However, terming Sedna a planetoid complicates matters and adds to the series of Sedna issues that will keep the space community busy for years. The question whether Sedna is a planet or a planetoid resurrects the question of Pluto's own status.

A few years ago, all outer-space aficionados were in robust argument over whether Pluto was really a planet. Pluto is the farthest known planet from the sun. With a diameter of 2,274 km, it is also by far the smallest, smaller than seven of the moons in the solar system - the moon, Io, Europa, Ganymede, Callisto, Titan and Triton. Owing to speculation about Pluto being demoted from its planetary status, the International Astronomical Union (IAU) clarified in 1999 that it had made no proposal to change the status of Pluto as the ninth planet in the solar system. However, the IAU added that a substantial number of smaller objects had been discovered in the outer solar system, beyond Neptune, with orbits and possibly other properties similar to those of Pluto. Therefore, Pluto was assigned a number in a technical catalogue of such trans-Neptunian objects (TNOs) so that observations and computations concerning them could be collated conveniently. This process was explicitly designed not to change Pluto's status as a planet.

Now, the criteria that would relegate Sedna to the status of a planetoid are shared by Pluto, which joined the planet's club in 1930. If astronomers rule that Sedna is a planetoid, they may have to demote Pluto as well. But picking on Pluto might not be well received by the public. With its cartoon doggy name, it can rally many defenders for its little-guy status. One American newspaper, which did not delay its verdict, said, "Despite its being slipped in the solar system with shaky qualifications under some sort of size-and-distance-challenged affirmative-action admissions policy, everyone agrees Pluto has risen to the occasion and done a bang-up job going round and round the sun and leaving us alone."

On the other hand, some Chinese astronomers have said that both Sedna and Pluto should be regarded as larger cosmic bodies belonging to the Kuiper belt, and that a "historical error" was committed when Pluto was named the ninth planet. Sedna then has raised the fundamental issue of what it takes to be a planet. P.B. Shelley, in his work Prometheus Unbound, refers to "the maze of planets struggling fierce towards heaven's free wilderness". If his poetic insight was correct, then there can be no delay in deciding the specs - planets are out there in a maze actually struggling to be recognised and counted.

With all the furore, a final decision will be made by the IAU, which usually takes a couple of decades to decide such matters. Incidentally, the IAU, at which the Indian National Science Academy represents India, has some 225 Indian astronomers as members in their individual capacities.

The IAU will also decide the name of the object. As of now, its official name is 2003 VB12. Once its orbit is tracked better, the team that discovered it proposes to recommend to the IAU Committee on Small Body Nomenclature, which is responsible for solar system names, that it be permanently called Sedna. Being located in the most distant and the coldest place in the solar system, it was appropriate that it be called Sedna, who is thought to live at the bottom of the frigid Arctic Ocean. In fact, it is suggested by the Sedna team that all newly discovered objects in the cold inner Oort cloud be named after beings from Arctic mythology.

Some astronomers wonder whether Sedna should be considered a part of the Oort cloud. Others have suggested that it could be part of a closer region, the Kuiper belt, which stretches from Neptune to just beyond Pluto, and that it had somehow been dislodged and sent off on a more distant orbit. Brown conceded that Sedna was closer than expected but suggested that it could reside at least part of the time in the cloud's inner sector.

The Kuiper belt and the Oort cloud define the outer regions of the solar system. The Oort cloud, at the extreme of the sun's gravitational and dynamic influence, is said to be a hypothetical shell of icy proto-comets in very loose orbits around the sun. This spherical cloud of comet material is situated about 50,000 to 100,000 A.U. from the Sun (approximately 1,000 times the distance from the sun to Pluto). It has an inner reach at the ecliptic from the Kuiper belt and is said to extend three light years, almost halfway to the nearest star, Proxima Centauri.

The cloud is named after Jan Hendrick Oort (1900-1992), a brilliant Dutch astronomer who worked at the University of Leiden and was noted for confirming the hypothesis that galaxies rotated. Then, in 1950, Oort proposed a model for the origin of comets. He said that a cloud of cometary material surrounds the solar system at an enormous distance, a concept now generally accepted. He was trying to explain an apparent contradiction: several passes through the inner solar system would destroy a comet, and hence if the only comets were those that existed at the origin of the solar system, then they should all have been destroyed by now. But the Oort cloud contains millions of comet nuclei, which are stable because the sun's radiation is weak at that distance. The cloud provides a continual supply of new comets, replacing those that are destroyed.

Sedna's remoteness has inspired scientists to conjecture about how much it could tell them about the far reaches of the solar system. The planetoid is more than three times away from the sun as Pluto, normally considered the edge of the planetary system. Moreover, it travels a widely eccentric orbit, taking 10,500 years to revolve around the sun.

With an orbit that ranges roughly from 76 to 850 A.U., it is much closer than originally expected and may belong to an "inner" Oort cloud. If this is so, it implies that the Oort cloud is both denser and extends much closer towards the sun than assumed earlier.

It also leads to another interesting hypothesis about the sun itself. Early in the solar system, objects in the outer extreme were being sling-shot into orbits further away from the sun when they approached closely giant planets like the young Jupiter. In their newer orbits, these objects began to come under the gravitational influence of distant stars, causing them to slow down. Sedna is expected to have suffered a similar fate. The remarkable conclusion that one can draw from this is that the other stars, which affected these objects, must have been situated much closer than envisaged. Thus there is a possibility that the sun was formed in a close group along with many other stars.

Of all the objects in the solar system, it has been found that only Mercury and Venus rotate more slowly than Sedna. Sedna rotates on its axis once every 40 days or so, leading to the hypothesis that the slow rotation could be caused by a moon. The Hubble Telescope is expected to step in at this point and check out whether there is indeed a tiny satellite revolving around Sedna.

The Californian team estimates from its thermal observations that Sedna is made of water ice or methane ice, just as Pluto and its only satellite Charon are. But other observations belie this conjecture. Why Sedna is so red, almost as red as Mars, has baffled space scientists.

As of now, Sedna is the only potential Oort cloud object to have been spotted. But how is it that suddenly we are seeing so much more of the solar system? Technology, of course. In 1930, Clyde Tombaugh's blinking persistence in Flagstaff, Arizona, using photographic plates, helped spot Pluto. However, for those plates Sedna-like objects were quite out of range. Now astronomers use silicon CCDs (charge-coupled devices), which turn light into electric signals, and are assembled even into digital cameras.

CCD technology was first demonstrated in 1969 at the Bell Laboratories. Since 1974, the Instrument and Sensing Technology programme at the National Aeronautics and Space Administration (NASA) has been using this technology on missions such as the Galileo mission to Jupiter, the Hubble Space Telescope, the Yohkoh Soft X-ray Telescope, and the Space Shuttle Electronic Still Camera. Every passing year, using complex arrays of CCDs, robotic telescopes can find "planetoids" with far greater ease.

With only 15 per cent of the sky scanned by Palomar in this survey, given the penetration of CCDs, it is likely that several other Sedna-like objects may float into view. And for every large object that is seen, there must be many smaller ones that do not get tracked. From Sedna, it is now conjectured that the inner and outer Oort cloud regions must have thousands of objects, far more than that in the Kuiper and asteroid belts combined. We have far more companions in our solar system than we thought.

With the new discovery, the solar system has suddenly become much bigger - bringing a sense of reality to what has so far been a remote frontier of hypothesis. Perhaps it has become stranger, too. Experts believe that as other similar discoveries are made, our definition of planets, comets and asteroids could change, and other defining characteristics would emerge as we continually learn that the solar system is not just the sun, nine planets and little else.