Earlier this year, it was discovered that Sedna is the tenth planet orbiting around the Sun. Is it really so?
IT is a question so simple that it should be part of the primary school curriculum. In fact, not many of us even think to give it a second thought. But, now that the question is here, not many of us know exactly what a planet is.
In this huge universe of ours, where millions of extraterrestrial bodies abound the dark sky, only a few have been labelled as planets. And this is the way it has been since astronomy became a science. However, it took just one, called Sedna, to kick in a storm over the definition of the word planet.
Recapping a little of what has been happening, the ‘tenth planet’ orbiting aound our Sun was discovered earlier in the year. Immediately titled, Sedna, after the Inuit goddess of the ocean, the latest celestial body had the whole world buzzing. But even before the hangover kicked in, questions were raised regarding the validity of the claim.
Measuring about 1,180 to 2,360 kilometres across, Sedna, or 2003 VB12, is the most distant object yet found orbiting our Sun. In fact it is three times further away than Pluto is from the Sun. However, its distance from the Sun varies during its 10,000-year-long circumnavigation of the Sun and can be as close as about seven billion miles from the Sun and as far away as about 93 billion miles.
Nevertheless, the fact that its rotation on itself is relatively slow initially suggested that the planet had a satellite in orbit that was affecting the rotation of the planet. Immediately the Hubble Telescope was pointed in its direction.
But detailed observation turned out to be untrue, which in effect, immediately put in doubt Sedna’s new found planetary status. And this in turn, opened a Pandora’s Box of new queries; what defines a cosmic object as a planet? Soon more disturbing questions were raised, the most extreme being, is Pluto big enough or independent enough to be labelled a planet, this despite the fact that it has Charon, a moon of its own.
The global astronomical community was in a fix and immediately referred all queries to the International Astronomical Union (IAU).
The IAU too was unprepared for any such challenge. After all, such a headache isn’t an everyday occurrence. So, in an effort to keep a lid on all proceedings, the IAU that among other things decides on the names and classification systems for all solar system bodies has deferred all queries to its General Assembly, scheduled to meet in 2006.
At present though, the IAU has no specific plans to directly challenge Pluto’s planetary status. Some hope that a decision by an IAU working group could have that effect. According to Iwan Williams, president of the IAU’s Planetary Systems Sciences, “Whether or not one needs to do anything about Pluto will depend on what the definition of a planet will be.”
But that hasn’t stopped the arguments and Pluto’s fate, and that of Sedna’s could very well be decided much before 2006. If a planet is defined as any large object whose own shape is rounded by gravity, as has been suggested by many, then our solar system could well be inundated with no less than 50 planets.
A controversy similar to Sedna’s dilemma erupted two hundred years back when in 1801 Ceres was discovered between Mars and Jupiter. Though initially called a planet, it was later stripped of that title and classified as an asteroid — one of many in that part of the solar system. And of course, years before Sedna made headlines, Pluto was causing more headaches to astronomers than those generated by their attempts to understand the discovery of black holes. In fact some Pluto’s planetary status has been was a mistake of observational history ever since it was discovered in 1930 by Clyde Tombaugh. For when the ninth planet was discovered, it was thought to be 12 per cent larger than the Earth, thus making it the fifth planet of the solar system in terms of size.
Ever since, however, it has come to be known as the smallest of the nine. But Pluto’s other characteristics make it unique in other ways.
Pluto is inclined 17 degrees to the main plane of the solar system in which the larger planets orbit. Other planets have inclined orbits, too, but none as much as that of Pluto. Its orbital path is also very non-circular; it ranges between 30 and 50 astronomical units (AU) from the Sun. And it has a moon. Sedna’s is even stranger; roaming at a distance of 76 to 1,000 AU from the Sun, but it’s inclined at 12 degrees, less than that of Pluto. One AU is equal to 150 million kilometres.
Of course the non-believers have never had any of this. Calls for Pluto’s demotion were voiced ever so vocally in 1997, following Clyde Tombaugh’s death. And it became a point of public discussion in 1999 when rumours made rounds that Pluto was finally going to be stripped of its planetary status. It is that only after public disapproval was voiced of such a move, that the plans were dropped, postponed some say.
Coming back to Sedna’s existence, there are those who call it a Kuiper Belt Object. The belt contains hundred of known objects, most of which are worlds of rock and ice. But then, astronomers haven’t found anything quite like Sedna.
There’s another argument that is trying to find a compromise between the ‘planet’ and the ‘not-planet’ camp. Label Sedna a ‘planetoid’, like a planet, but not exactly one. Now that’s diplomacy. Whatever the outcome of any debate that is going to go drag on for years to come, one thing is for sure. Many hope that an emotional attachment will leave Pluto’s status as it is while a newly constituted definition of planet will help any future definitions of the word planet. In the end, either the solar system will be left with just eight planets, or, more than ten.
Earlier this year, it was discovered that Sedna is the tenth planet orbiting around the Sun. Is it really so? 
