When I look back at the science class of my youth I can remember styrofoam balls painted blue, orange and red suspended from a much larger ball painted a brilliant yellow. Always, there were nine balls ranging in size extended from the yellow sphere, and ours was the swirly marble in position three. I'm almost positive I have one of the books I used to plan my model of the solar system somewhere in an attic. Now that book is a piece of history. A time capsule of the way things were in my childhood that will be one day laughed at by my children. Already I can hear a young voice giggling in disbelief,

You thought there were only NINE planets in the solar system when you were my age?

The recent discovery of a planetoid three billion kilometres from Pluto may cause science books to be rewritten once again, and children to look upon their parents as though they were the ones saying they believed the Earth to be flat.

With a temporary name of Sedna, after the Inuit goddess of the sea, the planetoid is the subject of some debate, not unlike Pluto was when it was discovered in 1930. The question of what makes an object a planet, and how large something must be to be considered a planet rather than a floating rock arises. Undoubtedly this question underscored the excitement of the team of astronomers led by Dr. Michael Brown of the California Institute of Technology when they located Sedna from Palomar observatory in November. Their findings were later confirmed using the Spitzer Space Telescope and Hubble Space Telescope .

The debate is sparked by the fact that Sedna is 300 km smaller than Pluto, which some still believe to be too small to be considered a planet itself. However considering the fact that seventy-four years later Pluto is still a planet it is likely that Sedna will also become a permanent fixture in future solar system models.

Facts about the planetoid:
  • Diameter: 2,000 km (roughly 1250 miles)
  • Distance from the sun: about 10 billion km (roughly 6.2 billion miles)
    At that distance the sun is but a tiny speck of light, not unlike many stars in our night skies. This distance places it in the coldest known region of our solar system. No doubt indicating the inspiration for the tentative name, the temperature never rises above -240C (-400F).
  • Distance from Earth: 13 billion km (8 billion miles)
  • Make up: rock and ice
  • Orbit: highly eliptical; takes over 10,000 years to complete
    At the furthest point in its orbit Sedna is 130 billion km (84 billion miles) from the sun. The last time Sedna was at its current distance Earth was coming out of the Ice Age, which might explain why we've never noticed it before.
  • Color: red; It is the second reddest object in the solar system, the first being Mars.

There is some speculation that Sedna is not a planetoid at all, but that it may be a part of an Oort Cloud. This is largely due to the unusual eliptical orbit it has, which is unlike those of known planets and more closely resembles the Oort Cloud astronomers say explains certain comets passing through the solar system. So far the only thing disproving this theory is that it is far closer, roughly 10 times closer, than the "predicted distance" of the cloud. Another factor working for the "Sedna! Our tenth planet!" lobby would be the existence of a moon around Sedna. Not much has been released about a satellite, though artistic renderings of Sedna include one. Unfortunately much more study will have to be done as Sedna grows closer to us in the next 72 years and it may take some time before this issue of "planet or not" is determined.

Update! 4.14.04
Thanks to images retrieved from Hubble we now know there is no moon accompanying Sedna. Brown had been so sure of a satellite due to the planetoid's slow rotation, but photos only show a distant star anywhere near Sedna. Although it is possible the satellite could have been on the far side of the planetoid when the photos were taken it is believed to be unlikely. How will this affect the 10th Planet lobby? We can only sit back and wait to see.

Update: 4.01.07
Sedna lost out on its planetary status and is now defined as a planetoid that may yet be termed a dwarf planet. The controversy over Sedna's size and the discovery of another planetoid, Eris, led to debate over the definition of planet. In 2006 the International Astronomical Union issued an official definition of what a planet was and the definition excluded Pluto. Pluto is now considered a dwarf planet, alongside Eris and Ceres and perhaps after further study Sedna as well. So while my science book is still a part of history that may one day amaze my children, it is for a different reason than we first thought. We've lost our 9th planet and are now down to an even 8. Though some felt Pluto should be "grandfathered" in that did not take place. Take heart, you lovers of Pluto, Eris, Sedna and other small rocks with gravitational pulls..the debate goes on and there may yet be a revote at the next IAU conference in 2009.