James B. Kaler is a professor emeritus of astronomy and an award-winning author. As an astronomer, Kaler has studied stellar evolution, including planetary nebulae – the colorful remnants of dying stars. As a popularizer of astronomy, Kaler has written 10 books and numerous magazine articles. He was interviewed by News Bureau Physical Sciences Editor James Kloeppel.
We’ve been hearing about the discovery of a new planetary object called Sedna. What’s the controversy about it?
New is relative in this business. Sedna was discovered going on three years ago, and in the meantime a number of other curious Solar System objects have also been found. Yet Sedna remains unique and not so much controversial as mysterious. It is the most distant object of the Sun’s family ever observed. It was found at a distance of 90 Astronomical Units (“AU” meaning the distance between Earth and Sun), more than twice Pluto’s average distance. A long looping orbit takes it from 76 AU to as far as 980 AU from the Sun and back over its “year” of some 12,000 Earth-years. Were you aboard at the maximum distance, you would receive but a millionth of the heat we get here from the Sun. Even on a nice sunny day, the temperature hovers at 400 degrees Fahrenheit below. Now appearing as a mere pinpoint 400,000 times fainter than the eye can see, Sedna’s size has yet to be measured, but estimates suggest up to 1,800 kilometers, three-fourths Pluto’s dimension.
Sedna lies beyond the limits currently established for the “Kuiper Belt,” a disk of debris that lies beyond the orbit of Neptune and that was too thinly populated to make a planet. Sedna is thus suspected of belonging to the vastly larger “Oort Cloud” that is occupied by frozen cometary bodies that were hurled out of the early Solar System by the large planets.
Why are some astronomers now saying Pluto isn’t a planet after all?
Pluto, while long considered the “ninth planet,” is really not so much the last planet as the first body of the Kuiper Belt, that disk of debris that extends from the orbit of Neptune (at 30 AU) to around 50 AU and that is the source of our “short-period” comets, those that orbit the Sun under a couple hundred years. Comets are small, dirty ice-balls that release gas and dust when they are tossed too close to the Sun, though Pluto – 70 percent the size of our Moon – is certainly too evolved and stable in its orbit ever to be one. Hundreds of these small bodies are known, and there may be tens of thousands over 100 kilometers in diameter.
Several things distinguish Pluto from the classic planets. Its orbit is highly eccentric and tilted, and most curiously, it is held in Neptune’s gravitational grip, orbiting twice for every three of Neptune’s turns around the Sun. Moreover, a large number of Kuiper Belt Objects share the trap. Moreover still, it is not the largest of them! That honor goes to newly discovered 2003 UB313 (informally called “Xena”) that may be some 20 percent or more bigger than Pluto. If Pluto is a planet, then I suppose so is “Xena,” and we get 10. Or more if we drop the size limit (why, after all, not Sedna too?). To my mind, a clear compromise would be to put Pluto in the Kuiper Belt where it belongs, but perhaps recognize it as a transition body that leads to the planets, and thus keep it as number nine – and last.
Should we expect that more planets are yet to be discovered in our solar system?
The answer depends on whether one considers Pluto, “Xena,” Sedna, and the rest, as planets. There seem to be no big ones like the inner eight, Mercury through Neptune, no “planet X.” Unless terribly far away, its gravitational effect on the classic planets, and on spacecraft, would be noticed, much as Neptune’s effect on Uranus led to Neptune’s discovery. But I would be amazed if astronomers did not find more bodies of the Kuiper Belt and beyond that did not match or exceed the size of Pluto. What we have learned is exciting stuff – the thought of what we have YET to learn is even more so.
