Photo by Dr. Horace Smith, MSU Observatory
An object was discovered using the Palomar Observatory's 48" Samuel Oschin telescope by astronomers Mike Brown, Chad Trujillo, and David Rabinowitz. It has the temporary name "UB313". It's in orbit around the sun out beyond Pluto, the ninth planet, and it appears to be larger than Pluto too. Does that make this thing a planet? The problem is that there is no exact definition of what a planet is. To the ancient Greeks, planets were the wandering stars that moved across the heavens. Webster's defines planet as "any of the large bodies that revolve around the sun".
But that's not precise enough for science. How large is large? There are many bodies that go around the Sun that are "large" that we don't count as planets. At least half a dozen asteroids orbit the sun that are larger than 200 miles in diameter. The biggest, Ceres, is almost 600 miles across. Pluto is bigger than that. It's about 1400 miles across. The earth's moon is larger still, about 2100 miles in diameter. The moon is going, along with the Earth, around the Sun. There are several other moons going around other planets that are even larger than our moon. Also, astronomers have discovered more objects orbiting the sun, out beyond Neptune, that are large. The recently discovered objects known as Quaoar and Sedna are both thought to be around 800 miles across. According to some researchers, there are at least 70,000 "Trans-Neptunians" with diameters larger than 60 miles.
The area of our solar system just beyond Neptune is refered to by some as the Kuiper Belt. It is thought to extend from about Neptune's orbit, which is 30 AU from the sun to about 50 AU. One AU is 93 million miles, the distance between the Earth and the Sun. This belt of objects is named after Gerard Kuiper who wrote a paper in 1951 predicting objects out past Pluto. Still others refer to this area as the Trans-Neptunian population. As telescopes and searching techniques have improved, quite a few of these Kuiper Belt Objects (KBO's) have been discovered. This new object's distance from the Sun varies between about 38 AU and 98 AU. UB313 takes about 560 years to complete one orbit.
This new object appears to be a KBO or Trans-Neptunian body. It's slightly larger than Pluto. Should it be counted as a planet? Many astronomers think not. If we count UB313, then we should also count the many other similar sized bodies in orbit around the Sun. Some people are of the opinion that Pluto is also Trans-Neptunian and should not be counted as a planet. But demoting Pluto is very unpopular. Pluto was discovered in 1930. If it were discovered today, there's no doubt that it would be thought of as just another Kuiper Belt Object. Everyone over the age of 75 has been taught that there are nine planets in the solar system. Should we count pluto as a planet for sentimental reasons? I think not. Planets have been demoted in the past. In the early 1800's Ceres, Juno, Pallas and Vesta were discovered circling the sun between Mars and Jupiter. Textbooks of the day listed them as planets. After the 1850's photography helped astronomers discover many more bodies in that region of space. By the late 1800's it was clear that there were far too many bodies orbiting the sun between Mars and Jupiter, to count them all as planets. We now refer to this part of space as the Asteroid Belt.
So the discovery of "the tenth planet" might not be the next planet after all. It's more likely to lead to a different understanding of the nature of our solar system; four small planets, followed by an asteroid belt, then four big planets, followed by another asteroid like belt. I'm sure the story won't end there.
Photo by Dr. Horace Smith, MSU Observatory
Photo by Dr. Horace Smith, MSU Observatory