This Is Not Your Parents’ Solar System
Question: What is a planet?
Answer: Something round that orbits a star, according to the new definition proposed last week by the International Astronomical Union. In the case of our solar system, that star is the sun.
Q: Didn’t we already know that?
A: Actually, no. Although many people thought they knew what a planet was, there had been no clear definition that all astronomers could agree on.
Q: So why do we need one now?
A: Recent discoveries of bodies in the Kuiper Belt, a huge region of icy objects beyond the orbit of Neptune, raised the question of whether they should be considered planets. The issue became more pressing last year when Caltech astronomer Michael Brown found an object that was at least as large as Pluto.
Q: What is that object?
A: It currently goes by the un-euphonious name UB313; Brown has nicknamed it Xena. An official name will be up to the Astronomical Union.
Q: Where is Xena?
A: Its orbit takes it as far as 9 billion miles from the sun, nearly three times as far away as Pluto. It takes 560 years to revolve around the sun.
Q: So does this mean there are more than nine planets in the solar system under the new definition?
A: Yes. At the moment, there would be 12. Besides Xena, the new planets are Ceres -- an object in the asteroid belt between Mars and Jupiter -- and Charon.
Q: Isn’t Charon a moon?
A: It was classified as one of two moons revolving around Pluto. Astronomers now consider Pluto and Charon a “double planet.”
Q: What is a double planet?
A: Two planetary objects spinning around each other that have similar mass, so that neither dominates the other. Both revolve around a point in space between them.
Q: What about Ceres? Is that a recent discovery like Xena?
A: No. It was first observed in the early 19th century, even before Neptune was discovered. For a time, Ceres was listed as a planet. When astronomers found it was part of a belt of asteroids, Ceres was demoted. Under the new definition, it will be considered a “dwarf planet.” Xena and Charon would be classified as “plutons.”
Q: Plutons? What are they?
A: They are a class of icy worlds in the outer solar system. Some Earth scientists are unhappy with the name because pluton is also a geological term referring to a type of rock that has cooled beneath the Earth’s surface.
Q: Could life exist on any of these new planets?
A: Unlikely. Charon and Xena are so far away that there is no day, just perpetual, sub-freezing night. Ceres has not yet been closely explored, but NASA is scheduled to launch a Jet Propulsion Laboratory mission next year to examine it.
Q: So is that it? Three new planets?
A: Probably not. Under the new definition, there could be dozens of others, perhaps as many as 53 planets. Even that number could be low. Astronomers are only a third of the way through mapping the Kuiper Belt.
Q: Fifty-three? Come on, nobody is going to memorize 53 planets.
A: It may be messy, but the experts who spent two years coming up with the new definition insist it’s scientifically sound.
Q: Instead of adding planets, why not just kick Pluto out of the club?
A: Good question, and one that a lot of scientists are asking. That includes Brown, who said the
IAU should “bite the bullet” and admit that Pluto should never have been considered a planet alongside Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The simplest answer for why they didn’t do that is tradition. Since Pluto’s discovery in 1930 by the beloved astronomer Clyde Tombaugh, Pluto has developed a devoted following. Past attempts to declassify it met with protests in the form of letter-writing campaigns by the public.
Q: Will textbooks have to be rewritten?
A: Yes, but textbook manufacturers say they’ve been following the debate and are ready with revisions. They say they are waiting, however, to see if the new definition gains general acceptance.
Q: So this is not a done deal?
A: The full IAU, comprising about 3,000 members, is scheduled to vote on the definition Thursday at its meeting in Prague, Czech Republic.
Some members are enthusiastic about this solution, but others are dismayed and have submitted alternative proposals. They say the IAU is twisting itself into knots and flummoxing the public just to save the reputation of an icy rock with the same name as Mickey Mouse’s dog.