Re: Are Sedna and Quaoar two new planets?

The question you ask is both easy to answer and also sort of tricky. The easy answer is that they definitely qualify as "minor planets," so in a way, yes they are planets. The category "minor planets" includes comets, asteroids, and Kuiper Belt Objects and basically means all of the natural Sun-orbiting stuff in the solar system that isn't a "major planet." The major planets are what we usually mean when we say planets: Mercury, Venus, Earth, etc.

Are they major planets? Well, no. That's the easy answer because they have not been accepted as major planets by the International Astronomical Union, the body who decides how astronomical terms will be applied.

There's a much trickier question lurking inside of your question: should Sedna and Quaoar be considered planets? (From here on I'll use "planet" to mean "major planet".) So far, we've been sort of working on a "we know a planet when we see one" definition in astronomy. But recent discoveries of extrasolar planets and large Trans-Neptunian Objects (including Sedna and Quaoar) have forced astronomers to recognize that what we mean by "planet" is not totally clear at times.

There are a number of ways you can define planet: size and orbital characteristics being relatively common suggestions. (For size, you can set an arbitrary minimum size for planets, usually set at around Pluto's mass, or you can say that any body that's large enough that its own gravity will force it into roughly a spherical shape is a planet. For orbits, you can say that if the orbit is more elongated than some limit or if the orbit is titled beyond a certain point, it isn't a planet. There are other things one can use, even with these two general categories, but you get the idea.)

If it sounds kind of messy, it's because it is. Luckily, most astronomers know that there's a problem with those terms, but they aren't getting too worked up about it. (If you consult the media, you might get the impression that we're almost having physical fights over this, but we're not. Most of us realize that we'll eventually figure something out. Science is good about that kind of thing.) As far as I know, not many people are seriously proposing that Sedna or Quaoar be named as planets. If anything, more people are suggesting that we reconsider , since these two objects are getting close to Pluto's size. But don't expect to see Pluto get cast out of the planet rolls soon.

So to conclude, Sedna and Quaoar aren't planets. But their discoveries (and the discoveries of others like them) are forcing astronomers to carefully consider what we mean by "planet."