Replies

I would think such a definition might be especially useful with extrasolar planets, given that AFAIK the only way we even discover their existence is by their gravitational influence on the stars they orbit.

Yeah, Geoff Marcy et al has co-discovered 121 of the known 200+ exoplanets, and sez:

At the telescope, we measure the change in the wavelength (color) of light coming from a star over the course of days, months, and years. This changing wavelength is the Doppler shift of the light, resulting from the star orbiting a common center of mass with a companion planet. For example, Jupiter's gravitational pull causes the Sun to wobble around in a circle with a velocity of 12 meters per second.

This is a very tiny shift. Another method is to watch for the slight dimming of a star's light as one of its planets transits the disk.

How does the mass of Pluto compare with the mass of the largest closer thing that isn't a planet, and how do their orbital radii compare?

The largest thing not classified as a planet is slightly larger than Pluto; Pluto's orbit is more out of the ecliptic (which is defined as the imaginary disk which transects the orbit of the Earth) than that of any other known planet, except possibly for 2003 UB313 Eris, because I haven't found that info. Discoverer Mike Brown sez:

The dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the planetoid discovered almost 2 years ago. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.

I think a simple definition is best, and that would be the one offered by David Levy (see above).

I wonder if it would be meaningful to classify objects as planets based upon how strong their gravity field is at the object they're orbiting? I would think such a definition might be especially useful with extrasolar planets, given that AFAIK the only way we even discover their existence is by their gravitational influence on the stars they orbit.

Yours sounds pretty good as well, since the shapes and masses of the extrasolar discoveries will be in question for a good while, in most cases. But again, where is the dividing line? Those in favor of dumping Pluto (which is a political stance, not a scientific one) would say that Pluto is out.

But again, where is the dividing line? Those in favor of dumping Pluto (which is a political stance, not a scientific one) would say that Pluto is out.

Well, I just looked at a table of asteroids, and it appears a few of the bigger asteroids would probably beat Pluto in the gravitational-influence metric (assuming mass is proportional to R^3). If one defined "planetishness" to be the cube of the radius divided by the distance to the orbited star, Ceres would yield a value of 1.01E+8 km^3/AU^2; Pluto would yield only 8.75E+6 km^3/AU^2. Vesta yields 2.24E+7 km^3/AU^2 and Pallas 1.85E+7 km^3/AU^2. I would guess Pluto is third after those two. So we just need to blow up those who rocks and then Pluto can be a planet again. Seriously, though, I would think such a metric might be a good way of defining planets, since it represents something which can be measured for distant planets even if we can't see them. Just have to accept that Pluto's an oddball.