In what way has Pluto not "cleared its orbit"?

Is it because Pluto and Charon orbit each other around a point between both?

For one, there’s that pesky planetoid, “Neptune” in the way.

Well if Neptune is in Pluto’s orbit, then isn’t Pluto in Neptune’s orbit?

To clear the neighborhood means that all the mass in the vicinity of a body’s orbit is either

  1. accreted to the main body itself (i.e., it become part of the planet), or
  2. in orbit about the main body, as a moon, or
  3. flung out of the vicinity into a new orbit, or out of the solar system altogether, or
  4. in an orbit that is dominated by the main body

It’s #4 that applies to Pluto and the other Plutonios. The main body is Neptune, which is more than 100 times the mass of Pluto, and all the Plutinos are in a 2:3 resonant orbit with it. The Plutinos orbit the sun twice for every three orbits of Neptune.

Several planets, including Earth, also have other bodies on the same or nearby orbits that are dominated by the planets, e.g., the Trojan asteroids.

Pluto’s orbit is significantly (for some value of ‘significant’ - it’s not really explicit) affected by Neptune’s gravity and by other objects in the Kuiper belt. Neptune’s orbit about the sun is not greatly affected by Pluto or by any other objects in the Kuiper belt.

By the way, yes, Pluto and Charon’s barycenter is outside Pluto, but I don’t think that’s relevant here. For what it’s worth, the earth and moon both orbit a point that’s not at the center of earth but rather a few thousand miles beneath the surface of the earth. This is generally true for any two orbiting bodies in the solar system - even the barycenter of the earth/sun system isn’t exactly at the center of the sun but a point that’s a bit outside the center of the sun.

Why is Neptune still considered a planet when Pluto crosses its orbit?

Thanks everyone!

Follow-up question: what’s a “3-2 resonance”?

Orbital resonance

If you’re thinking of Pluto/Neptune, it’s 2:3, not 3:2. Subtle difference.

Pluto’s orbit does not literally intersect with Neptune’s. When it reaches the points that are equivalent to Neptune’s distance to the sun, it is well below Neptune’s plane of orbit, due to the inclination of its orbital plane.

I’m happy to believe Pluto and Neptune are in 2:3 resonance. Yet if I Google for their orbital periods and perform division I see 1.505 or even 1.514 where I’d expect 1.500. What gives?
Will the period ratio switch to 1.49 after a century or three, so they can “catch up”?

It’s that whole leap year, leap second, daylight savings time kerfuffle. Pay it no mind.:smiley:

“Clearing” does not mean every tiny bit of cruft is cleared out. That’s just impossible. It means that all the significant stuff has been removed. Pluto and its moons are merely some of the insignificant junk that Neptune hasn’t dumped yet.

(Note, though, that the closet planet Pluto ever comes to is Uranus due to the resonance and the position of Neptune when Pluto is at perihelion.)

That would be an amusing bit of trivia if it were true, but it’s not. Right now, Pluto is closer to Neptune than it is to Uranus.

Note the word “ever” in my post. Obviously if the other 7 planets were on the opposite side of the Sun from Pluto the closest one could be Mercury if it was on the near side. That’s not “ever”.

ftg is not saying that Pluto is always closer to Uranus, he is saying that it occasionally passes closer to Uranus than it ever gets to Neptune. Interestingly, because of the orbital resonance, Pluto’s closest encounters with Neptune occur when Pluto is farthest from the Sun.

Perhaps we should look at what problem we’re trying to solve here … that we’re finding a number of objects beyond the orbit of Neptune similar to Pluto and in some cases actually bigger … are these all planets, and if so doesn’t that make the designation less useful … and the other problem of never formally defining this designation, what is a “planet” exactly …

1] Hydrostatic equilibrium (i.e. roundness) seems logical, this eliminates all the jagged chunks of rocks …
2] Orbits the sun (i.e. not a moon) again is logical, it’s a planet and not a moon …

Here we still have potentially dozens of bodies out there AND we have a couple of asteroid belt objects that qualify … Ceres and Pallas I think … which are not generally considered planets … so the third criteria was added

3] Sweeps out it’s orbit … which quite clearly Ceres and Pallas hasn’t …

Out there where Pluto and her sisters orbit is kinda junky … little bits of rock and what have you stuck in some circular cloud around the solar system, something akin to the asteroid belt … and these objects tend to be skewed to the ecliptic, Pluto is angled at 17º, Enis at 44º and Pallas at 34º where Mercury is the most skewed of the planets at 7º … all this speaks of a different formation pathway … were these dwarf planets aggregation points within the collapsing nebula that became our solar system, or is this all the riff-raff left over and condensed at a later time? …

On the bright side, if this new definition of a planet doesn’t work out, we can change it … I for one would welcome a universe where we can boast of 38 planets !!! … up yours Beetle Juice …

Basically, the effect of the “cleared its orbit” puts a size limit on an outer planet, because the Kuiper Belt appears to be shaped by Neptune. If there was (or is) a large planet beyond Neptune, its gravity would be affecting all the clutter in its neighborhood. If you put an earth-size planet out in that area, the effects of Neptune would probably insure that it could not clear its own orbit, the same way that Jupiter essentially prevents Ceres from clearing its orbit.

Earth gets closer to Saturn than Pluto can ever get to Neptune. In fact, if I recall correctly Pluto can get closer to Uranus than it can to Neptune.