You’re referring to the invariable plane. All planetary orbital planes wobble around the invariable plane, and the inclination of the Earth’s orbit has a 100,000-year cycle relative to the invariable plane, which varies from 0.1° to 3°. This cycle closely matches the 100,000-year pattern of ice ages.

It has been hypothesized that a disk of dust and other debris is in the invariable plane, and that this has influence on the Earth’s climate. The Earth currently traverses this plane around January 9^th and July 9^th, when there is an increase in radar-detected meteors and meteor-related noctilucent clouds.*

^*Source: Wikipedia — Milankovitch cycles.

Why? Shouldn’t it be the plane that defines the solar system’s net angular momentum? That would be (approximately) the orbit of Jupiter- with a dash of Saturn thrown in, and Mercury still fail.

KC: The LHB happened half a billion years after accretion finished. My point is that the assumption that planetary orbits must be stable is probably not valid.

@ Greg: Mercury does have a tail. It isn’t very bright, but there are plenty of pictures of it floating around the web. Pluto is an icy planet just like Triton, Ganymede, Enceladus, etc. It is big, round, and has a geologic history. Comets meet none of those criteria.

In the Kingdom of the Blind, the man with one good eye is King; however, where depth perception is required, he is handicapped.

If we make the #1 criterion for planethood be that an object orbits in the same plane as its entire planetary system, how do we classify exoplanet systems with planets orbiting in several different planes? Are none of them planets? Do we pick one and use its orbital plane, call that a planet, and the others not planets? By this criterion, we should demote Mercury, which has an orbit inclined to the plane of most of the solar system’s planets by about seven degrees.

Au contraire, mon ami. The planetary plane is determined by the Sun’s equator, not the ecliptic (which you’re thinking of), and the orbital inclination to the Sun’s equator of the 8 major planets are: Mercury — 3.38°; Venus — 3.86°; Earth — 7.25°; Mars — 5.65°; Jupiter — 6.09°; Saturn — 5.51°; Uranus — 6.48°; Neptune — 6.43°. All well within 8°.

Whereas the orbital inclination of Pluto to the Sun’s equator is 11.88° (O.K., not much, but a benchmark has to be set somewhere), and its orbital eccentricity makes it an oddball, as I have already stated above.

As for your hypothesis of extra-solar planets in eccentric orbits, I don’t think it is a coincidence that all the major 8 planets of the Solar System orbit more or less within the planetary plane — Protoplanetary disc.