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Name: Chris
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
Do comets orbit the sun in the same direction as the planets, ie., clockwise? If so, why?


Replies:
Most bodies, including comets, orbit the Sun ANTI-clockwise. That is, viewing from above the North Poles of the Sun and planets. There is a notable exception to this, and that is Halley's Comet.

Its perihelion, its closest distance to the Sun, is just 0.6 AU (90 million km; between the orbits of Mercury and Venus), while its aphelion, or furthest distance from the Sun, is 35 AU (5000 million km), or roughly the distance of Pluto. Halley's orbit is highly inclined (18°) to the ecliptic, with much of it lying below the orbits of the planets, again viewing from above the North Poles of the Sun and planets.

Howard Barnes


Dear Chris,

They generally do, and the reason is that comets are left over from the primordial solar nebula, and planets and comets and asteroids generally go in the same direction. There are some exceptions.

Good question.

David Levy


Chris,

I would be very surprised if all comets have the same orbital direction as the planets. This would suggest that comets have the same evolutionary history as the planets - which current theory say they do not. Whereas we could imagine that suns, planets, and comets are born out of the same swirling gases and might be thought of as starting to orbit in the same directions, comets, unlike planets only fall toward the sun when they are knocked out of their primary orbit. This would mean that cometary orbits can be in either direction - depending on the initial fall toward the sun. In fact, the most famous comet, Haley, is in a retrograde orbit - opposite that of the planets.

Greg (Roberto Gregorius)


Hello Chris,

Astronomy is a bit outside my particular field, but you've got a very good question. My initial reaction seems to bear out when looking around for an answer to this. However, all the resources I am finding are not what I would call reliable in terms of science. They look reasonable, but in the absence of finding anything directly from an authority, I am going to have to leave my answer at "probable, but not verified."

(What a way to say that I may not be right and that I tend not to trust the Internet!)

Anyhow, the answer to your question is many comets do appear to orbit in the same direction as the planets and many others appear to be random. It depends upon where the comets came from.

Short period comets (those with orbits lasting less than 200 years) typically orbit in both the same direction and in the same plane as the planets. Short term comets generated from the Kuiper belt, a large body of objects orbiting the sun in an area past Neptune. As the Kuiper belt is somewhat flat (disklike) and orbiting in the same direction as the planets, it is not too surprising that objects originating from within it could also possess these properties.

Long period comets typically originate from further out in a region known as the Oort cloud. These comets arrive in a much more (if not entirely?) random fashion. They are seen coming from all directions and not possessing a preferred orbital plane or direction.

The origin of this difference comes from the formation of the solar system. Objects close enough to the sun (and the orbits of the other planets and objects) should end up with orbits similar to those of the planets. Objects that are further (far enough) away from this influence would appear more random. Observations of the Kuiper belt appear to be more plentiful and reliable, while the exact nature of the Oort cloud seems to be less well established.

I hope this helps,

Michael



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