How does the orbits of the planets compare to the orbits of comets?
Planetary orbits are all nearly circular , comet orbits are highly excentric .
All orbits around the sun are elliptic, but the planetary orbits look more like a circle, while the comet orbits look more like a cigar (with the sun near one of the tips).
The earth's distance from the sun varies about 5% from closest to farthest, for a comet we see a factor of 30 times or more.
This means that planets stay more or less at the same distance from the sun, while comets travel as far out as beyond Neptune, and can travel as far in as Mercury. For us they are only visible when they come 'near' the sun, as only then a tail can develop through the sun's radiation.
Also the planes of planetary orbits are more or less the same within a few degrees of the Earth's orbit (called the ecliptic ), while comets (and dwarf planets) can move in quite different planes.
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