So the Earth's been orbiting the sun in a relatively stable orbit for billions of years now. during that time, it's been plowing through dust, and hit by comets and asteroids, presumably mostly from the outer reaches more than the inner parts of the solar system.

Even if the drag and increased mass should amount to a tiny, tiny fraction of Earth's mass, shouldn't that have been enough to cause our orbit to spiral inwards? Both due to slowing of the Earth's motion, and an increase in mass?



No not nearly enough drag. There just isn't much dust in space, and the Earth has so much momentum that the tiny amount of interplanetary gas has a tiny effect on the Earth's orbit.

There's only about 5 particles of mostly hydrogen gas per cubic cm in the region around the Earth. It has a drag on the earth of a few thousand Newtons. But the mass of the Earth is $5\times10^{24}$ kg, so the deceleration is of the order $10^{-20}$ m/s2.

At this rate it would take longer than the age of the universe for drag to make a significant change to the orbit of the Earth.

Things were different when the planets orbited in an accretion disc. At this time, large planets could make excursions from the outer part of a stellar system to the inner, due to friction from the dust that they encounter.

The Earth does gain a little mass from dust, rocks and asteroids that continuously hit it, but it also loses a little mass to atmospheric lose. The total is slightly negative (overall Earth loses a little mass) but this doesn't affect the orbit, as orbital velocity is independent of mass.

Large bodies do change the orbit very very slightly, but comet and asteroid impacts happen from all directions, and there is little cumulative effect.

The gravity of other planets does perturb the orbit of the Earth, but not to make it spiral into the sun, but to cause the elliptical orbit to become first more, then less eccentric, and change the orientation of the ellipse.

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    $\begingroup$ Radiation pressure from the Sun is nearly a million times larger. Of course that is mostly radial, but Poynting-Robertson drag is still 10-100 times bigger than dust drag I think. $\endgroup$ – Rob Jeffries Apr 19 at 8:07
  • $\begingroup$ I'll copy that verbatim to my answer. $\endgroup$ – James K Apr 19 at 8:09
  • $\begingroup$ Thanks for the detailed answer. One follow-up: you wrote "orbital velocity is independent of mass", which is, of course, true. However, if total mass changes, the kinetic energy no longer matches the orbital velocity required to maintain orbit. I also assume that more impacts occur from the outer rim and not from the direction of the inner system, further skewing the velocity vector? Maybe it's not enough to make a dent in the orbit, but coupled with fluctuating gravity pulls by other planets, I'm still trying to understand how stable the orbit should be $\endgroup$ – RonS Apr 24 at 20:23

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