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Over the following billions of years how much more massive will Earth have become through accretion?

Even over billions of years, the Earth is not likely to gain any significant mass.

Most of what we run into is debris from the asteroid belt, and the entire mass of that region is less than 1/10 of 1% of our mass. And only a small fraction of that will leave the asteroid belt anytime soon and of that fraction, most will run into Jupiter, not the Earth. So the total mass we might run into from that source is measured in hundredths or thousandths of a percent of our current mass.

We do run into the occasional cometary bit, but the mass gained that way is almost certainly less than the mass gained from running into asteroidal pieces. So again, the incremental mass involved is closer to zero than not.

There are, however, theories that suggest that the orbits of the planets are not entirely stable, and that Venus or Mercury might run into us, in which case the Earth would gain a few percent (in the case of Mercury) or a few tens of percent (in the case of Venus) of its current mass. Of course if such an event occurred, the impact would destroy all current surface features (including continents and ocean basins), and as a relatively minor side effect, all life as well.

Whether such massive collisions could take place depends upon how long it takes planetary orbits to become unstable. Most theories suggest times measured in tens of billions of years, and in only 7 or 8 billion years the Sun will swell up and vaporize Mercury and Venus, so odds are that such collisions are impossible. But a similar collision with Mars could still be a possibility, even after the Sun has collapsed to become a white dwarf.

To summarize, thinking in terms of 'normal' accretion, the mass of the Earth will always be essentially the same as now. However, if and when catastrophic changes in orbital motions occur, we could add a few percent to a few tens of percent of our mass by collisions with one or more of the other inner planets. But even if such a thing should come to pass, there will almost certainly be no one around to notice.  


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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