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I don't know a lot about physics, but this seemed like the correct place to ask my question. I apologize in advance if it is a ridiculous one.

It's actually a two part question:

  • If the population of humans and other live forms increases, does that add to the overall mass of Earth?

  • If that is the case could that extra mass ever be enough to change the orbit around the Sun?

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To illustrate just how little effect we have on the earth's orbit, you may find this page helpful/entertaining: what-if.xkcd.com/8 – thanby Jan 28 at 14:49
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Also interesting reading: scitechdaily.com/earth-loses-50000-tonnes-of-mass-every-year. Earth gains tons of cosmic dust, and looses tons of light gases, on a continual basis. Apparently (per the link) the net effect is a loss. – cobaltduck Jan 28 at 16:20
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Clue: what happens to corpses? – imallett Jan 29 at 0:21
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10 upvotes for this? Really? – Alec Teal Jan 29 at 16:52
up vote 19 down vote accepted

Mass is conserved (up to whatever small contribution nuclear decay has to the overall loss of mass). All biological matter is just created from materials from the environment (we do eat, right?). With exception of an occasional space probe, shooting stars and solar wind effects, the earth can be considered a closed system in terms of matter exchange. Whatever happens on our surface is just a local redistribution of resources.

It's quite easy to compute the energy required to move the earth away from the sun. It's about the same as the kinetic energy of earth around the sun: $2.7\times 10^{33}\,\rm J$. This kind of energy is hard to come by. Planets usually move around and change orbits because other planets have gravitational effect on them. Jupiter was throwing the planets all over the place before the solar system settled in this configuration. But... locally, we can't do anything, because the conservation of momentum holds: INTERNAL forces can't change the mass*velocity product, and mass remains constant, so does the velocity.

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It's quite easy to compute the energy required to move the earth away from the sun. It's about the same as the kinetic energy of earth around the sun: Is being about the same purely coincidence? – kasperd Jan 28 at 14:29
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@kasperd, No. The kinetic energy necessary to stay in a perfectly circular orbit is exactly 1/2 the kinetic energy necessary to achieve escape velocity at that same altitude. But the Earth's orbit is not perfectly circular, and thus the 'about'. – Aric TenEyck Jan 28 at 14:39
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If Wikipedia is to be believed, we have a minimal net loss of earth mass, adding together effects of increase by incoming meteors, dust etc. (+40,000 tons/yr) and decrease by gas loss etc. (-95,000 tons hydrogen/yr). – Dubu Jan 28 at 15:17
    
Yes, we're losing a bit of mass. Mars lost its entire atmosphere (no magnetic field, no protection from solar wind, and lighter gasses just evaporate anyway). And... we're also receiving a bit of "push" from solar wind and radiation. But... not that much. I think the main contribution to orbital shifts are still other planets and tidal coupling with the sun. – orion Jan 28 at 21:55
    
Might we say that all in all the Earth evaporate slowly in space through its exosphere? – daniel Azuelos Jan 28 at 23:21

Life forms are made up from materials already present in Earth. Thus, increasing population would not alter the overall mass of the planet, and can't impact its orbit.

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And even if the mass of newborn life forms appeared magically from nowhere, it would be so much smaller than the mass of the planet, that it wouldn't have any measurable effect. The planet gains and loses mass in much larger quantities (some air molecules escaping into space, meteors getting captured, etc.), and that is still small enough to be insignificant. – vsz Jan 28 at 14:54
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Strictly speaking much of the chemical energy in the bonds comes indirectly from the sun's energy and hence we are gaining mass from the sun. (Chemical bonds add weight). But of course that would require a ludicrous amount of life and ridiculous amounts of energy absorption from the sun. – Ramchandra Apte Jan 28 at 18:16
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Since the overall growing of life on Earth bring ground materials to a slightly higher altitude (around 0.5 m higher), might we say that due to the conservation of its angular momentum, the Earth rotation is slowing down? – daniel Azuelos Jan 28 at 23:29
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@ResidentBiscuit: Here's another thing that will blow your mind. Since most of the solid mass of trees are carbon, and trees absorb only water and trace minerals from the soil, and trees absorb carbon dioxide from the air that means trees don't grow out of the ground, they grow out of air! – slebetman Jan 29 at 3:34
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@slebetman There was a video of Feynman talking about how trees grow out of the air on YouTube, fascinating! Link: youtube.com/watch?v=N1pIYI5JQLE – npace Jan 29 at 13:44

As others have said, the mass of the Earth doesn't go up because we are eating food produced on Earth. Suppose we somehow were importing really large amounts of extra-terrestrial food, now what happens?

  1. We roast. Food contains a fair amount of carbon. Carbon from extra-terrestrial food is just as much a greenhouse problem as burning fossil fuels.

Should we somehow avoid that (say, carbon sequestration) and keep growing:

  1. We drown. Food consists mostly of carbon, hydrogen and oxygen. The hydrogen and oxygen combine to make water. The sea level rises.

Should we somehow avoid that (say, by importing material to build up the surface with) and keep growing:

  1. We get crushed. All that mass added to the Earth will raise it's gravity.

Should we somehow avoid that and keep growing:

  1. We get shaken to death. All that mass being added to the Earth will lower the moon's orbit and thus increase the tides. Eventually this will cause truly catastrophic earthquakes.

Should we somehow avoid that and keep growing:

  1. We get smashed and buried. As the moon is drawn ever closer it will eventually cross the Roche limit and be destroyed. Our rings would be far more magnificent than Saturn's but there would be an awful lot of mass up there bumping into itself. While much of it just gets ground to dust a decent amount gets deorbited--and even a small portion of the moon is still quite a layer on the surface.

Should we somehow avoid that and keep growing (note that we are now to the point where there will be a slight effect on our orbit--drawing us closer to the sun, not away):

  1. We go nova. We have a large unburnable core with a bunch of hydrogen (in the form of water) on the surface. That's basically the same thing as a white dwarf collecting hydrogen off it's companion--eventually the hydrogen ignites. (And, yes, our core would be degenerate at this point.)

Should we somehow avoid that and keep growing (and note that we are still orbiting closer, not escaping):

  1. We turn into a neutron star. There's nothing to hold off the pressure, we cross the limit for electron degeneracy and down we go.

Should we somehow avoid that and keep growing (and note that we are still getting closer, although now if anything the sun is orbiting us):

  1. We turn into a black hole. Same thing, degeneracy pressure isn't enough and down we go.

Should we somehow avoid that and keep growing (and note that we are still getting closer, although now if anything the sun is orbiting us):

  1. We fry. Our increasing mass brings the sun down on us just like we brought the moon down several steps ago.

Escaping ceases to mean anything at this point.

I would be very surprised if we could manage to eat enough to even reach the second stage of this list before the aging sun renders the whole matter moot.

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Plenty of others made it clear that babies aren't made out of matter from somewhere else. But if the earth's mass were to increase ten percent by magic, its orbit would not change. Its momentum—its tendency to fly off into space in a straight line—would increase by ten percent, and the counteracting force of the sun's gravity would increase by ten percent.

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Yes! But all those magic babies would have a significant effect on the moon's orbit. The perigee would be much closer, and the lunar month would be significantly shorter. Fortunately the resulting tidal havoc would be insignificant compared to the logistical burden of keeping ~10^23 newborn babies (a mole!) fed and cleaned. – TonyK Jan 29 at 21:39

protected by Qmechanic Jan 28 at 14:52

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