With solar wind constantly pushing us away and the mass of the sun decreasing as it burns hydrogen fuel, are we moving away? Is that process accelerating?
Well, I found this story :
The solar wind pushing the Earth isn’t doing diddly to change its orbit. The Sun losing mass, due both to the solar wind and to the mass-equivalent of the energy being radiated away, moves all the planets out a little bit. It’s not a major factor now, but will be when the Sun gets to the red giant stage and really starts to lose mass. However, that won’t be for another 5 billion years or so.
But the Earth is moving away from the Sun by about 15 cm/year. The main cause of this seems to be tidal drag – the same cause of the Moon moving away from the Earth. cite
If that recession rate stays constant, the Earth will move away from the Sun by .005 AU during the next 5b years. However, it’s not likely to remain constant. The Sun will gradually get hotter as it ages. That will cause its atmosphere to expand outward, which means the Earth will raise larger tides in it, thus increasing the tidal drag. OTOH, the expanded Sun will rotate slower, which will reduce the tidal drag. I don’t know enough about the math of tidal drag to say which effect, if either, will dominate.
As for what happens when the Sun becomes a red giant, there was a recent paper by Schröder and Smith: Distant future of the Sun and Earth revisited (warning pdf) which discusses this. Their conclusion is that the Earth will be engulfed by the bloated red giant Sun. While the solar mass loss will cause the Earth to move outward, the tidal drag will reverse its effect and counteract that outward movement. The reason is that the greatly expanded Sun will have essentially ceased rotating, so the tidal drag will move the Earth inward.
However, I should note that the cited paper does not take into account the current movement of the Earth away from the Sun. It probably won’t make any difference, but it’d be nice if they had accounted for it.
The orbits of the planets vary over time due to the gravitational interactions between them. For example, the Earth’s orbit varies from being nearly circular to mildly eliptical, in a roughly 400,000 year cycle. This affects the Earth’s climate.
Over very long timescales (hundreds of millions of years), orbits can change more radically. Simulations have discovered the possibility that Mercury will move outwards from it’s current orbit, eventually interacting with Venus. As a result, it’s possible that one of the inner planets could be ejected from the solar system within the next billion years, although the probability of this happening is low.
Current theory postulates that the giant planets formed much closer to the Sun then they are now, and that they migrated outwards early in the history of the solar system.
Going slightly off topic, the martian moon Phobos’ orbit is decaying, and is expected to break up into a ring-system within 11 million years.
It will all be fine until Jupiter emits another Venus-sized comet.
If that happens, all bets are off.
Don’t forget the Yarkovsky effect.
Everybody forgets the Yarkovsky effect. Can’t imagine why.
Thanks for the link to the Yarkovsky effect. Earth’s prograde rotation should contribute to an outward acceleration.
This part, I don’t get. If we can accurately measure the average Earth distance to the Sun this year, then the outward force of solar wind + Yarkovsky + decreased solar mass should result in a slightly larger distance next year. And I’m pretty sure a constant outward force is like thrust, so there should be acceleration. So I was thinking even if thrust is small, constant thrust over millions or billions of years would result in large changes.
But I guess what you’re saying is Earth is so massive, Yarkovsky and even YORP are negligible over billions of years.
Been reading some James P. Hogan?
What?
Has some other hack been mining Velikovsky for outrageous science-fantasy background elements?
Damn, now I’ll have to re-write a few chapters. I wonder if a Symmes Hollow Earth has any miles left on it…
To answer the OP: so far, so good!
Compared to what?
Obviously, it isn’t infinitely stable. But compared to anything else you can measure it against on this planet, it is almost perfectly stable in relative terms. It’s mor estable than a mountain, or an ocean. It’s a billion times more stable than you are when standing on a flat surface with no distractions.
So, yes, there are cosmological factors which affect the earth’s orbit. But the percentages compared to the time involved (1% in 5 million years) make it perfectly stable in any sense that is meaningful to us. You’re more likely to be hit my lighting 10 times tomorrow, survive every one, win the lottery every week for the rest of your life and become president of the world before the day is up than the earth’s orbit is likely to make a difference in yours, your children’s, or even your great-great-great-great=great grandchildren’s lives.
Basically, yes. Although I don’t know why you brought up the YORP effect, since it doesn’t contribute to outward movement of a body. I’m sure someone brought up Yarkovsky as a joke, since it’s a very, very small effect on a planet the size of Earth. Roughly about the same as photon pressure, which no one’s brought up for good reason.
Anyway, we know (well it still needs to be confirmed) that the Earth is moving outward at 15 +/-4 cm/yr and various people have tried to explain it in various ways. The only explanation that gives values in the right order of magnitude is tidal drag. Everything else is too small.
But even the .15 m/yr movement it’s currently undergoing is not going to be a huge change after 5b years. As I said above, it will only move the orbit out about .005 AU.
Yup.
No. Consider that gravity is a constant inward force. By your logic, since we can measure that, and it’s big, wouldn’t it also stand to reason that we should orbit at a significantly smaller radius every year?
The orbital accelleration we experience at this moment is the vector *sum *of all the forces. Hence the orbit we have over time is the sum of all the effects over time.
This sum includes ALL the forces. The big ones and the small ones. The ones we know about, and the ones we haven’t discovered yet. Gravity pulls one way, Yarkovsky, Solar wind, tidal drag, and a boat load of other forces pull the other way. There may even be other inward-going forces as well (quantum monopole doodad fields anyone?)
Whatever they are, as long as none are changing over time, the orbit we have is the orbit we keep.
If the solar wind suddenly somhow increased, say, 1000x, then yes, we’d see the orbit move out. And once all the new forces were in equilibrium, the orbit would stop increasing. It would be in a new stable state.
If the Sun’s gravity suddenly somehow increased 1%, then yes, we’d see the orbit move in. And once all the new forces were in equilibrium, the orbit would stop decreasing. It would be in a new stable state.
Is the Sun’s gravity constant? It’s turning mass into energy which then radiates out. Does the loss of mass affect the Sun’s gravitational pull on the earth? Or is it so small a loss of mass to be negligible for the purposes of this discussion?
Not quite. Forces like tidal drag, particle drag from the interplanetary medium, and the Yarkovsky effect can do work on the planet (i.e., they can act at least partially along the planet’s direction of motion), so they can in principle cause the orbit to change over time. This is why the Moon is getting farther away: the tidal drag couples the Earth’s rotation to the Moon’s orbital angular momentum; the Earth does work on the Moon and moves it into a higher orbit over time.
Central forces, on the other hand (those which act only along the Sun-Earth axis) do no work on the planet, so they don’t affect the orbital energy and hence don’t cause the orbit to change over time. Solar wind (ignoring its drag effects) and gravity fall into this category.
I brought up YORP because it can affect rate of rotation which can affect orbital stability. But if Yarkovsky is negligible, then YORP will also be negligible.
And being typical short-sighted humans, we won’t start making any plans for what to do about it until ~4,999,999,950 or so years from now.