So I recently read that Jupiter does not orbit the sun. In fact, it orbits a spot outside of the sun called a barycenter. This blows me away. Things that don’t make sense to me about this are:
How can a planet made of gas have that much mass?
If Jupiter’s gravity can pull the sun out of stability, why then when the earth is between the sun and Jupiter doesn’t Jupiter pull the earth way out of it’s orbit?
Although I’m fascinated by these things, I really don’t know much about them.
Jupiter is really big, and the gas is pretty dense.
Because even when Jupiter and the Earth are in opposition, Jupiter is around 600 million km from Earth, but Earth is 150 million km from the sun. Plus, the Sun is about 1000 times the mass of Jupiter, so the gravitational force of Jupiter is 1/16,000 the force of the Sun on Earth. So while there is certainly some perturbation, it’s not going to somehow throw Earth out of the solar system or anything.
Just so you know, all pairs of orbital bodies have a barycenter they both orbit. The Earth/sun system rotates around a barycenter as well, its barycenter just happens to be much closer to the center of the Sun than Jupiter’s, because Earth is much smaller than Jupiter and much closer to the Sun.
We had a discussion of something much like this a while back
In summary, Jupiter pulls both the Earth and the Sun almost equally. Because the Earth and Sun are not precisely the same location, there is a differential effect (the Earth gets pulled by a slightly different amount than the Sun gets pulled), and this differential perturbation does affect Earth’s orbit slightly, but it’s only a slight effect.
A three-body system (or 10 bodies) is chaotic enough there is no simple solution to what would make it stable. For now, the short answer is that the orbital periods of Earth and Jupiter are not in sync. If for example, a Jupiter year was exactly 10 earth years, then earth and Jupiter would line up exactly every 10 years, and Jupiter would be pulling the earth out of its orbit a bit each time. But instead, because they are not similar, think of the system as like a guy making a pizza doughball into a flat pie - They pull on a different part around the rim each time, thus keeping it circular. At this point it’s essentially stable, Jupiter is not pulling us further out than our orbital velocity compensates for. Everything is relatively balanced.
Also, the effect of Jupiter, compared to the sun, is minimal - the sun is 100 times the mass and 1/10 the distance
The Sun and Jupiter are both giant gas balls - get enough gas together and it will stay together by gravity.
Jupiter isn’t pulling the sun out of stability. There isn’t really any notion of the sun’s stability of position anyway. Not within the solar system. There are instabilities in the orbit of stars around the galaxy, and very occasionally stars can be ejected from the galaxy. Something that takes of the order of billions of years to happen.
Another way of looking it is that the barycentre is just the centre of mass of the two bodies. That moves linearly with the mass ratio of the bodies.
Gravitational attraction however drops with the square of the distance. The Earth is so much closer to the sun than Jupiter that even when Jupiter is nearest the effect of its gravity on the Earth is attenuated hugely.
Sun → Earth = 1.5 \times 10^{11}m
Sun → Jupiter = 7.50 \times 10^{11}m
Jupiter → Earth nearest = 6.0 \times 10^{11}m
distance squared ratio = 2.25/36 = 0.0625
Mass Jupiter =1.9 \times 10^{27} kg
Mass Sun =2 \times 10^{30}kg
So another factor of a thousand, to make Jupiter’s gravitational attraction to the Earth become, at best, 0.00006 times that of the Sun. It isn’t zero, and the effect is real, but it isn’t a concern.
There are quite a few videos out there that attempt to show how things really work. Here is one:
[hijack] There are a few hypervelocity stars (HVS) that have been identified. HVS are going fast enough to escape the glalaxy, even though they are currently within the galaxy. Eventually they will escape, but the galaxy is big, so it takes a while for them to actually leave. It should only take millions of years, rather than billions, to leave.
The current thinking is that these stars were in a close binary with a white dwarf. The white dwarf exploded in a Type Ia Supernova, which destroys the WD. The HVS keeps its obital velocity, which was quite large, but released from the gravity of the WD, it shoots off in whatever direction it happened to be going at the time of the supernova.
[/hijack]
Thanks all for the info and the links. I understand it somewhat better now.
I think this is bit is what initially threw me. My (flawed) thinking was… If Jupiter pulls the sun towards it and the earth is closer to Jupiter and less massive than the sun, Jupiter should have a really dramatic effect on the earth.
Not quite, Jupiter ranges from 365-601 million miles from Earth, The sun is a mostly steady 93 million miles. So the sun sits at about 1/5 the distance of Jupiter (on average)
As for mass, the sun is about a thousand times more massive than Jupiter.
Ultimately though, you may have overestimated the distance to Jupiter but you underestimated the mass of the sun so, the point stands. The effect of Jupiter compared to the sun is minimal.
Even if the effect is minimal, why doesn’t it have a bigger effect after millions and billions of iterations? I could see how the effect would be infatismal from year to year, but after so many orbital cycles, it seems like those would all add up to major changes in our orbit. Is our orbit kind of like a “groove” to where minor forces aren’t enough to to pull us out of that groove?
Jupiter is moving along its orbital path, so the perturbation events don’t all happen at the same point in the earth’s orbit. By the time the earth has completed one orbit, Jupiter has advanced about thirty degrees along its orbital path. So now the next perturbation of Earth’s orbit will happen about thirty degrees later than where the one last year happened. Over much time and many orbits, these perturbations are fairly evenly distributed along Earth’s orbital path, and its orbit remains approximately round.
Most of the effects cancel one another out. However, given enough time, yes indeed, it is possible for just the right confluence of effects to build up into something dramatic. The Earth could be ejected from orbit around the sun.
Note that the Milankovitch cycles do exist - periodic variations in Earth’s orbital eccentricity (over tens of thousands of years), due to the effect of Jupiter and other planets
At least based on some of the science shows I watch, Jupiter and Saturn were instrumental in determining where the planets are all located today. Apparently Jupiter began migrating inward towards the Sun, essentially causing havoc by swallowing up most of the material that comprises the inner rocky planets. However, Saturn’s massive gravity eventually pulled it back to where both planets are now. This allowed for some material to remain,which eventually set the stage for several collisions that led to the formation of the earth and other inner planets. I can’t google now, but I believe the “Grand Tack” and “Grand Attack” theories are what posit this history of the solar system.
This all apparently happened during the very early years of the solar system over 4 billion years ago, and we’ve been in relative equilibrium since that time.
ETA: By the way, some parts of the theories are based on what we observe in other star/planet systems, where “hot Jupiters” and “super Earths” seem to be common, and where most planets’ orbits are much closer to their stars. We don’t have those, so the theories evidently explain that as well.
Ever seen one of those scale models of the solar system, not the silly little desktop models? There’s one near me where the Sun is about the size of a basketball, or maybe a small beachball, Earth is the size of a BB, and Jupiter is at best as big as a golf ball. That alone is quite illustrative of the size difference, but what’s even more eye opening is the insane distances involved. At that scale, Neptune is nearly a mile away. The space between Jupiter and Earth is about 500 feet, and remember, that’s only when they’re perfectly in line with one another. Normally because of the orbits not being in sync, the planets are much farther apart, to the degree that the planet closest to all other planets is always (on average) Mercury. That further shows how the perturbations would tend to cancel out over time. Also, there’s been some four billion years for all this to get mostly sorted out, so we’re left with only the most stable parts of the system.
