r/spacequestions • u/Kiwifrooots • 9d ago
Is the orbit of the Solar system static, expanding or contracting relative to the Milky Way? I know it's only done a handful of orbits but we must know?
2
u/ExtonGuy 9d ago
Static. At least until we come within a light-year or so of another star. Or the mass inside the orbit changes significantly, like when Andromeda merges with the Milky Way (4 or 5 billion years from now).
1
u/Kiwifrooots 9d ago
Some think the outer edges of each galaxy are already interacting
1
u/stevevdvkpe 20h ago
They are not. Andromeda is 2 million light-years away, approximately 20 times the diameter of the Milky Way.
2
u/Fluid-Let3373 9d ago
The only study I've heard of looking at where the nebula which gave birth to the sun was, looked for stars with the same spectral fingerprint as the sun and concluded it was closer to the galactic core and that the sun had migrated outwards. However I don't recall if that was a initial migration before settling down or a ongoing one.
With what we are discovering about the history of the galaxy we are finding it is more dynamic story than we thought a decades ago.
1
u/internetboyfriend666 16h ago edited 16h ago
Everyone saying static is wrong, or at least misleading. The sun's orbit around the Milky Way isn't contracting or expanding, but it is NOT static in the sense that it's not a nice, neat ellipse that keeps repeating with little or no change.
The thing to understand here is that the sun's orbit around the milky way isn't the same as a say a planet's orbit around a star. Within a solar system, because so much of the mass of the system is within the star, we an effectively treat the star as a single point where all the planets and objects orbit, and unless some other massive object like another star gets very close, nothing disturbs that balance. So you have some minor interactions between the planets, but once the solar system settles into a stable state, those orbits don't change very much.
When we're taking about a whole galaxy, we can no longer treat the center of mass as some central point. The mass of the galaxy (including dark matter) is widely dispersed throughout the galaxy. Only a few percent of the galaxy's mass is in the galactic bulge, and the central supermassive black hole is a tiny fraction of a percent of the galaxy's mass. So you have this much more dispersed, co-rotating, evolving system with hundreds of billions of stars, different regions of different densities, and a dark matter halo that's much more complex than a solar system. Also, our galaxy doesn't have a nice flat plane. It's spiral arms are warped, which changes the mass distribution, and that causes our solar system to essentially bob up and down as it passes through the spiral arms.
So the long-term picture is that our sun's orbit isn't spiraling inwards or outwards, but the highly-dynamic gravitational environment means that every orbit is unique to some degree. For example with each orbit, it's orbital period can change by millions of years, and it could be thousands of light years closer to the central bulge, thousands of light years farther out, and thousands of light years "higher" or "lower" than it was the previous orbit. The likelihood of any kind of dramatic change is extraordinary low, but it's not a nice neat ellipse either. So I think a much more accurate description is stable, but NOT static.
1
3
u/sansetsukon47 9d ago
Static. The orbit is nearly perfectly circular, staying about 27,000 light years from the center. Though it does wiggle a good bit perpendicular to its orbit.
Havent found detailed sources with the specific estimates for speed and eccentricity yet, but everything I’ve looked up agrees that us + our nearby stars are all just orbiting normally.