Would the Theia impact have changed earth's orbit?

[Stonius (Markus)]

Theia is the hypothesised minor planet that smashed into the earth 4.5BY ago, creating the earth-moon system.


The question is whether the impact would have changed the earth's orbit around the sun.


For example, I think I'm right in saying that if the earth weighed twice as much, it couldn't follow the same path around the sun as it does now.


So if the earth suddenly gets a moon, does that extra mass in the Earth-Moon system shift the orbit?


I guess then you have the vectors of the actual impact too which would alter the orbital dynamics.



But presuming it's fair to say that the earth was once in a different orbit. Is there any way of surmising what that ancient orbit would be?


Markus

[astroron (Ron)]

Originally Posted by Stonius

Theia is the hypothesised minor planet that smashed into the earth 4.5MY ago, creating the earth-moon system.


The question is whether the impact would have changed the earth's orbit around the sun.


For example, I think I'm right in saying that if the earth weighed twice as much, it couldn't follow the same path around the sun as it does now.


So if the earth suddenly gets a moon, does that extra mass in the Earth-Moon system shift the orbit?


I guess then you have the vectors of the actual impact too which would alter the orbital dynamics.



But presuming it's fair to say that the earth was once in a different orbit. Is there any way of surmising what that ancient orbit would be?


Markus

Do you mean 4.5 Billion years ago?
Cheers

[Stonius (Markus)]

Originally Posted by astroron

Do you mean 4.5 Billion years ago?
Cheers

Yes, indeed, Ron. Typo. I'll edit and fix

[multiweb (Marc)]

Originally Posted by Stonius

But presuming it's fair to say that the earth was once in a different orbit. Is there any way of surmising what that ancient orbit would be?

There's a very good two part BBC serie presented by Brian Cox called the planets. My understanding is that Jupiter runs the show in term of everything else that orbits the sun as it coalesced at about the same time in the original gas cloud. Everything else that influences orbital mechanics is pretty insignificant in orders of magnitude. The gas giants orbits tightened closer to the sun before moving back to where they are now so the inner objects were many and colliding a lot more.

[Stonius (Markus)]

Originally Posted by multiweb

The gas giants orbits tightened closer to the sun before moving back to where they are now so the inner objects were many and colliding a lot more.

This is what I'm getting at - I thought orbits were a function of mass and velocity?


How can Jupiter change orbits without changing its mass or velocity?


How can the additional mass of an extra moon *not change the earth's orbit?


Given such a large impact, I'm amazed our orbit isn't more eccentric.


But maybe, as you say, we have Jupiter to thank for that. But even so, presumably the earth would have been in a different orbit pre-Theia?



Markus

[multiweb (Marc)]

Originally Posted by Stonius

This is what I'm getting at - I thought orbits were a function of mass and velocity?


How can Jupiter change orbits without changing its mass or velocity?


How can the additional mass of an extra moon *not change the earth's orbit?


Given such a large impact, I'm amazed our orbit isn't more eccentric.


But maybe, as you say, we have Jupiter to thank for that. But even so, presumably the earth would have been in a different orbit pre-Theia?



Markus

Orbits changed because the sun output changed a few times at the very beginning if I recall it was colder then got hotter.

[JA]

Originally Posted by Stonius

Theia is the hypothesised minor planet that smashed into the earth 4.5BY ago, creating the earth-moon system.


The question is whether the impact would have changed the earth's orbit around the sun.

Yes it would have. In terms of trying to understand or model it, the extent to which it would change would depend on the nature of the collision, whether it was for instance: front-on, rear, glancing or some sort of non-contact gravitational effect. It would also depend on the relative masses and velocities of the 2 colliding bodies as well as how much ejecta was generated or whether the colliding body was completely subsumed into the earth and how much energy was lost in heat, sound etc.

Originally Posted by Stonius

For example, I think I'm right in saying that if the earth weighed twice as much, it couldn't follow the same path around the sun as it does now.


So if the earth suddenly gets a moon, does that extra mass in the Earth-Moon system shift the orbit?

I don't know about suddenly acquiring a new moon as there would be some immediate transient effects, when it popped into existence, but barring that the Earth and Moon would begin orbit around their common centre of mass. Now because the Earth is 80+ times more massive than the moon, that Centre of Mass probably wouldn't be that far displaced from the centre of mass of the Earth alone***, so you probably wouldn't notice much/any orbital variation due to the presence of the moon, from that perspective and nor would you notice any orbital variation resulting from the increased coupled mass (of the earth and Moon) now orbiting the Sun. Why? Ask Newton ......

The laws that govern the circular orbit of a satellite/planet around a star interestingly enough, given a massive difference in mass (which exists: some 333,000 to 1), the parameters than govern the type of orbit are velocity, Mass of the Star (not the satellite), orbit radius & the Universal Gravitation Constant. Too high a velocity and the object/satellite/planet will escape orbit around its star, too low a velocity and it will crash into its star, just right in a certain range and it will orbit somewhere from circular or elliptical also influenced by other nearby/massive planets.

Originally Posted by Stonius

I guess then you have the vectors of the actual impact too which would alter the orbital dynamics.

If you have the impact data then it becomes a collision mechanics problem, with a lot of variables

Originally Posted by Stonius

But presuming it's fair to say that the earth was once in a different orbit. Is there any way of surmising what that ancient orbit would be?


Markus

With the collision data, you'd have a chance atleast. it wouldn't be that easy and require many assumptions to model.

Best
JA

***, The calculated Centre of Mass of the Earth - Moon system is approximately ~5000km displaced from where it would be without our current Moon. So the Centre of Mass moves from the Centre of the Earth to ~5000km closer to the Moon, i.e: still within the Earth itself.

[Stonius (Markus)]

Originally Posted by multiweb

Orbits changed because the sun output changed a few times at the very beginning if I recall it was colder then got hotter.

Ah, radiation pressure. Guess that would make sense.

[Stonius (Markus)]

Thanks for the detailed analysis JA - So it seems you're saying that the additional mass wouldn't have made much difference, it's more about the vectors involved in the collision itself, and without knowing that it's not really possible to surmise where the earth might have been orbiting before. Cheers and thanks for the explanation.


Markus