Okay, so that's a possible explanation for these hot Jupiters, but what about these eccentric giants like 70 Virginis?
These things that are further out with very elliptical orbits. What can we do about them?
Well, is it just on it's own? Is that just an isolated case so we don't have to worry about it or is it pretty common?
Well, here's the data again, we've got a time lapse showing how these things were brought up,
now we're plotting, once again, how far out they are, but now we're plotting the eccentricity.
Eccentricity of zero means a perfectly circular orbit, eccentricity of one would mean a parabola,
so the more up here, the bigger the ellipticity is, the close and further it goes.
What you can see is that the ones that are close in actually aren't very eccentric. Eccentricities of 0.2 or 0.3 or a bit less.
But that kinda makes sense because if you're a big planet and you're near the sun,
you're going to get bent all out of shape by the tides where the sun's pulling on one side of you and then the other,
so that's going to cause friction, which isn't going to mean you're going to try to limit that
the best way to do that is to go in a circle.
Yes, so it turns out that if you do the calculation, you bring a planet closer and it'll raise big tides in the star
and these tides will actually tend to, what's called, circularise the orbits.
So, you expect that anything this close in be pretty much in circular orbits, and indeed they are.
But you look at the ones further out and, my goodness!
Yes, so this is 1AU, so that's the distance of the Earth, and here we have Jupiter and we can see all these things.
I mean, these things are coming in on comet-like orbits, they don't look at all like planets.
They certainly don't look like the Earth or Jupiter.
Yes, everything in our own solar system, Jupiter has an eccentricity of about 0.05;
which will mean that almost everything we're seeing is far more eccentric than almost anything in our own solar system.
That's a bit weird, how are we going to produce this?
The problem is, you're forming things out of a protoplanetary disk, the disk is going to be in circular orbits.
It has to be because gas can't be in elliptical orbits because it'll bump into other gas and equalise out.
You're forming things out of a circular spinning disk, they should, damn it, be in circular orbits!
So, what happens if you just go out and try to simulate it and see, make sure that you're intuition is right.
So, now we're going to do the reverse.
What we were talking about last time was that we were using the disk to cause things to spiral in.
It does that, very slowly keeping them in a nearly circular orbit as it spirals in,
but let's say we form the planets and get rid of the disk.
In this case, there's nothing left to stabilise them, if you've got enough planets close together, they'll start influencing each other.
Here's a simulation. I'll give you the code so that you can play with it yourself.
So, I put a bunch of things in random orbits and you can see them all interacting with each other.
Like this one had a close collision, and, presumably, when things interact, they get too close, one of the ones gets thrown out.
Usually the smaller of the planets.
You can see that this green one here is now in a pretty eccentric orbit. Oops! It crashed in. Oh well, it wasn't anymore.
But this whitish one over there is now in an eccentric orbit.
So, in this case, everything will start off in a circular orbit and you did actually end up with some elliptical orbits
simply because of the interactions, but it doesn't always work.
Here's another one of the same code. Once again they start in circular orbits.
So, here you've done exactly the same thing, you've just started things out with a different random set of conditions.
That's right, and if you have the code you can play for yourself and see what you get. What are we getting over here?
Lots of collisions again, oops! There's another collision in the middle, and this time you end up with this on, oh it crashed into that one.
That's one way to make a big new planet.
So, there we've got something that's actually fairly big, fairly close in, but it's in a fairly reasonable circular orbit, not too eccentric.
Maybe this will do it, like a game of Billiards or a game of Crap Shoot if you like, where there's random interactions.
You need to have lots of things, get rid of the disk and then have them interact by their own gravity.
Okay, so you end up with one object here, it seems to be that whatever you get, you're going to end up with one object
because everything else has kind of 'goneski', it get's thrown out of the system, it doesn't look at all like out own solar system.
This is out own solar system and you can see that the things are pretty damn-near circular.
So, once again, the problem is that it might be too good an explanation.
If you do get these huge numbers of planets all playing Billiards, first we get lots of collisions,
and secondly, you don't get anything like our own solar system.
Right, I mean, the advantage of going in these nice circular orbits is that they don't cross each other, and you don't play billiards.
As soon as you start throwing planets in on these elliptical orbits,
there are going to be collisions and you're going to rearrange the system in pretty short order.
Guaranteed.