We've learnt a lot about close-in planets, but it would be really interesting to find out about ones that are further away.
Things like Kepler are telling us wonderful things about things that are orbiting within 50 day periods.
As they get more data, it might get out to a hundred or a few hundred day periods, maybe out to Earth radius or beyond,
but both transits and radial velocities don't tell us much about things at five, 10, 15 astronomical units out
where Jupiter, Saturn, Uranus and Neptune are in our own solar system.
Yes, it's going to be pretty hard as well, because these planets have many, many decade long orbits
and so you can go through and you can beat on them for decade after decade so you can see the whole sine wave.
But you also have the problem that things like Uranus and Neptune for example, they're a long way out
and so the the amount that they perturb the sun is very small and so the reflex motion,
very long periods mean it's not a good strategy to go out and find them this way.
Yes, for transits, the odds of something that far out being edge on enough to cause a transit are very remote.
You'd have to look upon millions and millions of stars to find one and then you'll see a very small dip that recurs every 40 years or so.
Right, so even if you do find one, you won't be sure it wasn't just a butterfly that flew in front of your telescope.
Odds are you've retired before the next one happens
and you really want your four or five of these things before you've got to say very much.
So, clearly we need a different technique here. (Yes, we need another technique.)
The technique that's being used for this is something we've talked about before
when talking about Dark Matter which is gravitational lensing. So, here's the basic idea and the fundamental equation.
Let's say you've got a beam of light that's passing near an object of mass, M, passing at some distance, R, from it.
Then, as an approximation valid when R is fairly big, it deflects the light by an angle in radians,
that's equal to four GM over RC squared, so G is the gravitational constant, M is the mass of whatever this thing is here,
R is how far the light passes from it and C, the speed of light all squared.
This really comes back to the foundations of general relativity
and how we realised general relativity came into being, Einstein, when he had the idea
(Yes, he came up with this equation) and he came up with this equation and was tested.
He then went out and found that the sun deflected the light of background stars
by so much during the eclipse during the first World War. (Right.)
So, it's fundamental physics here.
We've seen this before for gravitational lenses, in this case we're looking at a background galaxy
that's been lensed by this cluster of nearer galaxies over here so you see the multiple images and they're on different sides.
So, its been very powerful in cosmology, but it could potentially, one could imagine, maybe using it here to look for planets I guess.
How would this work?
Well, here's an edge on galaxy, it would look a bit like our own galaxy,
what the Milky Way would look like edge on, and you see that these galaxies puff up in the middle.
This is called the 'bulge' and most spiral galaxies have one of these bulges including our own.
Now, we're out here in the outskirts of the galaxy and the idea would be that we'd look at the bulge.
Our line of sight would pass just skimming the top of the disk of our galaxy.
If you look at the stars here, we might look at big stars, giants or something over there, bright stars.
Then every now and then a planet or a dwarf star somewhere on the line of sight will pass in front of it and cause some amplification.
So, the reason why we're looking out here is because our galaxy is full of dust and that dust means you can't see through things very well
and so we need to kind of skirt around that dust so we get a clear view of as many stars as possible.
Here's a view of the galactic bulge from Mount Stromlo.
That's the dome of the 74 inch telescope in the foreground, and what you can see up here is the bulge.
The middle of the galaxy is hidden behind this layer of dust along here, but you can see the bulge poking above and below it.
So, what you do is you find somewhere along here, and what that gives you is very large numbers of stars that are quite a distance away.
The bigger the distance away is, the smaller the mass can cause the lensing
because it gives a very small angle and still focus things on the Earth.
So, looking at lenses with focal lengths of tens of thousands of lightyears, these are very small lenses.
You want the biggest distance you can get away with and also,
because you're skimming just above the surface of the disks as you look out there,
there's a fair chance there's going to be stars and planets in the foreground to see it.
So, you get a lot of stars in the background, we get some in the front and occasionally we can hope for these things to line up.
Yes, so let's calculate how big the lensing angle is going to be in this situation.