So, Paul, I have to admit, I'm just overwhelmed.
You know, even 10 years ago, I thought we wouldn't know anything like this about planets.
This is amazing and it just seems to me that it's going to be hard to improve much on this.
Yes, and in fact, this is a gift that keeps giving, there's another surprise as well.
Remember, we were talking about what a planet would look like if it was all by itself.
You get a magnification pattern like that and as the Earth drifts across the image plane,
you will get some pathetic little quick spike like this.
Now, this is much less amplification than you get if the planet is near a star, because then you start getting all the caustics,
it's a much bigger signal, many more things you can measure.
The other benefit of a planet near a star is that you see the increase of brightness
because of the star lensing event and therefore can trigger all the follow-up.
But none of this, in principle, you might be able to see something from planets that are a very long way out.
Well, you'd have to be pretty lucky to find one, yes.
Yes, I mean, it would only be possible if there were a lot of them out there and, curiously enough, they have been found.
Not very many of them but sort of less than 10 but here's a couple of examples.
Less than 10, I mean, if there's 10 of these, these seem like they're almost phenomenally almost impossible to imagine they'd happen.
Let's see, let's explain this first one. See if you can convince me.
So, we've got data, this is the data of many years from both the MOA and the OGLE surveys.
The MOA gets more data with bigger arrow bars and the OGLE gets the tighter arrow bars but less data points
and what you can see is a lensing event, but it's a very small magnification, only about 40 percent.
It takes a couple of days to go across, that's quite short.
But look at this one, another one over here.
Look at that, nothing, nothing, nothing WOAH! Nothing, nothing, nothing, nothing, nothing.
Up by a factor of 30!
But only for, this is one day there, so this is only happening over hours.
Wow, that almost seems impossible to fit. Are we absolutely sure that's microlensing?
Well, I mean, the star doesn't do a lot else, does it, over many years and it fits the microlensing shape.
They had, luckily this was up when New Zealand was observing because it went up and down during the night.
That's a remarkable event, and this one is doesn't look like there's any way to get around it because everyone's seen it, perfect shape, wow!
So, we have 10 events seen with these short little periods which means they have to be little tiny things, so what does that mean?
What it means is that you need an awful lot of planets that are not near a star.
Because these are so hard to see because the image plane around it is actually so small,
you have to get very lucky for the Earths track to go near it.
So, seeing even seven or eight of these things that we actually see, it's telling us that there must be huge numbers of these things out there.
Probably about 1.8 for every star, so these things outnumber stars.
Everything else, you're talking about maybe 30 percent of stars have these things, these are more than one per star.
They have to be not near the star, they have to be sort of out at Saturn's orbit or probably further out,
maybe 30 or 40 astronomical units out, otherwise we start seeing all of these causticly things.
Okay, so how are we going to make these things? It strikes me as being slightly problematic from what I know of stars.
Yes, I mean, one possibility is that these are outer planets, Neptunes or something like that.
But that far out, you might actually be able to see them by direct imaging.
We're going to talk about that in the next lesson, but we don't see those things in enough numbers.
Odds are that these things really are free-floating planets.
These are not, even at a distant orbit around a star, they actually are drifting around the galaxy all by themselves.
Okay, so if we go to a place where we're going to make stars and planets, for example, the Orion Nebula,
one could imagine making a bunch of little tiny things that are smaller than what we call a brown dwarf,
so there's this limit of about eight hundredths of the mass of the sun,
where you make a star and a star is defined as something that is able to fuse deuterium into helium and has an energy source.
But, below that, we have brown dwarfs and thanks to infrared surveys of the sky,
we now know more or less how many brown dwarfs there are out there.
There are a lot of brown dwarfs but there are not that many brown dwarfs.
Yes, so there's only one possibility. These things are basically the small failed stars.
They form the same way a star forms, we've talked about giant molecular clouds collapsing like this one in Orion,
and we know that there are stars down to maybe 120 times less than the sun, which we call these brown dwarfs,
and maybe there are ones that are smaller still formed in the same way.
But, as you said, the numbers don't quite seem to work out.
Yes, we count how many stars there are, we seem to actually be going and, as you get smaller and smaller, you get fewer and fewer of them.
So, you're going to have to have some little mechanism, something made of a bunch of little ones.
Yes, so if we extrapolate from the bigger ones, we can see that there wont be enough of these things. So we need another mechanism.
Another possibility is, in fact, these aren't failed baby stars, but in fact they are escaped planets.
So, you remember this planetary Billiards simulation, which we've shown many times,
as a way to get the elliptical giants and to get the hot Jupiters,
and maybe to get the stars going backwards we saw from the Rossiter-McLaughlin effect.
But if you look at the simulation, you see that every now and then you lose things.
Yes, so we actually lose quite a lot if we form lots of planets like this early on.
Yes, so it could be that this Billiards explains both the eccentric giants and it explains these few floating ones.
In most solar systems you had a lot of planets, then you lost a few.
They got flung out and they're now wandering the galaxy poor and lonely,
whereas other ones stayed in and got warped into these orbits closer in.
Well, you don't just lose a few, it sounds like we need to have a couple of these things per star.
There's big ones, because already these are like Jupiter and there may be a bunch of smaller ones as well.
Who knows how many little things are out running around.