At long last we actually know that this thing really is planet size. It actually looks like Jupiter.
This is actually a real scale picture of the thing with all the right properties, which means it's almost invisible,
but you can see the transit every now and then and dimly make out the planet as it whizzes past.
But we now know its radius, it really is not just a back hole or something, it really is a planet of about Jupiter size.
But Paul, this is only one object, and there aren't that many planets discovered by the radial velocity method
and if we're going to go out and look at all those objects, the odds of finding another transiting planet are not that large.
Maybe we'll find one or two more, but we really need more because we can get caught out by only using single objects.
Yes, a lot of people are scratching their heads and wondering how we're going to find more of these things.
We can't just rely on following up the radial velocity ones because most are not going to be edge on.
But for a radial velocity measurement we can only measure one star at a time, it takes a big telescope a lot of time.
But imaging, you're taking pictures of these things and trying to measure the brightness.
That needs much smaller telescopes to have bigger fields of view and look at many more stars at a time.
That was the idea, you might pick some region of the sky with lots of stars, like this one up here,
and point a medium or small telescope like this at it and take a picture
and you might measure a few hundred thousand stars at a time and do that over and over again and look for the transits.
So, the only problem with this is that when you try to take pictures of big pieces of the sky,
you have the problem of actually doing the photometry as accurate. We need to do it, it looks like, to better the half the percent accuracy.
That's the trouble. Photometry is the art of measuring how bright something is
and these dips are only like a one and a half percent, or maybe even smaller if we get small planets,
so you have got to get very high precision brightness from maybe a hundred thousand or a million stars across the whole field of view,
and that's difficult for all sorts of reasons.
I mean, one reason is that you've got your detector which has pixels and let's say the light's falling over here.
It'll fall on this bit of this pixel, every pixel with slightly different sensitivity.
Of course, as time goes on, it's going to move around because of the atmospheric seeing,
so it's going to be jumping from pixel to pixel to all over different sensitivities.
In fact, it's even worse than that because even within an individual pixel,
that bit of the pixel might be less sensitive than the middle of the pixel.
Right, and so we really do have the problem of trying to deal with what, unfortunately, this is what a star really looks like to a telescope,
it doesn't look like the beautiful pointy thing that you often see in pictures
unless you stare at it for a long, long time and are able to average over things.
Well, a solution to this was to actually put the telescope deliberately out of focus,
which is what they do, so replace it with something like that.
In this case, if it's jiggling around, it's going to be almost the difference because it's spreading over so many pixels.
Each pixel might have very different sensitivity, but if you average over a hundred pixels, that's going to average to not much.
So, this is the first approach.
Consider, it goes against everything we know as an astronomer to deliberately put your telescope out of focus,
but these things are so bright you can measure them well anyway.
Well, the advantage, of course, is that these digital detectors can only count up to, typically about 65,000
and, for some of these stars, you're literally getting millions of photons per second;
so this is a good way to count up to a bigger number which means you can average over better in addition to just the atmosphere itself.
The atmosphere itself is another problem.
Here's a time lapse at Mount Stromlo that I took a couple of nights ago, and you can see a number of things here.
First of all, there is a whole bunch of clouds around and there's some kind of clouds moving up there.
Those are fairly obvious ones, but at night you're taking your data of something and some tiny little wisp of Cirrus goes,
or you might not even notice especially if it's a dark night, so that can give you variations of brightness as well.
So, I think "yay! It's a planet!" What you've discovered is a cloud in the atmosphere above you.
Secondly, even when it's a perfectly clear night, there's dust in the atmosphere
and you're looking for these transits over multiple nights and on different nights there's different amounts of dust.
For example, the observatory in the Canary Islands has trouble with this dust being blown off the Sahara Desert over them.
Depending on the wind direction, there's a lot of dust coming over, you can't see it but then it drops the light.
Then again, if you imagine looking at a particular star, you can see all these stars around here,
the whole sky is rotating, compared to the Earth, it's moving down.
So, you follow a given star, as time goes on it's getting lower and lower into the sky,
we're looking at more and more atmosphere which is going to cause more and more absorption.
So, you've got all these things that are going to cause the brightness to vary with time. What can we do about that?
Well, it strikes me that we're going to have to be very clever about how we measure things,
but if we looks at a big part of the sky, we can do everything to sort of inter-compare with everything else we see,
so we don't really care about absolutely how bright the star is,
we only care how it's brightness changes over time relative to everything else.
Because the vast majority of this is that we're not going to have a transit at a particular moment,
so if we average across all of the other stars, that will vary as the atmospheric emissions vary,
and then you look at the brightness relative to that.
If, for example, a cloud comes over, it's going to affect not just one star, it'll affect all of the stars around it;
and so you can say oh well, this star is still this much less bright than that star
and you can inter-compare everything and maybe be able to get rid of this problem of the atmosphere.
So, people using this technique has developed this.
The current state-of-the-art is to actually not look at a small bit of the sky with a fairly powerful telescope,
but have a whole bunch of much smaller telescopes.
This is the HAT-South Telescope centre at Siding Spring Observatory in those boxes over there,
and when they open up you see, once again, a bunch of fairly small telescopes.
The benefit of the small telescopes are that they can look at a very wide area of the sky
and because you've got a very wide area of they sky, you can pick out some really bright stars.
You don't need a big telescope to get decent counts on them and also, because they're so bright, they're going to be really easy to follow up.
Yes, and so this is a project which we're doing up at Siding Spring and each time it looks,
it's actually looking at four identical pictures offset in the sky and we can look at almost 250,000 stars at a time with this telescope.