Stopping Starlight to See Planets
Ben - How do we spot planets in the glare of their parent star? Ben Oppenheimer is associate curator of astrophysics at the American Museum of Natural History and he studies exoplanets and a type of failed star called brown dwarfs...
Ben O - Well there are a number of ways to detect these things. What I'm working on is trying to see them directly. I want to see what these things look like and when you're trying to actually see a planet around a nearby star, you have this terrible problem that the star is many millions to billions of times brighter than the planet. And so, the planet will be lost in the glare of the star. So, what do you do? Well, you can study these things indirectly which is largely how people have done this to date. You look at the star and you look for tiny modifications to the star's light or the star's position that tell you that a planet is there, and there's quite a bit you can do with that. It's exciting work. First of all, you can discover that there are planets around these stars. You can start to discern a few things about their atmospheres, but you're limited. You really need to separate the light of the star from that of the planet, and then we can get into really doing the astrophysics of these objects.
Ben - So, you need to find some way to block out from that star?
Ben O - Absolutely, yes. So the real trick and largely what I'm working on these days is a technique called coronagraphy which is essentially creating an artificial eclipse of this distant star. It's much like when you're up on stage or say, late at night, a car is approaching you, you'll hold up your hand so you can see a bit better. We do this in a slightly more precise way in our instruments with distant stars to try to see very faint objects next to them. So right now, I've got a project which partially involves the University of Cambridge working with Ian Parry here and we built an instrument that does exactly this, and we use it regularly at the Palomar Observatory in California.
Ben - Could you not just do exactly what you just said and put something in the way between you and the star, perhaps a large disc, and just hold it up between your telescope and the star itself?
Ben O - Absolutely. In fact, one of the techniques is exactly that. You place a large - what we call - star share out at a tremendous distance away from the telescope. This is tens of thousands to hundreds of thousands of kilometres. The problem there is that you have to position these things very carefully. There are some plans to do space missions like this. For example, you could take even the Hubble Space telescope and put one of these shades in a separate space craft. That's a large technical problem and of course, you now have to coordinate two different spaceships and all this sort of thing. So we tried a little bit easier way in the sense that we have very small optics, tiny 1-inch size pieces of glass that we use inside the telescope in a camera that's attached to the telescope.
Ben - So rather than interrupting the light before it gets to your telescope, you're tweaking what the telescope actually receives in order to cancel out the light from the star.
Ben O - Exactly. We're essentially manipulating the starlight at a very, very precise level. This glare that you see around a star is highly amplified by any defects in the optics of your telescope, and in the camera that you're using to image things. So, we have to control those defects very, very carefully. A little bump on one of these mirrors at a level of a few nanometres is enough to disrupt the light so much that you just won't see anything, even like Jupiter which is a rather large planet around a distant star. So we have to be very, very careful with these things. The benefit of a star shade as you mentioned before is that the light doesn't even get into the telescope in the first place, so you're starting out at a much better situation.
Ben - So it sounds like a very big technical compromise in order to block out the light that you don't want. How much light do you actually get from the planet? I'd imagine it must be miniscule.
Ben O - Well, you'd think these things are ridiculously faint, but actually, they're not. Around bright stars, say you're looking for a reflected light off of a planet - although Jupiter would be about 10 to the 8 to 10 to the 9 times fainter than the star itself, that's not that faint in terms of the sorts of things that astronomers typically look at. If you talk to a cosmologist, they're looking at things that are so ridiculously faint that might be less than a photon for a couple of minutes. You're looking at things that are just ridiculously faint in comparison to the planets. It's not the intrinsic faintness of the planets that makes them hard to study. It's that damn star light!
Ben - What sort of planets can you see around this? You mentioned things to the scale of Jupiter and as you said, these are very big planets. Could you see anything Earth-sized?
Ben O - Obviously, everybody wants to know, are there places like earth out there and if there are, is life prevalent on them. For a number of years, I worked to define the science goals and how they feed into some of the technical aspects of a mission called the terrestrial planet finder. The purpose of that is to make a coronagraphic telescope in space that would be able to actually see a planet like Earth around say, the closest hundred or so stars. There, the problem is that these Earth-like planets, if they're really like Earth, they're going to be 10 to the 12 times fainter than the star. Now, you're really looking and talking about a needle on the haystack problem. So for every 10 to the 12 photons coming from the star, only one is coming from that planet. So it's a tricky business. What we can do now are warm, very young planets, sort of like the size of Jupiter. These things are still hot. They're so young that they haven't quite cooled down to what you might call steady state temperature. And because they're hot, they're much brighter.
Ben - Can you use the same techniques to block out infrared light as well as looking in the visible?
Ben O - Yes. In fact, this project that I have going on at Palomar is operating solely in the infrared. One of the benefits of that is that you can image other types of things. For example, many young stars have debris discs around them. This is dust that is presumably in some cases, beginning to form planets and one of our discoveries a couple of years ago was around the star called AB Aurigae. This wonderful structure in the disc around it that seemed to indicate that something was forming there so you see a little hole in the disc and some clumps, and it looks like maybe there's something that we haven't quite seen yet, just starting to accrete material and form, who knows - maybe something like Saturn or Jupiter. We'll have to wait quite a long time to see what that is.
Ben - So, if you can get a relatively broad spectrum view of these planets that you couldn't see because of the glare of the Sun, can we start to infer some things about the atmosphere based on the reflected light?
Ben O - Absolutely. In fact, that's where the real science is. The science here is taking the light and dissecting it, making the spectrum, so measuring the brightness as a function of colour. And when you do that, you can actually detect the presence of molecules like water, methane, carbon dioxide, carbon monoxide, even ozone would be detectable. This is exactly what we want to do. This is how you really get to the physics of it and this is why we need the direct detection of these things. There are some objects that show spectra very similar to planets in the night sky and those are brown dwarfs. These things have things like methane, water in the form of steam and carbon dioxide, and one can actually study these and determine all kinds of things about the atmospheres. You can determine wind speeds, upwellings, disequilibria between various chemical species, and in fact, that's probably how life will be detected outside of our solar system - by looking for a disequilibrium in a couple of chemical species say, methane, water, oxygen. We know that our own atmosphere on Earth would be very, very different if all life just disappeared. This would be a very unpleasant place to live actually.
Ben - I'm assuming that brown dwarfs, which in themselves are fascinating things, not quite big enough to be a star, a little too big to be really considered a gas giant. They're not somewhere we should be looking for life.
Ben O - Probably, not. Although there could be moons around these things, you know, that are possibly habitable. I mean, one of the crazy things is that, outside of Earth, in our own solar system, some of the moons of for example Jupiter like Europa may in fact be a great place for life to live. There's tons of water there. There may be an ocean under its very icy surface and who knows? Maybe there's a fish there. You know, I have this dream that there was a probe designed to go there and melt a hole. In the dream is that they turn on the camera and there's a fish looking back. It would be a wonderful discovery!
Ben - A wonderful discovery perhaps but definitely a bit of a shock both for us and for the poor fish. That was Ben Oppenheimer from the American Museum of Natural History, explaining how small and precisely designed optics can cancel out the light from a star, making it far easier to see the planets that orbit it.