Artist illustration of an extrasolar planet. Image credit: CfA. Click to enlarge.
Listen to the interview: Microlens Planet Discovery (6.2 mb)
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Fraser Cain: Can you give me some background on the planet that you helped to discover?
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Grant Christie: There’s still a bit of analysis to do on it to figure out exactly all its parameters, but it’s in the order of about 15,000 light-years away. That’s still being worked on, the distance. It’s quite a massive planet, probably in the order of about 2-3x the mass of Jupiter, and it’s orbiting at about 3 astronomical units away from its parent star. It’s not exactly like a familiar object, but if you could see it up close, it would probably look a bit like Jupiter. It would be about 3 times heavier, but not that much bigger because it would be more compressed by its gravity.
Fraser: The planets that have been discovered to date are within a few hundred light years of Earth. How were you able to find one 15,000 light-years away, expecially using backyard equipment?
Christie: With this discovery, we’re just part of a cog in a wheel, we’re part of a team, but it was using a method known as gravitational microlensing. That sounds like a bit of a mouthful, but essentially it uses a star as a lens to magnify a more distant star. This works if the two stars are exactly lined up as we see them from Earth. So we have a situation where we have a distant star somewhere in the halo – or the bulge – of the galaxy maybe 20,000 light-years from Earth. By chance, another star has come almost exactly in line between us and it. That intervening star’s gravity works like a lens and it amplifies the light of the more distant star. We can’t see them as two stars, they’re so close together, and no telescope on Earth can. But what we see is the magnification, or the amplification of the light from the distant star as it goes through that lens. All of that’s fine, some 600 of these microlensing events are detected each year currently. They in themselves aren’t that unusual, but it turns out that if you have a planet orbiting the lensing star – the one that’s intervened between us and the more distant one – then that planet hugely changes the characteristics of the lens. It changes the light amplification greatly. What we’re doing is simply measuring the brightness changes of the lens as these two stars come into alignment and then move out of alignment. It turns out that the one we were observing, the light was magnified by something like 50x over and above what was there before the lensing started. That brings faint stars that we normally couldn’t see with a small telescope up within our range. In the current case, the amplification brought it up to magnitude 18 in the visual wavelengths. That’s very close to our limit, but we were still able to do it.
Fraser: Was your team expecting to find evidence of a planet before you began any observations, or was that just a happy outcome?
Christie: It is largely a happy outcome. There’s a team based in Chile, a Polish team from Warsaw University let by Professor Udalski, and their job, their main function is to find microlensing events. They monitor millions of stars every night looking for stars that just seem to rise in brightness in a way that you’d expect from a lens. There are obviously lots of variable stars as well, which they have already tabulated, so they know about those. They’re detecting microlensing events. They’re detecting about 600 a year. They started observing this event in about March 17th, or thereabouts, and they noticed this star just starting to brighten – it had never brightened before – and they followed it. Each night as they took an observation, it appeared to brighten more and more, and as this process goes on they noticed that it was following a particular brightening curve that you’d expect from a microlensing event, so they were confident that it was a microlens. And then as we got closer into April, it started to show signs that it was departing from a pure simple lens you’d get from a single star all by itself; that’s a mathematically defined shape and if the photometry’s good, you can usually tell whether you’ve got a single lens or not. Around April 18th they started to notice a significant departure from that simple lens model, these are the guys running the OGLE team. They put out an alert that went to MicroFUN, who is a group we’re associated with. They run out of Ohio State University, led by Professor Andrew Gould there. We then received notification saying, it looks like there might be an anomoly with this microlensing event; try and observe it as much as possible. That’s really where we started our observations. By that stage it was faint, but it was still within reach of our telescopes. We were surprised that it actually was observable. I would have thought that it was too faint. Now I know that we can do work at a fainter limit than I’d previously thought. It was known by about April 20th that this microlensing event had a strong anomaly in it, which is the term they use, and we followed it for the following few days – probably about 3-4 days. It went through some very strong anomalies that really were a sign that was a planet present causing those anomolies. Most of these events you observe – I’ve done quite a few, probably 20 at least myself – turn out to be a simple lens, and there’s nothing surprising in them at all. The excitement of doing this sort of work is that you simply don’t know, nobody knows what you’re going to find. You start following one of these microlensing events as it reaches its maximum, and it’s at the maximum point, or close to it when the maximum sensitivity to a planet is going to be. We’re just not that interested in looking at them until you get very close to that maximum. And that’s when the networks really come are really start to saturate the light curve by covering them.
Fraser: So the stars have to be lined up quite nicely for the effect of the planet to show up.
Christie: Yes, they need to be nearly perfect. That creates a very high amplification. Some of the ones we’ve looked at have had amplifications where the light is magnified 800x. They’re not common, but when you get a very high amplification lens like that, when the alignment is nearly perfect, that’s when you’re most likely to find a planet if there’s one present.
Fraser: How sensitive can this technique be?
Christie: Some of the experts have said that had this planet not been bigger than Jupiter, it was the size of the Earth, these observations still would have detected it. I know there’s some debate about that amongst the academics in the teams, but broadly speaking, that’s probably an indication that this method can be very sensitive. And this event actually didn’t come up to be that bright. We’ve observed ones which have come up so bright you could see them in a little 6″ telescope.
Fraser: That’s amazing, though. I know people have been discussing different techniques that they might be able to see Earth-sized planets orbiting other stars, but to know that we might have a technique available right now is pretty impressive. I wanted to talk to you a bit about how amateurs can get involved in the discoveries in astronomy. Where are some avenues that people can get involved?
Christie: There are lots of ways you can get involved in observational astronomy, but in talking about photometry, which is a measurement of star brightness, you basically just need a telescope with as much aperture as you can afford. A decent sort of mounting and a CCD imaging camera. For below $10,000 you can set up a system that’s very capable, and can actually be really useful. There are lots of other things you can do in observational astronomy that don’t require that, but to do this sort of work, that’s what you’d need. We do work other than this microlensing work, we also measure the light changes of objects called cataclysmic variable stars. These are interesting objects that do a lot of flickering, and all sorts of things, and we’re part of a worldwide network that follows that kind of object. Generally, the common denomenator is the measurement of brightness over time of some star or object. That’s called photometry, and that’s primarily what we do.
Fraser: Congratulations on your team’s discovery of this new planet, and good luck with your work in the future.
Christie: You’re very welcome. I’d like to pay tribute to my co-worker here in New Zealand, Jennie McCormick, who uses the smallest telescope of all, and has done way over a thousand hours on this kind of work and deserves the recognition from her efforts put in.