Gemini Planet Imager: Seeing exoplanets

19 May 2014

Interview with

Scott Thomas, Cambridge University

This week astronomers announced that they now have a powerful new toy to play with - a system called "the Gemini Planet Imager" that is enabling them to actually "see" planets - known as exoplanets -

' alt='The distant exoplanet TrES-2b, shown here in an artist's conception, is darker than the blackest coal.' >in orbit around other stars outside our solar system.

Scott Thomas is an astronomer from Cambridge University; he explained the Gemini Project to Chris Smith...

Scott - Extrasolar planets can be spotted in a few different ways. The ones that's been the most successful is called the transit method. You might've heard of the transit of Venus when Venus passed between us and our sun. Well, this is a similar sort of thing. We look at another star and we wait for the planet orbiting that star to pass in front of the star. When that happens, the light from the star dimmed ever so slightly, just a tiny amount - 1%, 2% - we're talking about. And so, that's how we know that something has moved in front of the star and we know there's a planet around it.  So, this transit method has been very successful and there's a spacecraft called Kepler which up until it failed quite recently to take possibly thousands of potential planets around other stars. There are other methods as well - the radio velocity method which is where you watch a star and you see if it wobbles very slightly due to the gravitational influence of the planets moving around it.

Chris - And I suppose another constraint of these sorts of studies is that they tell us that a planet is there. We can infer from either the way the light changes or the way that the star wobbles, a bit about what the planet might be in terms of its size and what it's therefore made of. But actually, we can't physically see it, can we?

Scott - Exactly and when people hear about the way that planets are discovered, the first thing we think is, "Oh! We see it on the sky" but that's not just the case. However, the Gemini planet imager are actually able to directly see the planets. So, they're able to look at the sky, look at the star, and produce an image with the star in the middle a little spot next to it, and that little spot is the planet.

Chris - Is the difficulty that the star you're looking at is a billion times brighter than the planet? 

Scott - That is one of the difficulties definitely. This instrument uses what's called a coronagraph and a coronagraph is basically when you block out the light from the star, you put not quite a disk of paper, but you've put something over the image of the star to leave only space and the planet around it.

Chris - Sounds pretty easy. Why is this paper published in an international journal this week?

Scott - Well, because there's a lot of engineering that goes into this and in fact, the issue of the star being a billion times brighter than the planet is not the only one. In order to be able to see the planet, the distance from the star we're talking about here is what we call an arc second, an arc second being a very tiny unit of measurement, 1/3,600th of a degree. I think in order to see a planet at the distance from a star is like, trying to see a human hair perhaps at the end of a football field. So, you're dealing with very tiny distances. At these distances, we have trouble with things like atmospheric turbulence. So, any sort of heat, any sort of cloud, anything that's going to distort the air between us on the ground and space where we're looking, that's make it difficult to separate the planet from the star.

Chris - So, with that in mind, where is the Gemini platform and why is that location been chosen?

Scott - The general instrument is on a telescope in Chile because the high deserts there mean that you get a location that's very cloud-free, very clear atmosphere. In fact, it was on the telescope that was already there. I think this is an important distinction to make because when we build a telescope, it's a significant investment in money, in engineering. And so, we want these things to be like Swiss Army knives. We want them to do as much as possible so we'll build a telescope. But we'll make it capable of having several different bits that can be tacked onto the back. It's like if you had a digital camera and you could leave the lens in place, take off the back and replace it with a new camera. So, this new camera in this case is the Gemini Planet Imager.

Chris - What is it delivering? They've turned it on this week.  What does it look like it's going to do?

Scott - The press release this week is about an image that have taken of a system called beta pick. Beta pick is a star. The beta means that they sit in brighter star and the pictorous constellation. Beta pick has a disk around it. In this disk, there was a planet.  The planet is called beta pick b because we name planets with names like A and B because we're very imaginative as astronomers. And the team behind the Gemini Planet Imager, turned on the Gemini planet and they pointed it at beta pick.  Within 60 seconds, they had an image of this planet. So, that's really a pretty astounding sensitivity.

Chris - And what will being able to see these planets at, to what we can currently learn about them just by using existing techniques?

Scott - If you can see a planet directly, you can take a spectrum of it and that means that you can split up the light coming from it and to all its component wavelengths, all its component colours. And then using that, you can draw some information about what's in the atmosphere. So, spectroscopy, the taking of spectrum like this is hugely important for astronomers because it lets us figure out what the components of the atmosphere are. It's the water, it's the oxygen, hydrogen, helium. We see lines in the atmosphere in the spectrum that tell us about this. This is really important because we want to know what these planets are made off, we want to understand something about the chemistry, about how they're formed. Hopefully, tie it back perhaps to our Solar System as well.

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