Searching for extrasolar planets

Looking for planets around the stars of the night sky
03 December 2013

Interview with 

Didier Queloz, University of Cambridge


Chris -   Hello. Welcome to the Naked Scientists at the Cambridge Science Centre. I'm Chris Smith and this is another of our live events. Welcome! The whole point of these programmes is that this is where you, the audience become the interviewers. It's your opportunity to give our guest panel - who we will meet in a minute - a really good grilling. This week, we're going to talk about space science.  All of our guests this week are connected in some way by the field of space or space science and space exploration.  It's quite pertinent because in recent years, we've actually heard stories about people planning to mine asteroids.  We've also heard about people trying to build a hotel, an orbiting hotel, in space.  We've had a call for people to join a manned mission to Mars. They don't want elderly people though.  It's amazing the lengths the government will go to to cut the care in the community bill.  And more recently, they've actually announced just last week, a proposal was put forward to build a barbers on the moon.  It's going to be called E-clips. It is of course nearly Christmas time, but there is some concern this year over whether father Christmas is going to be able to make all his deliveries because there was a bit of a slay accident recently during a test flight.  Father Christmas collided with an asteroid and NASA did catch this with the Hubble space telescope and when they first saw it, they said they think it's a UF ho! ho! ho!  Of course, during this programme, we will be obeying all of the laws of physics.  Very important because the penalties for not doing so are extremely harsh.  In fact, just a famous astronaut Buzz Lightyear discovered when he broke the laws of gravity, he got a suspended sentence.  Let me introduce our panel.  Please welcome this week, we have with us, Didier Queloz.  He is from the Cavendish Laboratory.  We have sitting next to him, Alan Tunacliffe who is from Chemical Engineering and Biotechnology, and Gerry Gilmore is on the end.  He's from the Institute of Astronomy.  They're our panel this week.  Sitting primed on their experimental bench, Ginny Smith and Dave Ansell, who will be doing lots of exciting interactive demonstrations and experiments for us.  Let's kick off with you Didier because you're famous for having discovered the world's first exoplanet.  So, you're going to tell us what one of those is.

Didier -   Yeah.  Well, it was almost 20 years ago and first of all, I think we should start with the beginning.  I mean, what is an exoplanet?  So, we have planets in our solar system orbiting our sun and the next question is, we have plenty of suns everywhere.  We call them stars.  The next question is, what about the planet orbiting these stars?  So, these planets that now we have found: there are about 1,000 of such objects found.  We call them exoplanets, and that's my work.  I'm trying to understand this object, detect them first, and then to try to understand how they form, and how they are.

Chris -   Let's put some numbers on this.  So, in the universe, how many stars are there, apart from Rod Stewart and Tina Turner or Elvis?

Didier -   It's difficult number to capture because the universe is so big.  So, it's difficult to figure out. We have galaxies; it's the biggest structure that we are in, and that's about 100 billion stars.  So, in the Universe, there are zillions of galaxies.  So, it can make up of a very, very big number here.

Chris -   With that very, very big number, how many planets might there be around all those different stars then?

Didier -   Well, it was a big question 20 years ago.  We had only our own planets at that time and the big question is, are we unique or we part of a more bigger set of planets?  So, I think we have clearly answered this question.  There are planets almost everywhere and it's pretty common to find planets around other stars.  What is a bit more difficult here is trying to understand what they are like because as of today, we have not really found a planet or a system like our own solar system.

Chris -   Like the Earth is what you mean.

Didier -   We have found kind of object that look like the Earth, but they're not really exactly like the Earth.  That's a big problem then.

Chris -   How do you find them?

Didier -   Well, I think here, this is a good time for the experiment.  So, the problem we have with the planets is a very simple one.  I mean, the planet is orbiting a star and the star is extremely bright.  So, you could take any means to make a picture, you will only see the star.  So, you have to find tricks to try and see something which is almost invisible.  So, in a practical sense, when you have the planet orbiting the star, what you do have - and this nice experiment here - because the planet is very light, Jupiter, which is the biggest planet we have is only one thousandths of the weight of the sun.  So, it's very tiny.  Then when you have the planet orbiting the stars, what you do see here is practically, all the system is orbiting around what we call the centre of gravity of the system which is in the case of the sun, almost in the middle of the sun.  It is still inside the Sun, but it's a little bit to one side.  So, there's a tiny motion that the sun is doing, by the fact there is a planet.  So, you do obviously see the motion of the planet; that is what we call an orbit.  We know that the Earth takes one year to go around the sun.  But because of the Earth, the sun is doing a tiny bit of motions and then these motions, this is what we have been using to detect a planet.

Chris -   So, when you said there's a little bit of motion, the sun wobbles backwards and forwards a little bit because the planet going around it exerts a gravitational influence on the star.

Didier -   That's a perfectly the right way to explain this. Exactly.

Ginny -   So, we're showing this over here.  Dave setup this wonderful example.  It's not quite what you'd see if you had an actual star in an actual planet.

Chris -   It's a door knob on a stick, is it?

Ginny -   Yeah.  So, we've got a nice gold door knob to represent the sun - shiny, bright, and we've got - is that ping pong ball on the other end?

Dave -   It's a ping pong ball, yes.

Ginny -   We've got a ping pong ball on the other end of a stick.  What Dave is doing is he's got that on a piece of string and he's spinning it.  You can see that this means that the ping pong ball is going round and round the door knob.  The piece of string isn't ending up in the middle of the stick like you'd expect if it was a similar way to both ends.  It's actually orbiting around the centre of mass.

Dave -   So, this means if you look at the door knob, the door knob is wobbling in the same way that the sun is orbiting because the Earth is orbiting it.  There isn't actually a stick between the Earth and sun of course, but the physics is very similar with gravity and the stick in this particular case.

Ginny -   So, the sun is actually doing a little bit of orbiting itself.  This seems very simple with just one planet.  I can imagine it, it must get very complicated if there's more than one?

Didier -   Yeah, it's a bit like a watch after.  I mean, we have the needle that takes the hours, the other one, the minutes, and then the second.  So, like watch, you can read through it if you have enough time to watch the whole motion of all system.  That's what we need to do if we want to capture the longer period system.

Chris -   What do you use to spot these wobbles?

Didier -   Okay, that's the second big question because what we have seen here is the principle of the motion.  But the problem is, what do you do to detect that motion.  That's not at all obvious.  So, a single trick that we can use, which is related to the light - I mean, the light is about like the sound.  It's a wave.  If you move an object, the sound will change.  So, if there's a tiny difference in the wave, we can see there's a change of colour and then because of these motions, this would create kind of a tiny change in the colour in the waves.  So, we can see this experiment.  I just tried to show this instead of having the light which is just too fast, but using the sound.

Ginny -   So, we've got another really high tech experiment for you here.  We've got a ball of bubble wrap with some kind of alarm inside it, you said?

Dave -   It's a buzzer I bought this afternoon.

Ginny -   So, we've got a buzzer in some bubble wrap and we're going to put that in a canvass bag.  You can see how high tech this is.

Dave -   So, it's going to make a horrible noise.  I'm going to explain what I'm going to do first before I turn it on.  I'm going to whirl it around my head and if you listen to it, you should be able to hear a slight change in the pitch of a sound as it goes around.  So, let's turn it on.

Ginny -   I'm going to get out of the way because I don't fancy being hit in the head.


Ginny -   Did everyone hear that?


Dave -   So, as the bag is moving towards you, the pitch went up slightly and as the bag moved away, the pitch went down.  As Didier was saying, the same thing actually happens with light.  The physics is slightly different because Einstein and relativity gets in the way.  If it's moving towards you, the light gets slightly blur.  If it moves away from you, the light gets slightly redder.

Didier -   That's exactly what we're doing.  We're just detecting this, but then it's a tiny change of the light.  That's pretty tough...

Chris -   You're not using a shopping bag.

Didier -   No, we don't use that.  I mean, this is where the difficulty here because we're talking about tiny motions and digging up such a tiny motion - I mean, if you take Jupiter for example, it's the speed of the running man.  So, it's very tiny compared to the speed you encounter in space.  So, it's really tough and then the equipment that we needed to build to measure the speed change - I mean, couldn't be built before, so we had to wait so long to build this such first experiment that was doing that and that's what happened 20 years ago.

Chris -   So Didier, you've said very nicely how you detect these planets.  How do you work out whether a planet is, say, like the Earth or not?

Didier -   Yeah.  That's another aspect of the detection.  So, in some case, you can be lucky enough to have the orbit of the planet, crossing exactly the disk of the stars and it's called a transit.  This transit, it's a kind of a shadow that is produced by the planet on the stars.  So, you don't see the shadow itself because the star, we don't resolve it, so we don't any picture of the star.  But you see a tiny change in the light of a star, like if you have clouds passing in front of the sun during the day and you don't really see the clouds, but you feel that there's a tiny difference in the light.  So, it's exactly the same and from that slightly drop of light, you can get the size of the planet.  So, if you can get some way to weigh the planet, the technique that was described before called the Doppler technique and some wave to size the planet, so you get two very fundamental parameters of the physics.  One of them is the mass and the other one is the size.  So, if you get the mass and the size, you can build something which should be magic which is called the density.  With the density, I can tell you whether it's like the water, whether it's like a gas, or whether it's like a rock.

Chris -   Okay, let's take some questions.  So, you need to get your thinking caps on for your questions.  What have you got in the email, Ginny?

Ginny -   So, I've got an email in from John Black who actually picks up exactly on what you were just talking about.  So, he asked, if you're using that kind of method to detect planets that are travelling in front of a star, so you're looking for the shadow, does that only work for planets that happen to line-up with the telescope and the star?  Can we find out about other planets that don't line up?

Didier -   Well, yes.  You have to be lined up to get the shadow effect.  Otherwise, you don't get the shadow.  But there is something interesting which is called the phase effect.  I mean, we do see the light from the moon changing because the position of the moon is not the same.  It's not the same light from the sun.  So, you do have such effect as well on other planets.  So, you don't need exactly to have a transit, but at some time of the planet, the planet gets a lot of lights and the other time, the planet show you only the dark phase.  So, if you have good telescopes, very accurate because this is a very tiny change, you can see that.  It's called the phase effect and it's where to probe the clouds on the planet.  In that sense, we do a little bit of weather on these planets by doing this way.

Chris -   Amazing!  Let's take your questions.  What's your name?

Helen -   Helen.  I'm from Cambridge.  You were saying, of the exoplanets that we've found, how big is the largest group that all going around the same star?

Didier -   We have a lot of what we call the multiple systems and what we do see is when we have small planets, because we're finding small planets between the Earth's mass and Neptune's, they are very all fine with others.  So, we have really a series of what's called compact systems made of many planets.  So, I think the system that has the most planets is maybe 6 or 7 of them.  It doesn't mean we have found all of them.  We just usually find the easiest one which is the one we know by the stars.  So, there may be much more, and we clearly do not know how many planets maximum we can give in a system.  But it just depend on the mass and there's a lot of mass in the planet at the beginning when you form a system.

Kyle -   My name is Kyle.  I'm from Cambridge.  Is there any way to tell if say, they's a like liquid within a solid, like a lava in the centre of the Earth?

Didier -   This is a very good question.  So, we would love to know this.  I mean, the only thing we can do right now is to get broad perceptions whether it's a big planet like Jupiter made of gas or if it's a rocky planet like the Earth.  Now, if you start to mix up all the thing together, you cannot have a mix of everything because you can have a planet that have a core.  That could be very heavy like the iron core and then you have what's called an envelope which is a big glass.  It's like the lava you have on the Earth.  Then you can have an atmosphere that could be made of water.  You can imagine a planet that doesn't exist in our solar system where you have a huge, huge, watery oceans that's not only 10 km, but it can be 500 km.  So, we can have a lot and lot of many mix to ways to build your planet.  So, answering exactly your question will take some time.  We designed an experiment right now to trying to probe and to detect some - what's called feature, what's on the surface of the planets.  I'm trying to exactly know that because the water for this reason is kind of a training prospect to get water on other planets.

Chris -   You're listening to a special edition of the Naked Scientists recorded with this amazing audience at the Cambridge Science Centre.  My name is Chris Smith and with us this evening, Ginny Smith, and Dave Ansell, our experimentalists, and our panel of esteemed experts.  We have Didier Queloz, we also have Alan Tunacliffe, and Gerry Gilmore.  In a second, we're going to be finding out about life in outer space and what can survive the rigors of space.  But right now, we're talking with Didier about finding exoplanets.  Ginny, what else have you got on the email there?

Ginny -   So, I've got an email in from Angelita Mills who wants to know why poor Pluto was demoted from being a planet.

Didier -   This is a very interesting question.

Chris -   Let me take a vote on that first.  I mean, who thinks Pluto deserves to stay as a planet?  Overwhelming response.  I'm just very disappointed.  Were you disappointed when Pluto got demoted?  Yeah.

Didier -   So, I have to tell you why then.  We have to go back with the history of the solar system.  It's a very fascinating story.  You have to realise that for a long time, the knowledge of the planets was only related to the capability to see the planets by the eye.  And then it took a long time to move away from that time with the telescope and it expanded and we get Uranus and then using Uranus, you detected Neptune.  And then it took quite a long time for people to figure out exactly what was next.  And then the problem of Pluto is, it's not alone.  I mean, the region of Pluto had been probed when Pluto was been discovered, but they also probe other systems, other planet at that time that was demoted as a planet.  One of them is called Vesta or Ceres.  This was a planet.  These are objects that are as big as Pluto there.  So, all these objects are not planets at all.  So, Pluto is exactly the same situation because it took a long time after to figure out that there are other kind of object like Pluto.  So, the problem is then, what do you do if you start having not only 1, 2, 3, but hundreds of systems like that?  And that's these hundred systems deserve a specific name which is called the transneptunians.  So, if we want Neptunes that is not only Pluto, there is tons of object and that's the reason why at some point, we have to decide what we do there.  Do we continue to count them?  And then we ended up with 100, 200 of such object, or do we define a planet as an object that is kind of isolated?  And that's exactly what happened in these big discussions.  So, people have agreed that at some point, to qualify as a planet, it's a bit tough discussion, but it's kind of clean.  It has to clean everything around it.  So, there are other objects that you find between Mars and Jupiter.  It's called the asteroid belt.  So, it doesn't qualify as a planet and that's the reason why Pluto has been demoted.

Chris -   Now that's an astronaut's favourite music, Neptunes.  Question down here...

Paul -   Hello.  My name is Paul from Cambridge.  If we're looking for planets, I suppose that's probably because we all know what a planet is, but have you, in your searches found anything that's very different to what you're expecting?

Didier -   Well, I think we have been surprised from the beginning and we keep being surprised.  The reason why is that when we started all these, we have this obvious mental picture of solar system planets.  And all the planet that we're finding are very different.  The first one was a big Jupiter, very close to its stars.  Kind of a burning world.  So, there is no counterpart for these systems.  We have what's called super Earth.  It's kind of a very rocky, very hot planet, kind of different from the Earth.  We think that this is a big core of rocks and with a atmosphere of lava.  And again, it's a big surprise.  So, they all have planets, but they are not the planet the way we see it in our solar system.  I think this is just the beginning of the story.

David -   David from Cambridge.  If we were to hypothetically position ourselves and the nearest star in our galaxy, and using technology that we have available today, look towards our own solar system, what would we be able to understand about our solar system?

Didier -   I think today, we would not see anything because we're just on the verge to detect a system like our own.  We have found a lot of other systems and the interesting part of it is, we know that from all the system that we have found, that the vast majority of the stars, they did do have such a system.  So, we know already that the solar system is not the vast majority of the other planet.  But it doesn't mean that there is not such a system.  So today, we would just fail.  The big problem we would have is, Earth is very tiny and then even Jupiter will be a big problem because to get Jupiter, you will need to observe our system long enough and then you would face the activity of the sun.  There is a solar activity, it's a life cycle, and we know that the sun, when there is a high activity level can be pretty active.  A lot of spots on the sun and that prevent us to really do something good to the big planet.  So today, I must admit that we will just miss the solar system.

Chris -

That's reassuring, isn't it? No one is visiting too soon. Ladies and gentlemen, Didier Queloz.


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