Astrophysicist Didier Queloz on the how to find exoplanets, their impact on astronomy, and the uniqueness of our Solar System.
An exoplanet is made of two words, one is planet and we know what a planet is. We have planets in the solar system. And we had ‘exo’ to mean that these planets are not in the solar system. So, these are planets on other stars. Our sun is a star, it’s a star like all the other stars that you see at night and then these stars have also planets. And they have the specific name of exoplanet. So, it’s a kind of new object of the astronomy. The first exoplanet was found a bit more than twenty years ago. When the very first was found orbiting an object which is not a star, that is a pulsar star, it’s called a neutron star it was in 1992 and 1995 was discovered for real the first planet on the star.
Since then we have found a lot of exoplanets on other stars. And now we are convinced that there are plenty of these orbiting the stars. But we don’t really understand the whole diversity of them. The solar system has a very distinct kind of planet. The one which is the smallest one, and the Warth belongs to it is called the Terrestrial planets and we have the giant planets like Jupiter, Saturn. The planets we have found on other stars, the exoplanets are the easiest ones and the easiest ones means the biggest. And we have found quite a lot of other Jupiter’s, we have found very few small planets but whatever we have found they are different from the planets on the solar systems.
All these exoplanets that we have found are in a way asking a lot of questions about our own system. Why are we exactly like we are? Why are all these other systems different? And that’s practically the main topic of astronomers right now. We know that there are plenty of other planets on other stars, I think in a way that is a solved problem. It was not the case 20 years ago. And a long time ago, someone that would claim this, I mean could have real trouble. And we know the stories of people being burned, because claiming that there are other worlds. The concepts of other world or other planet, is a kind of old concept still. The Greek philosopher they had mentioned in texts that maybe there are other trees, other oceans, other lands on other stars. The concept is quite around there since a long time. But, it took us a long time to demonstrate that these planets exist.
The reason why it took a long time is because it’s difficult to detect a planet. Well it’s difficult because it’s small. A planet like Jupiter, is one thousand times the mass of the sun and it’s a big one. Well, it’s also difficult because it doesn’t shine. A star is shining, there is a nuclear reaction going on in the stars with a lot of light. So, in a way the fact that you have these planets, orbiting a star makes them difficult to find because you are blinded by the star’s light. It’s a bit like you have a light from a car and you want to see a mosquito next to it. It’s about the same. So, it’s really something difficult and the process to find these planets requires quite a lot of technology to detect these tiny objects. So, the trick we have been using for a long time is instead of trying to see the planet directly we have tried to look for a sign of the planet by looking at the star. When you have a planet orbiting a star there’s a couple of things that can happen on a star. One of the first effects you have is the motion of the planet changes a little bit, the motion of the star. So, you see the star a bit moving around tiny bit, small motions. So, if you have instruments sensitive enough then you can pick this and you can detect this. And, by detecting this you would find that the only way to have these tiny motions from the star is because you have an orbiting planet. Well, if you are lucky you can have the planet just going in front of the star. So, for a very short amount of time there is a kind of shadow and there is a decrease of the flux, it’s called a transit. That’s also a way to find planets. So, that’s all the tricks that we have been using since 20 years. There are a couple of other ones but those are the main tricks we have been using and right now I think the number of planets found that way are of the order of four to five thousand. We have also for few systems a kind of a picture, because there are some techniques some ways to use big telescopes and to try to suppress the light of the star. It only works if the star, if the planet is bright in a way, so the planet has to be young, has to be brighter. When the planet is brighter it’s younger, and the planet has to be quite far away, not too close. So, there are a couple of them, but most of the knowledge we have right now on exoplanets comes from these two techniques.
There’s a lot of problems right now with all these findings. The main is we have not really detected a system like our own solar system. It does not mean that they don’t exist, it just means they are difficult to find. I mean a planet like the earth is 300 times smaller than Jupiter. So, it has a tiny effect on the star making the earth tremendously difficult to be found. And the chance to have an earth transiting in front of a star is marginal or zero. If the earth would be closer, it would be much easier and actually we have found a lot of earth size planets much closer. We have found also a lot of planets like Jupiter closer. Well practically all planets we have found on other stars are way closer [to their star] than the closest planet in our system which is Mercury.
So, we have detected practically thousands of planets that are all different from the solar system. That’s interesting because it shows us that maybe there is not only one way to make a planet. There may be a different way, like there is different weather. So, there is some places with more rain than others. So, there is some places where you would form a system much closer to the stars. Where there’s a lot of consequences with this is life, because if the planet is too close, it’s too hot; if it’s too hot there’s no way you can have life. Any molecule will be destroyed. So, there are a lot of interesting questions about this, the fact that we are in a system that looks different from the other systems we are seeing. The fact that our system has a very well structure, with the small planets inside, the big one outside. All this has to be related to the formation mechanism of these planets and how special may be the solar system, how different may be the others. And that’s the whole idea of the whole field of exoplanets, and that’s the whole interest of that field. That’s why the astronomers are looking for the systems, because they want in a way to better understand the solar system. That’s the best way to understand the solar system. To look at other stars and to find out how many of these stars have a planet like the solar system; how many are different and why are they different.
So, there are different kinds of categories of planets. Practically, that depends on the structure. Some planets are practically made only of rock, like the Earth, like Mercury, like Mars. They are very small. Other planets are much bigger but they are not really made of rocks, they are made of gas. Those are planets like Jupiter. There are planets in between the two like Uranus and Neptune. They have some rocks, some gas, some gas because it’s very cold are kind of icy. So, it’s a kind of icy, rocky planet which is what we call a Ice Giants. There are also plenty of other planets in between. There are planets that are not really made of gas, not made of rocks but a mixture of the two. Or maybe, a planet with a lot of water. There are planets which are a mixture of different kinds like a rocky planet with a lot of water. We call that the water world. There could be also a rocky planet with a lot of gas. That would be a planet with a very thick and big atmosphere. You can have a planet which is made only of water. If you take Neptunes, and bring it closer most of the ice in the planet would melt and it would become liquid. So, you see when you look at the planets of the solar system if you change a little bit the ingredient, if you change a bit where the planets is, you change the temperature, you are changing how the planet looks like. So, this is something we have learned by looking at all these exoplanets. There is a range of diversity, there is not only this very strict category of planets you have in the solar system. When you have on one hand the terrestrial, the giant and the ice giant. There is plenty of mixing, depending how you can move around the planet or how you can mix the compositions. And I think we still have a lot of surprise ahead with a lot of findings that we’ll make in the next couple of ten years.