[MUSIC] Welcome. This week we are going to discuss a specific case on exoplanet which are the transiting exoplanet. To start with, we will open the chapter which is the configuration to understand how it looks like the transiting planet. So practically amongst the many possibilities to have a planet orbiting the star you may be in a situation where the planet is just crossing the planet orbit, is just crossing the stellar disk. This produces a special event that helps you to get parameters on the planet, and that's what we are going to understand this week is how we can make use of this special case of transiting planet. We have with the solar systems, very well known situation which called the transit of Venus, which is, is a historical element that has been used in the past to understand the solar systems. But give you also a nice comparison with extrasolar planets. In the case of Venus, our own planet transiting the sun, you see that there is a tiny spot spot, dark spot. But it's like a kind of a shadow on the on the stellar disc. And and that affects the way you see the sun. So for the extrasolar planets it's exactly the same, the, the transiting planet will affect the way we see these stars. So, there's an additional element that we can pay attention, which is, when you start the transit you see there is a lot of different color in the stellar disc, which is due to the fact that you don't see exactly the same kind of structure of the atmosphere of the, of the stars. And that produced some kind of a tiny effect you have to account for when you have a transiting planet. The other element you see on the fissure, on the figure, which is around the planet, there's a thin disk which is rela, related to the atmosphere of the planet that you see through the light of the star. So this is also a specific element that will be discussed and again used to probe the atmosphere of the planet. Speaking about the configuration of the transit, practically this looks like a bit the moon orbiting the earth. You have phases. There is time where there is the time of the dark side facing us and the time of the of the transit when there is the dark the, the bright, the bright side of the of the planet. So when you look at the effect on the star, these would produce a flux change, the fact that you have an orbiting planet around it would produce a kind of magnification of the flux, and then you have two specific events. The time when the transit, the planet transits the star, then the shadow produced by the planet on the star, which is this event. And you have the time when the transit goes behind the star, which is called the occultation. These are two different events. That can help you to get two different kinds of informations on the planet. And, we'll come back on that later in the discussions. So a transit is a special configuration of the orbit. So, you have to see it in a very special way, otherwise there is no transit. Or how likely it is. So you practically need to have this shadow effect, so you can compute this. You have to have the planet practically aligned with the star that you see. So the way to do that is, to consider that the, there is a shadow zone, from the star. And if the planet is in the shadow zone, then you have a transit. Of course, this depends how close you are. If the planet is very close to the star, the chance to be in the shadow zone is higher. If you move the planet a bit away, you see that the angle of the shadow zone will be decreased. So there is a high impact on the probability of the transit that depends on how far you are from the star. So the very close short period planet are more likely to transit, that the one a little bit farther away. That's very important element that you can use when you want to detect transit. And it's also an element that affect the kind of planet you detect, because your chance to detect a very short planet is higher, you will have much more short period planet detected by the transit, that the planet would be farther away. Excepted, if there is so many, so many planet farther away that compensate for the lack of probability. So it's a, it's a bit something a trick and we'll come back on this probability discussions later. So to go a bit into numbers, to understand what, what, what probability is. How likely? Is it 1%? Is it 1 over 1 million? So you have to go through this angle. So, practically, you can see through the angle that this depends of two simple parameters. The size of the star, plus the size of the planet which is marginal, because the size of the planet is one 10th typically for Jupiter compared to the Sun, for the sun. So it's, it's a tiny corrections depending how these orbit. But divided by the orbital distance a, which is a semi-major axis. This ratio gives you the probability of the transit. In addition to that, it depends on the shape of the orbit. And if you have an eccentric orbit, which is being described by the e parameters, this completely change the probability. So, it's interesting to consider the eccentricity because it can increase or boost the probability to have a transit, by comparison of the non eccentric planet. So if you put numbers into this, well, you can come up with a typical figure to have in mind. If you take the size of the sun as a typical star a typical star, if you take the orbital distance of the Earth, this is one AU, then the probability of the Earth to be seen eclipsing the, Sun, from another star is less than 1%. Actually it's 0.5%, so it's extremely rare. Technically, you would need 2000 planet like the Earth to have a chance to find one. So it's very rare event. So the eccentricity here can help you. There is a very famous case that connect the detection with the eccentricity. This is one star HD80606. It has a very extreme eccentricity. The orbit is extremely elongated, and the eccentricity is 0.93. So if you plug this number into the equation I showed you before, then you will increase the probability by more than seven times. So, in this case, the transit probability would be very low, but because of the eccentricity, it becomes very high. And, actually, on that very specific case, a transiting planet has been found, because of this number. So, at the time of the transit, this is the detections of the transit, by many different teams because this needed quite a significant number of measurements to detect it. And you, you see the decrease of the flux of the star at the time the planet enter, and the transit. And there is also the other event, which is called the occultations, when the planet goes behind but has been seen. So this is one case where you can increase th probability to find a transiting planet. But the goal case is not the eccentric planet. The goal case are the hot Jupiters. The hot Jupiters were not expected at the beginning of the search for planet. And it has completely changed the way we understand the formation of the planet. And it also change the hope to find transiting planet. If you take a hot Jupiter, they are so close, they are within the orbit of Mercury, then practically they are 20, 20 times closer than the Earth orbit. So you can multiply this 0.5% I mentioned before, by 20. If you do that, you find that the probability of a hot Jupiter to transit its star is 10%. So, it's not anymore 2000 stars you need to have, it's only ten. So it's completely changed again, and that's why, after the detection of the first hot Jupiter, 51 Peg, a lot people got interested into the transit, because they realized that then, the chance to find a transiting planet, it may because of the hot Jupiter, much, much higher than expected. And what happened in the past is by the time the discovery has happened from 51 Peg, the number of the hot Jupiter has increased. This is all the detection that has been made through the radial velocity techniques, as I described before. So, it's just a matter of time. One of these, at some point, we'll be convincing us of exactly what people thought of at that time. And actually after five transiting planets known, the sixth was the good one. The sixth was this one, HD209458. It's a very famous star because, using very small telescopes, knowing that there is a planet detected on that star through radial velocity techniques, then, practically, you come up with these two diagrams. On the left side, this is the detection of the transit. It's very easy even to detect, typically 1%. And the other side of the diagram, it's the radial velocity curve that we've described before that tells you that there is a planet of the mass of Jupiter orbiting that star. So that was a time where everything has changed. The reason why is first, we detected a transiting planet was more than 13 years ago. But then it brings a completely new way to see the hot Jupiters, because at that time, after four years, four years after the detection of the 51 peg, we have a different view on these very weird systems. And some part of the community didn't believe that all these planets were real at the time, and that was the final proof. Because if you come up to the same conclusion with two different techniques, there is no other way to understand, what's going on apart the fact that is a hot Jupiter orbiting these these stars. So the transit at that time was a key techniques that's been used to demonstrate that all this weird hot Jupiter were real. And starting from this point, in the next chapter, we are going to discuss how we can use these hot Jupiters, all this detections, to get parameters on the planet. [MUSIC]