00:00Here's another very interesting discussion.
00:02Essentially, we don't have to enlarge the stars too much to see what their surface areas are.
00:08Currently, astronomers can measure a star's area from the apparent brightness, the star's absolute luminosity,
00:14and very importantly, the temperature of the star.
00:17In fact, the temperature of the star determines the star's absolute luminosity through the Stefan Boltzmann law.
00:23Well, I won't go into details here, but it's enough to know that astronomers can tell how big the star
00:29is without magnifying it.
00:31This allows them to determine the exoplanet's size.
00:34We still need another important parameter, which is usually the mass of the planet.
00:39Planet mass is typically measured indirectly.
00:42Namely, let's say, for example, by the Doppler method, meaning the exoplanet orbits around the star,
00:48but actually, it orbits around a common center of mass.
00:51Both of them do. The exoplanet, as well as the star, orbit around a common center of mass.
00:57That means the star moves. It goes forward and backward, forward and backward.
01:01And then, the light coming from the star will be red-shifted when it moves away from us,
01:06or blue-shifted when it comes toward us.
01:08So if we measure the light from the star, the spectrum, we will see an oscillation.
01:12The spectrum shifts toward red, then toward blue.
01:15And it's very important that the amplitude of this oscillation is a measure of the ratio
01:20between the mass of the planet and the mass of the star.
01:24Because, you realize, if the mass of the planet is very small,
01:27then this oscillation has a small amplitude because the star stays roughly in the center.
01:32It's true that a star has several exoplanets.
01:35So, this Doppler motion has several components,
01:38but we can decompose this motion into frequency components,
01:42and of course, we're interested in the frequency component
01:45that has a period equal to the orbital period of the planet around the star.
01:50So, basically, from this motion, we can also deduce the mass of the exoplanet.
01:54And the last thing we're interested in is the planet's atmosphere.
01:58We take advantage of the fact that when the exoplanet is in front of the star,
02:02the light passes through the exoplanet's atmosphere.
02:05And in this way, the absorption spectrum of the light inside the atmosphere can be measured.
02:12Some calibrations are done there.
02:13Basically, the area of the planet is calibrated against the area of the star,
02:18and it's subtracted.
02:20So, that calibration, and if you subtract those two values, basically, you get...
02:24You're left only with the light that passed through the atmosphere.
02:28And that light that passed through the atmosphere,
02:31well, that specific intensity.
02:33You divide it by the intensity of the light coming from behind,
02:37which gives you a kind of calibration from the star,
02:40and this way you can get the absorption spectrum of the atmosphere.
02:44And after the absorption spectrum of the atmosphere is measured,
02:47the molecules that are present there are identified.
02:50Because each molecule has its own distinctive signatures,
02:53some absorption bands at certain frequencies.
02:55And in this way, methane, water, and all kinds of other molecules
02:59have been found in the atmosphere of distant exoplanets.
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