What are exoplanets and how do we find them?
Dr. David Charbonneau is a professor of astronomy at Harvard University and an astronomer at the Harvard Smithsonian Center for Astrophysics. His research focuses on the development of novel techniques for the detection and characterization of planets orbiting nearby Sun-like stars. He led the first studies of the compositions of exoplanets and of their atmospheres, and he is a member of the NASA Kepler Mission to find Earth-like planets.
Dr. David Charbonneau: The first thing to know when you’re thinking about how we study planets around other stars is that we never get to see the planets directly. The way that most planets have been found is we wait for the planet to pass in front of the star. When it passes in front of the star, it blocks some of the light from the star; we can see the star get a little bit fainter and a little bit brighter as it passes out from our point of view.
Then, based on that we can infer, for example, the size of the planet. If it’s a bigger planet, it will block more light. In terms of understanding the properties of the planets, what we really would like to measure are their sizes, their masses, and, if we put those two ideas together, therefore, their density and maybe what they’re made of. Are they made of dense things like rock, like the Earth, or are they made of puffy things like gas, like Jupiter?
The way that astronomers learn about the mass of a planet is through the dance of the planet and star. Think about it as if you’re watching two dance partners on the dance floor, but one dance partner was 10 or 20 or 100,000 times heavier than the other one, but that heavier dance partner would still do-si-do back and forth. We can study the light from the star, see that it’s dosi-doing back and forth, and we call that the wobble method or the Doppler method. That allows us to know that there’s a planet there, even if we don’t see the planet. It allows us to measure the mass of the planet because a heavier planet would cause the star to wobble more.
Furthermore, we like to figure out the temperature of the planet and, fortunately, that’s very easy. The temperature really is set by the distance from the star, and we can infer the distance of the star by measuring how long it takes the planet to go around in its orbit. We’ve been able to measure the size and the mass and the temperature for thousands of worlds, and now we have a very good understanding of which of those planets are a little more like Earth, both in terms of being rocky and being temperate, and which are really not like the Earth, maybe because they have a lot of gas or maybe because they’re much, much, much hotter or much colder.
Once we get past the stunning reality that we can discover new planets and identify their properties in our solar system and beyond we have to ask important questions like how in fact can we discover and identify properties of new planets? The physical properties of a new planet may seem unattainable as we are unable to even see these planets directly but in fact we are able to gain valuable and rather extensive information about these planets.
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astronomy, science and religion, How we discover exoplanets, stars, wobble method, doppler method, Dr. David Charbonneau