Eli5: how can they tell the composition, mass etc of planets in other solar systems?



For example in they found a planet called “55 Cancri e” which is believed to have a diamond core and its 40 light years away.


A planet called “HD 149026b” which has a temperature of 2,000C and is approximately 250 light years away.

How do they work out this information from so far away?

In: Physics

Depends on the method they use. The usual method is looking for stars that are wobbling, because they are wobbling due to the gravity of an orbiting planet. Once you know the size of the star, the period of the wobble tells you the planets orbital speed. Since orbital speed is directly tied to how far away from the star it is, you can then calculate the planets mass by how much it pulls on the star. Temperature is also related to distance so that’s easy to figure out too.

The other method is if the planet passes between it’s parent star and the Earth. How long the star’s light dims once again tells us orbital speed, which means orbital distance. You can then use that data plus how much the star’s light dims to tell the diameter of the planet. You can also still use the wobble method above to work out mass. An additional benefit of the transit method is when it passes across the star, it absorbs more of some light than others based on it’s composition, creating what’s known as a Lyman-alpha line.

For 55 Cancri e specifically, the spectral line showed what looks like Carbon, and because of the transit, we know both its mass and diameter, and thus it’s density, thus we can tell what form that carbon should reasonable take given our knowledge of planet formation.

There are different ways of figuring these out.

For example if they already know the distance / mass of a different star. And this planet passes the star, they can calculate mass through light reduction.

Composition can be detected through the radiation in the air or deduction.

An easy one for example is Earth. We have oxygen so we must have water / carbon. (This works both ways, we have water so we must have oxygen)

You know how Sherlock Holmes can walk into a crime scene, look at it really hard, from plenty of angles, and start telling you about what time the murderer woke up that morning, that he’s a tall black haired fella with a stutter and he’s a coffee drinker?

That’s what astronomers do. They can’t take an atmospheric sample and run tests on it, but there are clues that, when put together and correlated, can tell you an awful lot.

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