# Eli5: how are temperatures of exoplanets measured

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I read in an article that they compared the light of the star and the planet together with that of the star alone, when the planet moved behind the star.
How does this work ?

In: Planetary Science

So most stars are just points of light in the sky, even with our best telescopes. They’re just *that* insanely far away. So aside from some relatively nearby exceptions, we just can’t see the planets. The further something away is, the bigger a telescope you need to see it because there’s a fundamental limit to how well you can focus light.

But! Our instruments are *really* good at analyzing what little light we do get, even if that light is a blurry splotch. It’s a matter of precise color analysis. Specifically, we analyze the light of a star, and then we compare that to the light of the star with the planet blocking a tiny bit of the star’s light.

Now, the actual math is a good bit more complicated because of all the error-correction you need to do, but basically, you take the unblocked starlight and subtract the dimmer blocked starlight, and what your left with is the difference in that light caused by the planet’s presence, including the absolutely tiny sliver of light that passed through the planet’s atmosphere.

Then, you use your base starlight again to work out what the star is made of, which then helps you figure out what the planet is made of by comparison. The knowledge needed for that comes from shining a whole bunch of different lights through different substances at different temperatures and cataloging the results (a VERY labor intensive process getting all that data).

You also need to figure out the planet’s mass and orbital period. For the period, you can just watch to see how long that takes, and for the mass, you can figure out how big the planet is by how much light it blocks and how much it makes its parent star wobble during its orbit.

Add it all together and run it through some mind-bogglingly complex mathematical models, and you get out an estimate of the planet’s physical and chemical makeup and how close it is to its star, which you can then use to calculate how much light it gets from its star and how much it absorbs as heat, and therefore how hot it is.

All of this is possible thanks to hundreds of thousands of astronomers, chemist, physicists, mathematicians, and engineers working together to collect and analyze an utterly obscene amount of data even before we point our telescopes at a particular star system.