# Eli5: How do we know what other planets are like?

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I would like to know how do scientists know what other planets are like billions of light years away from us, i know there are powerful telescopes but even then the planets on the pictures are super tiny

How do they know there is water there or that it has an atmosphere,

In: Physics

We can see what kind of materials it *might* be made of by the spectrum of light that is reflected. Keep in mind, this is light outside the spectrum visible to us.

Think of it like gems; we know what diamond, sapphire, emerald, ruby, etc. are because of what color they appear to us. Similarly planets made up of largely hydrogen will appear a different “color” than ones that are made up of largely iron or nickel.

This requires us knowing the spectrum of the light source, so first we have to analyze the planets star.

Usually this process is done by monitoring the planet passing in front of the star and observing what spectrums are blocked by the planet.

There are many different tools scientists have to make estimations about what other planets and stars are like.

We can get an idea for a planet’s mass based on the orbit it has around it’s star, and how far away it is from the star. We can watch how far it orbits and how long it takes and use math to figure out the Mass from there.

If the planet passes between us and it’s star during it’s orbit we can also detect it’s size! We calculate how much light was being blocked from our view when the planet was in between us and it’s star. Kind of like a solar eclipse, but much farther away. We can calculate it’s diameter from that, which leads us to it’s size.

Now that we have size and mass, we can calculate density. This greatly narrows down the possible things the planet could be made of. If it’s a large low density planet, it’s likely a gaseous planet like jupiter. If it’s a high density planet it may have lots of metal.

In addition to the other answers we don’t know what planets are like billions of light years away. We can only really get good information about exoplanets around stars close to us as our instruments are less sensitive to them the further away they are. Pretty much all exoplanets we know anything about are within a few thousand light years which is much much less than a billion.

To answer the actual question we use spectroscopy, which means measuring the spectrum of light from an object. When light passes through the atmosphere of a planet some colours get absorbed by molecules in the atmosphere, every molecules has a unique fingerprint of wavelengths it absorbs. We can detect the light from the exoplanet either directly if it is separated enough from its host star, or we can observe the spectrum of the star before and after the exoplanet goes between us and it, then the difference in the spectra lets us work out the contribution from the planet.