I started writing the first #SpaceTalkTuesday thread about planetary habitability, but quickly realized you all need some background on how we *find* planets first!

So sorry to everyone who voted for habitability, but we’re doing HOW TO FIND AN EXOPLANET 🔭 today!

I promise this will make the habitability thread next week make even more sense (1/)

So, what’s an exoplanet? It’s just a planet that orbits a different star. Unfortunately, Planets are really small compared to stars and this makes them hard to find.

In our Solar System the smallest planet is Mercury which is only 0.3% the size of the Sun. Jupiter is the largest planet in the Solar System and it's 10% the radius of the Sun.

Earth is 0.9% the radius of the Sun

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Not only are planets small compared to stars, they’re also not as bright! Stars are fusing hydrogen into helium (and later more things that I won’t get into…) and the energy released from that is why they are so bright.

Planets are not fusing elements, so they’re not as bright. BUT they are warm, and warm things also emit light. Not at colors of light we can see with our eyes like stars do, but at longer wavelengths of light like the infrared (where #JWST will observe).

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So, #exoplanets are small and dim. Meaning until recently we couldn’t just take out a telescope and stare at a star and hope to see a planet around it. The light from the star would just overwhelm everything!

Back when people started thinking about actually finding planets there were two main suggestions: (1) look at how the planet’s gravity pulls on the star and (2) watch for the planet to cross in front of the star and block out a little bit of light

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We call the first method the Radial Velocity method.

You may think of an orbit as the planet orbiting the star, with the star stationary at the center. This is *almost* the truth, but even though planets are small compared to their stars they “tug” on the star ever so slightly causing it to move around.

This happens in our Solar System too! Jupiter is the biggest culprit - check out this gif (not to scale) of how Jupiter tugs on the Sun.

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Now we don’t always visually *see* the star moving, but what we do see is the impact of the star being pulled towards and away from us on the *color* of the star.

In the Universe, things that are moving away from us are “redshifted” and things that are moving towards us are “blueshifted”.

This is similar to the Doppler shift you hear in sound when a siren is moving towards/away from you. Things moving *towards* you are compressed, and for light this means it turns blue!

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For planets and stars this happens on a very small scale

We don’t see the color change visually, but we do see the spectrum of the star shift *ever so slightly* back and forth during the orbit. This corresponds to a speed that the star is moving with

Because we often already know the mass of the star, we can use orbital dynamics to figure out how big an object would have to be to cause the star to move with that speed

If it’s small enough, behold you’ve found a planet!

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The very first planet was found with this radial velocity method!

51 Pegasi b is a Hot Jupiter that was officially discovered in 1995. A few years ago, this discovery of the first planet outside our solar system won the Nobel prize!


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Now not all detection methods are equal, with RVs we learn about the *mass* of the planet because of how it tugs on the star. And we learn about the *orbit* because we can see just how long it takes for it to orbit the star

The orbit can tell us something about how hot the planet probably is but we can’t measure the radius of the planet or anything about the planet’s atmosphere with this method

For a while, this was the most popular and successful way to find planets!

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However, in 1999 the first planet was found via the Transit method and the world has never been the same (I am definitely not biased! 🤗 )

I’ll be back later with more on Transits and why they are THE BEST

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