I started writing the first 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


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 will observe).


So, 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


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.


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!


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!


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!


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!


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


AH I got distracted by a book last night. I will add more to this about finding exoplanets via the transit method today! I guess is turning into 😂

OKAY! It’s time for TRANSITS! I suppose this isn’t nearly as exciting as the picture of SgA* that was release today but oh well…

A transit happens when an passes between us and the star it orbits. Like I said earlier in the thread, planets are small, but that doesn’t stop them from blocking out a small amount of the star’s light when a transit happens ⭐

(Extended) (11/)

So how much light does the planet block out?

Well let’s imagine for a second that the star is perfectly uniform and is emitting exactly the same amount of light from every region.

Let’s also imagine the planet is a solid sphere that passes in front of it and that light can't travel through it.

Larger planets will then block out more light than a smaller planet because they cover up more of this hypothetical uniformly bright star.

Extended (12/)

Just like before, we generally already know the radius of the star because of reasons I won’t get into.

The amount of light the planet blocks out is proportional to the size of the planet compared to the size of the star.

So since we know the size of the star already, we can determine the size of the planet that is blocking out the light.

And we can figure out the length of time it takes for the planet to orbit based on how frequently we see the transit

Extended (13/)

How does this work in real life?

We have telescopes that stare at stars in the sky waiting for the amount of light measured from them to change!

Currently TESS (Transiting Exoplanet Survey Satellite) is looking for planets all over the night sky by staring at one region of the sky and then moving slightly to look at another region

Extended (14/)

Things can get complicated when there are multiple planets in the system that all have different sizes and orbital periods, and sometimes you can even have multiple planets transiting at once!

But telescopes like TESS (and the now decommissioned Kepler) are great because they can stare at a star long enough for us to figure out which signal is coming from which planet

Extended (15/)

Transits have been the most successful method of finding exoplanets so far!

Once the Kepler Space telescope launched in 2009 the number of we had discovered absolutely skyrocketed

We've discovered that are very different than the types of planets in our Solar System!

Extended (16/)

We can broadly classify the types of we've found like I've done on the graph below, compared to the Solar System planets

Most interestingly there are these classes of planets called Sub-Neptunes and Super-Earths which are bigger than Earth but smaller than Neptune. These are the most common but we have nothing like them in our Solar System! (And these are what I'll talk about next week for the Habitability !)

Extended (17/)

(You may have noticed that there are a couple other detection methods listed there, but I'm not going to talk about them here because they haven't found as many planets)

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@_astronoMay "Hey hey hey! Spectroscopy is back! Lets read the report! Wow, it's just a single word... "Stinky." Awww..." LOL. :)

@_astronoMay Thank you, love this style of "talks" 💖 💖 💖 💖

@_astronoMay thanks so much! I found the part about the sun moving absolutely fascinating. Space is awesome!

The fediverse is for fun, not obligation. Life must go on!

@_astronoMay Finishing my Train Talkin' Friday has slipped to ... well, it will probably be Train Talkin' the Followin' Friday.

@_astronoMay Have you ever heard the story of Ken Thompson and the transit of Mercury he predicted that the Naval Observatory missed?

@_astronoMay what’s with the bimodal distribution of radii? Is that an artifact of detection methods (some are good at small planets, some at large, but no methods are good in between)? Or is there a physical explanation? (Sorry if I missed it elsewhere in the thread!)

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@_astronoMay hi, looks interesting. How about extending the right graph a little to the left, would be interesting to see the number of planets more in our region of size. And, question: could it be that we're seeing more bigger ones nee just because they are easier to detect from here?


Is that graph showing that the planets of our solar system have unusually long periods for planets of that mass? If so, what does it mean?

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@_astronoMay This is enabled by prefixing the desired technology with the word 'space' thereby making anything possible

@_astronoMay I observed an exoplanet transit recently!

On February 19, I along with 3 other citizen scientists pointed our backyard telescopes at Qatar-8b, a gas giant 1.3x the size of Jupiter located 918 light-years from Earth, & captured it transiting its G-type star! 🤩

Check out our light curve data 🤓

@_astronoMay this reminds me of the 2012 Transit of Venus that I've watched through an actinic glass at work 😄 (i've also watched the 2004 transit but Venus passed on the southern hemisphere that time... if I remember that right)


The transit of Venus was so cool to see! Can't believe this was 10 years ago now. This event is why I decided to study exoplanets :)

@_astronoMay How do you find the mass? I'd understand the redshift to be correlated with the tug, but isn't it scaled by the cosine between planet, star and us -- where we don't know either?


Nice catch! Yes, we technically only measure a "minimum mass" because it does depend on the angle we're viewing the system at, but we generally don't know that angle very well unless we also know that the planet transits (crosses between us and the star)

@_astronoMay 51 Pegasi b is the planet that orbits its star in only 4 days right? That's insanely fast!
And that's four Earth days right?

@fdfolson There are planets that even orbit their star in less than one earth day!

@_astronoMay I don't know about your Mastodon instance, but has a 10MB limit on GIFs.

I seem to hit it at the worst times.

@_astronoMay it's because the resolution of that image is not supported, but if you resize it... voilà

@ranx ohhh thanks! Yeah it was yelling at me about the number of pixels or something and I just gave up 😂

Given different dense objects (think neutrino stars, black holes etc) what would be the threshold?

You mentioned 10% for Jupiter above. Is that an upper limit?


Larger objects will definitely tug on a star more. This is part of how we can figure out if it's a planet or something like a Brown Dwarf (between a planet and star) or Star that's causing our main target to move around.

The 10% for Jupiter is the radius, In terms of mass Jupiter is only 0.09% of the Sun

@_astronoMay How do you compensate for the speed of the earth around the sun, the sun around the galaxy, etc... this seems so hard

We know those speeds really well and can basically subtract them out!

@_astronoMay is this all based on the assumption that a stars light is normally white? Are there no stars that would naturally emit red light and thus trick us into believing they are moving away from us?


Objects can be any color for this to work! In fact many stars are red, and I'll talk about that next week in the habitability thread.

We can measure what colors of light absorption lines or emission lines from specific elements like Hydrogen would be at rest in a lab, then we can observe how red or blue shifted they are in different objects in space to work out how fast they're moving

@_astronoMay In a course I took in Uni we did this analysis on mock datasets by looking at the red/blueshift of the hydrogen line in the spectrum. Is that how this is done on real data too?


That's exactly how it's done! We usually use lines from lots of elements together though - the more absorption lines you observe the more precise your measurement of the shift is.

@_astronoMay Yeah, that makes sense. This was just an undergrad class, so I guess the professor decided to go for the simpler case. 🙂

I also did a project where I calculated and plotted the wobble of our sun. I always loved all the astro stuff we did!

I recall this!

Several planets in our own solar system were discovered this way, weren't they?


Sort of! Neptune was predicted before it was found because of variances in Uranus's orbit they couldn't otherwise explain

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