Erin M. May, PhD @_astronoMay@mastodon.online

Because remember, until we find that atmosphere, we cannot call that planet habitable! All it is is a "terrestrial habitable zone planet"

Now of course even finding the atmosphere doesn't mean it's *inhabited* because that means finding very specific gases or molecules that are even harder to measure.

I need to go get some work done, but I'll be back later for more on measuring biosignatures so we can say if a planet is *inhabited*!

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Because these are really small signals we haven't really been able to look for them yet!

But remember, #JWST just launched recently! And with JWST we will be able to start looking for atmospheres around terrestrial planets.

I'm on a team that has one of the largest amounts of time on JWST over the next year or so to do just that: determine which terrestrial exoplanets have atmospheres! This will allow us to definitively say that they're habitable or not.

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Now for really small planets, these are really small signals.

When we first go looking for atmospheres around habitable zone terrestrial planets all we want to see is: Are there differences in the size of the planet at different colors of light?

If that's a yes then there is an atmosphere! If it's a no then maybe there are clouds blocking our view or the atmosphere is too small for us to detect or it's just not even there.

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Thanks to the fearless efforts of laboratory physicists and chemists, we know exactly what colors of light different types of gas and molecules absorb.

For an exoplanet this works because the planet looks ever so slightly bigger (blocks out more star light) at colors that are absorbed by the atmosphere

So when we see the planet look slightly bigger at certain colors, we can say "hey, there must be Water or Oxygen or CO2 etc!"

(graphic from @/ExoplanetPete on birdapp)

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How do we measure an atmosphere?

We use a method called Transmission Spectroscopy that looks for the tiny signals from an atmosphere while the planet transits the star.

Last week I told you to imagine the planet was a solid sphere that blocked out star light. Now put an atmosphere on top of that sphere that starlight can pass through and interact with!

When light passes through gasses like an atmosphere, some light is absorbed and we don't see it anymore.

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Okay, now we get to the third of our steps towards finding an inhabited #exoplanet, does it have an atmosphere?

In our Solar System we have terrestrial planets with no atmosphere (Mercury, and from a habitability perspective, Mars), and terrestrial planets with atmospheres (Earth, and though it's too hot, Venus also has one)

This means we can't just assume all terrestrial planets have atmospheres. And Venus, Earth, and Mars were all at one point in the habitable zone!

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I'll restrain myself and not rant about this today, but just know that K2-18b is not a habitable exoplanet, it's too big! 😉

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This is something I could rant about for DAYS because some scientists like to call planets that are larger than two Earths "habitable"

But they aren't terrestrial, rocky, planets! They're much more likely to be like Neptune than Earth. No light would reach the surface, which means no photosynthesis!

In my opinion, this is the most overlooked step in identifying an inhabited planet, but one of the more important ones to consider!

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Well using lots of math and physics that I won't get into here, we're pretty sure that once a planet becomes about 1.5 times the Radius of Earth it is too big to be just a rock with a little bit of atmosphere like Earth.

This means that any planet bigger than about 1.5 times the radius of Earth will have an absolutely crushingly thick gas atmosphere. I'm talking at least TENS OF TIMES MORE PRESSURE THAN THE BOTTOM OF EARTH'S OCEAN *just* in the atmosphere

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But how big is too big?

Remember last week when I talked about how planets called Super-Earths and Sub-Neptunes are the most common type of #exoplanet that has been found so far?

In our Solar System, Earth is the largest rocky (terrestrial) planet and Neptune is the smallest gaseous planet.

We know that at some point as a planet is forming it will get too big and end up grabbing lots of gas. We know this must happen somewhere between Earth and Neptune.

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The second step, measuring how big the planet is is also easy! Remember the transit method from last week?

The amount of light blocked out during a transit is related to size of the planet compared to the size of the star. We "simply" need to find a very small transit!

Since it's related to the size of the star, we often find these small planets around smaller stars, it's just easier!

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The first one (finding an #exoplanet in the habitable zone) is easy enough, check out my #SpaceTalkTuesday thread from last week!

⬇️ ⬇️ ⬇️ ⬇️

https://mastodon.online/@_astronoMay/108278987770019772

Okay, it's not *easy*, but this part isn't my job! 😂

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So to find an inhabited #exoplanet here are our steps:

1) Find a planet in the habitable zone of its star

2) Measure how big it is - is it even a terrestrial planet?

3) Determine if it has an atmosphere (and now we know if it's habitable!)

4) Measure specific gasses and molecules in the atmosphere that are "biosignatures", or signs that there must be biology present (and now we know if it's inhabited!)

Sounds easy enough, right? 🙃 😉 🔭

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So after a brief break for me to grab lunch, let’s talk about how we go about determining which #exoplanets are the best for searches for life!

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OKAY. Now that we’re all on the same page about what habitable zone, habitable, and inhabited mean, we can dive a little deeper.

Before we do that, I’ll drop this handy summary of these three words for you to keep in your back pocket:

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