That’s right, it’s time! This week it’s all about ~exoplanetary habitability~

What does habitability really mean? Do we know of any habitable ? How will we be able to study them now and in the future?

Follow along for answers to these questions and more!

(fair warning, this will be a long thread. the threaded posts are unlisted as a best practice, but you can still boost them)

First I’m going to start with some definitions so we’re all on the same page about the terms I’m going to be using - specifically habitable zone, habitable, and inhabited. Sometimes these are used interchangeably, but they all mean very different things.

I’m also going to be heavily focused on *Earth-like* life, because we can’t search for things we don’t know and we’re more likely to not recognize the signals of non-Earth-like life if we detect them.


SO: The Habitable Zone. What is it and why does it matter?

The habitable zone is the region around a star where water could exist as a liquid on the surface of an . It’s mostly about the amount of light/heat the planet receives from the star and that’s about it.

You may have heard it referred to as the “Goldilocks Zone”. If a planet is too close to the star, it’s too hot, and if it’s too far, it’s too cold.


Now there are actually many ways to calculate a “habitable zone”. For example, the type of atmosphere an has also matters when we’re talking about how hot a planet is.

Did you know that Venus is actually the hottest planet in the Solar System even though it’s not the closest to the Sun? That’s because it has a really thick atmosphere with a strong green house effect that traps more heat from the Sun.


There are actually MANY scientific definitions of the habitable zone depending on the model used and assumptions made about the planet atmospheres.

Traditionally we think about it as the region where an Earth-composition atmosphere could maintain a liquid water ocean.

For example an can be further from the star if it has more CO2 in the atmosphere than Earth and still be warm enough to have liquid water.


But you can just think of the habitable zone as a range of distances around a star that is the right temperature for liquid water.

Importantly, just because an is in the habitable zone, doesn’t mean that it’s *habitable*


SO: The term “Habitable”. What does this really mean?

First, to be habitable, the must be in the habitable zone (But remember, being in the habitable zone alone isn’t enough!)

Second, the exoplanet needs to have an atmosphere.

Third, the exoplanet needs to be a terrestrial planet and have the *right size atmosphere*.

Being habitable does NOT mean the exoplanet is definitely home to life.

Let’s talk about these last two more:


2. The needs to have an atmosphere.

The atmosphere serves a few purposes: It both makes it not blazing hot during the day and freezing cold at night (like on Mercury, for example). And an atmosphere also protects an ocean and life from the most harmful parts of stellar radiation.

For example, the Ozone layer on Earth is crucial to the safety of life on Earth!

(Keep reading to find out about how we determine if an exoplanet has an atmosphere!)


3. The needs to have the *right size* atmosphere.

(This is where I could RANT FOR DAYS)

An atmosphere needs to be thin enough that light can penetrate through the atmosphere and hit the surface to provide heat/energy to life on the surface. (for example, photosynthesis!)

And if it’s too thick then the atmosphere is quite literally crushing.

(Keep reading to find out how we determine the type of atmosphere an exoplanet likely has!)


The last “definition” is the word inhabited.

that can be classified as inhabited once we know that they're in the habitable zone, are habitable (so they have an atmosphere that is the right size), *and* we've measured signals of specific gasses or molecules in their atmosphere that can *only* be explained by biology.

This is something that requires a LOT of discussion around, so I’ll come back to this later


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:


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


So to find an inhabited 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? 🙃 😉 🔭


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

⬇️ ⬇️ ⬇️ ⬇️

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


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!


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 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.


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


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!


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! 😉


Okay, now we get to the third of our steps towards finding an inhabited , 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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@_astronoMay hm - why would light have to reach the "bottom" to ignite photosynthesis? Why wouldn't we be able to detect large molecules hanging around in wuthering heights and produce energy from radiation? I understand that there IS some training that "life" ought to be oxygen based. But why on the ground? The atmosphere could be retarded as a sea...

@_astronoMay I didn't get to the end of the thread yet, but since you said you're not ranting about it right now, I'm guessing these might not already be answered below (unless someone else asked the same thing 😅 )
Three questions for now, I guess:
1) If the atmosphere is that thick, would it also be denser and easier to be bouyant in? Could there maybe be things that float high enough to get light?
2) Earth has lots of things in our oceans way deeper than light goes. Getting energy from geothermal stuff or something like that. Could these thicker "atmospheres" have similar things?
3) What does "surface" even mean on a gas planet? The outside of whatever outermost solid layer it does have? Or is there some dividing line between which gasses are part of the "inside" vs "atmosphere"?

@wolf @_astronoMay If it was smaller than earth, then earth would be a gassy planet. if it were larger than neptune, then neptune would be rocky. hence, "in-between". 🤔


Great thread so far!

I spend a lot of time day dreaming about this kind of stuff, keep it coming!

@_astronoMay hmm, Venus has an atmosphere, but quite toxic. Mars has an atmosphere, and even a door (photo published on the net today) 😉

@_astronoMay SciFi shows get his wrong too. As they open the door to the shuttle, imaging 1000atm crushing in. Thanks for sharing this!

@_astronoMay I’ve wondered why scientists seemed to focus more on finding Earth-like life. Thanks for explaining this!

@_astronoMay perhaps a thread on second point? There’s increasing interest among people about “completely different” life forms.

For example people find squids to be completely different when they think about it and so on. Lee Cronin etc are trying to create life in labs. Perhaps you can satisfy a wide verity of people 😁

@_astronoMay Amazing how much time and energy you put into this. Idea: take all the toots later and put in a markdown or pdf document and you'll have a nice introductory book!

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