We need to talk about K2-18b.

You know why.

We shall begin with a plot re-cap.

K2-18b is an extrasolar planet, orbiting a dim star known as a red dwarf. As the planet slid across its star, light from that central ball of stellar fusion passed through the planet’s atmosphere. This was detected with the Hubble Space Telescope, which noted that wavelengths of light typically munched up by water molecules were missing. Thus occurred the first detection of water vapour in the atmosphere of an exoplanet that is smaller than Neptune… and that orbits in the habitable zone.

Is this exciting? HELLA YES.

Should we all be focussed on the last five words of that re-cap? HELLA NO.

Why?

Because the habitable zone means absolutely nothing for this planet.

And you would die there.

Now at this juncture, I feel you burning to stop this planetary crusade. “Wait a minute!” —you shout— “We’ve heard this spiel from you before. The habitable zone is just a region around a star where the Earth could support liquid water. Any old Bob, Dick or Henrietta of a planet can saunter in and orbit without any other Earth-like properties whatsoever.”

Average temperatures of your favourite habitable zone worlds. The moon doesn’t really do average temperature, as it lacks at atmosphere to do the averaging. So lunatics get a scorching day and frozen night. They die. Just like you would.

That is true. Well done for remembering.

“Buuut this time, we’ve detected water vapour!” —you persist— “So we know that the planet HAS ITSELF SOME SEAS! And everyone knows, that makes for some sweeeeeet alien lattes!”

No. NO. NOO! I try to inject, but you blaze on unperturbed:

“Sure, habitability is complex and we need more than water to thrive. But the detection of water on a habitable zone planet makes K2-18b the absolutely scrumptious best candidate for a world teaming with small furry creatures we’ve ever ever evvvvvvveeeeerrrr seen!”

At this stage, I post a cute meme to the internet and wait for you to collapse in an exhausted heap of alien world ecstasy.

( • . •)
/ >🌚yes, I will tell you all I know about planet diversity.

( • - •)
🌚< \ stop... stop calling them all Earth 2.0.
— Elizabeth Tasker (@girlandkat) September 12, 2019

Now that we’re settled, let me tell you how you are guaranteed to die on K2-18b (if you want a cup of tea, go and get it now).

K2-18b weighs in at 8 times heavier than the Earth, with a size 2.3 times that of our planet. This gives the planet an average density around 3.3 g/cm³.

This density is tricky. It’s similar to Mars (3.9 g/cm³) which initially seems promising; Mars being a rocky world that we believe may have been habitable in the past. But Mars is a squiffy excuse for a planet. It only has 1/10 of the mass of the Earth. As planets beef up, gravity exerts a greater squeeze that ups the density. The Earth has an average density of about 5.5 g/cm³ and if K2-18b had a similar composition, we’d be looking at a density of ballpark 10 g/cm³ [1] <-- these are references at the bottom of the article, see how professional we're getting?.

To lower that density, we’ve got to mix our Earth-y silicate existence with something light and fluffy. For this planetary cookery class, there are three main options:

Fatal Option #1: A thick atmosphere of light gases such as hydrogen and helium.
Fatal Option #2: A truck load of water.
Fatal Option #3: A funky hybrid mix of the fatal hydrogen and fatal water.

Excited? Who wouldn’t be?! Let’s start with Bachelor Planet #1.

While the Earth doesn’t have a strong enough gravitational pull to hold onto light gases such as hydrogen and helium, there’s no real question that K2-18b has a hefty stockpile in its atmosphere. The detection of water vapour was published in two independent studies [2] and both find the data is best matched by a hydrogen-dominated atmosphere. Moreover, previous empirical eyeballing of exoplanets [3] has found that once you start approaching a radius of 1.5 Earth, planets swell in size but not mass, which is conducive to acquiring a thick cloak of light gases. If we claim that the low density of K2-18b is entirely due to these light and fancy-free elements, then a mass fraction of about 0.7% in hydrogen and helium should give us the required density [4].

So less than one percent? I bet you’re thinking that’s no big deal! What feeble life couldn’t handle that?!

You. You couldn’t handle that.

It turns out a splash of hydrogen goes rather a long way. Writing in the Astrophysical Journal, Eric Lopez and Jonathan Fortney offer a particularly delicious analogy [4]:

A 0.5% Hydrogen/Helium atmosphere leads to a surface pressure twenty times higher than that at the bottom of the Marianas Trench (deep. You can’t live there), and the temperature would be more than 2700°C (hot. You can’t live there).

“WAIT A MINUTE!” —you shout— “The planet is in the habitable zone! How did we get to surface temperatures of thousands of degrees?!”

The problem is that the habitable zone (as used for exoplanets discoveries) is defined as the amount of starlight the Earth needs to keep temperate surface conditions. Our planet can adjust the surface temperature (on rather long geological timescales) by altering the level of carbon dioxide in the atmosphere through the carbon-silicate cycle. As carbon dioxide is a greenhouse gas that traps heat, lowering its level cools the planet while letting it accumulate gives the planet more of a cosy blanket. Within the habitable zone, this thermostat works well. But beyond its edges, the carbon-silicate cycle can’t manage its job and the planet either boils or freezes.

Within the habitable zone, the Earth can adjust the level of carbon dioxide in the atmosphere to keep surface conditions comfortable.

So the habitable zone is the region where the carbon-silicate cycle can put the right amount of carbon dioxide into the Earth’s atmosphere to keep the surface temperature comfy.

Got a planet with no carbon-silicate cycle?
Or a world with a different atmosphere composition?

Then that habitable zone don’t mean jot. (And you all really know this as Mars and the Moon are in the habitable zone but don’t offer lakeside retreats.)

The fact that atmosphere of K2-18b is dominated by hydrogen therefore utterly invalidates our habitable zone ticket. Like carbon dioxide, hydrogen is a greenhouse gas, giving K2-18b an extra thick thermal coat that it can’t shrug off. Therefore even if the rest of the planet was hypothetically entirely Earth-like with a carbon silicate cycle, the planet’s surface would be far far warmer within the habitable zone than the Earth.

Hydrogen is also a greenhouse gas, making planets too hot in the classical habitable zone.

In short, you’re squashed flat and roasted. Got it? Excellent. Let’s move on to Planet Bachelor #2: the truck load of water.

…. where we are going to use the same logic to die horribly. Again.

If we ignore the hydrogen detection in the two discovery papers (or assume it’s somehow suuuuuuper low), then we can match the low density of K2-18b by employing a thinner, more Earth-like atmosphere but mixing-in a whole load of water into the silicate rock. The problem is that the amount we need… is rather more than what’s in your kitchen sink.

To match the density of K2-18b, the planet’s mass would need to be as much as 50% water [2]. By contrast, the Earth has less than 0.1% water by mass. And while all life on Earth needs water, too much can geologically murder the entire planet.

The carbon-silicate cycle works best when there’s exposed land. Before we reach 1% water, the planet will likely become an ocean world and the land sinks below the waves. If the sea is shallow enough, a more pathetic carbon-silicate cycle can still work with the sea floor. Ramp that up by pouring more water into the planet, and the weight of the ocean on the seabed will trigger the production of deep sea ices. These ices seal off the silicate rocks from the water, shutting down the carbon-silicate cycle as carbon dioxide can no longer be stashed away in the ground. Not only does that mean the habitable zone shrinks to a thin strip as the planet becomes unable to adapt to different levels of starlight, but it also prevents the cycling of nutrients such as phosphorous from the planet interior to the ocean. Even with perfect positioning, life therefore gets throttled due to lack of nosh.

Up the water content on the planet to several percent of its mass, and the pressure of the water can shut down plate tectonics. The exact role of the motion of our crustal plates in the habitability of the Earth is not fully understood, but it is thought to be a key player in nutrient cycling and magnetic field generation that protects our atmosphere. In short, by the time you have shut down plate tectonics, water has rendered your world well and truly geologically dead.

There is a possibility that an ocean world could develop alternative mechanisms outside the carbon-silicate cycle for temperature modulation [5]. But different mechanics may require a different levels of starlight, resulting in a new habitable zone that has different boundaries from the classical carbon-silicate cycle definition. An ocean world K2-18b in the regular carbon-silicate habitable zone is therefore not a mecca for alien lattes[**].

Could we rescue this situation with hybrid Bachelor Planet #3? Mishmash a splash of hydrogen atmosphere with mega ocean but avoid the pitfalls of both?

No.

Because you can’t minimise both the ocean and the hydrogen atmosphere, and even much smaller abundances than that suggested above would kill the planet.

The bottom line is that K2-18b is not a potentially habitable world and it is not where we would focus our resources in hunting for biosignatures. Measurements of the planet’s density require either a thick hydrogen atmosphere and/or a deep ocean. These un-Earth-like environments mean that the habitable zone does not apply to K2-18b and moreover, they suggest a world too hot and geologically dead to support life.

So is the discovery of K2-18b notable at all?

ABSOLUTELY. For three main reasons:

We’ve detected water in the habitable zone. While K2-18b is not a potentially habitable world itself, the presence of water suggests one of life’s key ingredients may be easy to come by on more Earth-like rocky worlds.
Being able to detect a planet’s atmosphere is crazy hard. But it’s this information that tells us what a planet is truly like, from surface conditions to geology to potential biology. And we’ve just done it for a planet in the habitable zone. It’s going to be the start of an amazing slew of information, not just about potentially habitable worlds but ones that might be far more alien than anything we’ve dreamed about. And on that note…
Planets like K2-18b that are larger than the Earth but smaller than Neptune are called ‘super Earths’ or ‘mini Neptunes’. They appear to the most common size of planet in our galactic neighbourhood but we… ain’t got one. Are these large worlds more like giant terrestrial planets or teeny gas giants? Do they form in-situ or migrate from somewhere else? Can they have an active geology or are they all crushed by water or gas? K2-18b orbits a bright but small star and has an atmosphere not hidden by clouds. It will be the perfect planet for observations with up-coming instruments such as the JWST, which will be able to detect far more molecules in the planet’s atmosphere and shed some light on this mysterious of all planet classes. Frankly, this excites me the most. After all, for Earth 2.0… well, we already got one.

The water vapour on K2-18b is an awesome discovery: it’s a huge step to discovering what planets beyond our own Sun are really like. But it’s not the habitable world you’ve been searching for.

You would die there. Stop thinking about going.

[**] Edwin Kite (University of Chicago) reached out to me to note that a possible cycle exists in which carbon dioxide could be exchanged directly between ocean and air. While very different from the carbon-silicate cycle, the level of starlight required turns out to be similar, potentially allowing a water world to be temperate within the regular habitable zone. Could this save K2-18b? It’s not clear, as this is very new theoretical territory, with no terrestrial example. The planet would still have to lose most of its hydrogen atmosphere in order not to have a profusion of greenhouse gases that would boost its temperature. But it would be a great location to start investigating alternative planetary cycles to our own.

Unfashionable facts:

Estimate based on Weiss & Marcy, 2014 for a rocky planet without a thick atmosphere.
Detection papers [Benneke et al, 2019] and [Tsiaras et al, 2019].
A bunch of papers have noticed this break around 1.5 Earth radii where rocky planets seem to acquire deep atmospheres, including Weiss & Marcy (2014), Rogers (2015), Chen & Kipping (2016) and Fulton et al (2018).
Based on Figure 9 in Lopez & Fortney, 2013.
For example, ‘the ice cap zone’ by Ramirez & Levi, 2018 or a cycle just between ocean and air by Kite & Ford, 2018.