JWST Finds Signs of Life on K2-18b — But It’s Likely Just a False Alarm

 Data from the JWST indicates that a planet 124 light years away could host life. However, it’s more speculation than fact. 

Artist's concept of K2-18b as a hycean world; Source: Wikipedia

A few months ago, NASA made an exciting announcement: the James Webb Space Telescope (JWST) may have found signs of life on the distant exoplanet K2-18b. 

According to this data, the planet might be a Hycean world — a planet covered in a global water ocean beneath a rich hydrogen atmosphere. The JWST discovered an abundance in methane and carbon dioxide but a lack in ammonia, strengthening the hypothesis.

But what it also found is a possible detection of dimethyl sulfide (DMS), a molecule which on Earth is mainly produced by phytoplankton in our oceans. 

Sounds promising, right? Since DMS is only produced by life, this must surely mean that K2-18b is an inhabited world! 

Before scientists can confirm anything for sure, the JWST has been directed once more at the system to confirm if DMS is actually present in the atmosphere. We’ll have to wait a few months before the results come out. 

But even now, it may be very unlikely that K2-18b is a habitable world. 

Atmospheric composition as detected by the JWST; Source: Bored Panda

In 2015, astronomers announced the discovery of K2-18b in the constellation Leo, orbiting a red dwarf 124 light years from Earth. Two years later, another exoplanet would be found, K2-18c. 

At a distance of just 0.06AU from its star, K2-18c’s “year” just lasts nine days. In comparison, 1AU is the average distance between the Earth and Sun. Simulations show that the planet is a mini-Neptune, five to eight times as massive as Earth.

Mini-Neptunes are a type of planet we don’t have in our own Solar System; they are intermediate in size between Earth and Neptune and have a gas-rich composition. Super-Earths, on the other hand, are rocky planets bigger than Earth. 

Though we have an idea of K2-18c’s composition, K2-18b is more of a mystery. 

It has a radius of 2.37 Earths, but a mass of almost nine Earths. It orbits the red dwarf at a distance of 0.14AU, which means it finishes one orbit in 33 days. At this distance, the planet lies on the inner edge of the star’s habitable zone. 

Now, the planet’s density is intermediate between Earth and Neptune — so it could be either type. It either is a rocky planet with a thick atmosphere or more Neptune-like, with ices in its composition (though “ices” is kind of misleading, because in Neptune-like planets ices are extremely hot and have high pressures). 

As noted, it has been theorized that the density indicates that K2-18b is a Hycean planet. In 2019, astronomers announced that the Hubble Space Telescope (HST) found water vapor in K2-18b’s atmosphere, prompting further observation with the next-generation JWST. 

So, with the preliminary findings of the JWST, the public has fixed on the assumption that the planet is inhabited or, at least, habitable. 

However, researchers have interpreted the data in different ways. In one study, it was found that just as likely as it is to be a Hycean world inhabited by microbial life, it could also just as likely be an uninhabited gas-rich planet. 

Being gas-rich would explain why K2-18b’s temperature is so cool — only -8°C. For a Hycean planet, it can be expected that a runaway greenhouse effect heats the planet up. 

Artist's concept of K2-18b; Source: CNN

In another study, it was found that K2-18b could have a magma ocean, which would make it too hot to sustain any life. 

So what exactly is K2-18b? An inhabited Hycean planet? A normal mini-Neptune with a gas-rich composition? A world covered in magma? 

Well, even if it’s a Hycean planet, there are many reasons why K2-18b isn’t a haven for life. Here’s why. 

1. So far, Hycean planets are merely theoretical. Though we can theorize that a certain planet is indeed a Hycean world, there’s no substantial evidence yet to prove they exist. 
2. An ocean-covered world with a thick atmosphere sounds like paradise — but because there’s so much water, there’ll also be a lot of water vapor. And water vapor acts as a greenhouse gas. That means Hycean planets might not be able to hold on to their oceans for long, as they are literally boiled beneath the thick atmosphere. 
3. Assuming a Hycean world capable of holding on to its water, these oceans would be extremely deep. This also means that the pressures would be extreme — especially since for a planet like K2-18b, the estimated surface gravity is 1.27g. 
4. It’s thought that K2-18b is tidally locked, meaning that one side of the planet is constantly exposed to its star’s radiation, while the other side would be covered in permanent darkness. 

In other words, there are many reasons why K2-18b is too extreme for life. Or, too extreme to have a substantial surface at all. 

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