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Complex Numbers #1064595 added February 21, 2024 at 9:33am
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That's Not Life
Periodically, astronomers announce the discovery of an exoplanet in its star's "habitable zone," and, inevitably, like the old game of Telephone, this gets filtered down to us garbled as something like "Scientists Discover Evidence of Alien Life."
Worse, this gets understood as "sentient alien life," because we've all grown up with Star Trek and Doctor Who, both of which put sentience on top of an evolutionary pyramid, like it's inevitable once you posit life. Much as I love those franchises, I understand that they're not documentaries.
I’m sorry, everyone, but we need to talk about Hycean worlds and dimethyl sulfide.
Okay, I have some idea of what dimethyl sulfide is (in that it's gotta involve two CH3 groups and sulfur), but I had to follow their link to "Hycean worlds." The link goes to an earlier Big Think article (same source, different author). Basically, they've got a lot of hydrogen. "Hycean" is a portmanteau of hydrogen and ocean (where "ocean" is understood to be a water ocean, and 2/3 of the atoms in water are, of course, hydrogen. But there may also be methane oceans, and 4/5 of the atoms in a methane molecule are hydrogen, but... whatever, I digress.)
I also checked up on dimethyl sulfide and, yep, a sulfur atom connected to two methyl groups. It's a bit like water, structurally, with sulfur instead of oxygen and methyl ions taking the place of hydrogen. Except that it's not like water at all. Among other differences, water doesn't usually burn, unless you live in Cleveland.
This planet, K2-18b, was indeed observed by the JWST, and did have a fantastic spectrum taken of its atmosphere, revealing many fascinating details about it.
The fact that we can do this for a planet 120 light-years away is wondrous enough in itself.
However, there is no evidence that K2-18b is a Hycean world at all; no water was detected. There’s only dubious evidence for dimethyl sulfide, and even if it does exist in the atmosphere, assigning a biological cause to it is an incredibly dubious proposition.
You said "dubious" twice. Now I'm dubious.
Yes, that word can describe both unreliable observations, and our reaction to them. Love English.
Yet if you’ve read headlines from around the internet, it isn’t just the usual suspects like the New York Post or the Daily Mail with outrageous, alien life-driven headlines, but normally reliable places like National Geographic, the BBC, and right here on Big Think.
NatGeo hasn't been reliable since Fox bought them (though Disney later acquired that property along with lots of other Foxy things, though not the "news" arm); I've been starting to wonder about the BBC; and while I like Big Think as a source, it's still subject to many of the problems all internet-only sources have.
Let’s take a look at what’s really going on with exoplanet K2-18b.
The article proceeds to do just that, but I won't reproduce it all here. But, in summary:
None of these possibilities describe K2-18b, because it’s massive, puffy, and more Neptune-like than Earth-like.
Does this mean it definitively doesn't harbor life? Of course not. But let's not get ahead of ourselves.
K2-18b is about 2.6 times the radius of Earth and 8.6 times the mass of Earth. This means its density is less than half of Earth’s density, implying that it has a large envelope of volatile gases surrounding it.
Let's not be in too big a hurry to compare it to Neptune, either. Neptune is classified as an ice giant, because it's cold. K2-18b probably isn't. But I'll run with the Neptune analogy, because a) the article does, and b) I won't mention the name of the other ice giant in our solar system.
There's a bunch of technical details that follow, which I certainly won't reproduce here, but the article is an easy read and the link is right there.
While a water-covered Earth-sized world would be an incredibly interesting place to look for life, and in particular to look for the biosignatures associated with the processes that occur in ocean waters, it’s an enormous stretch to apply those same criteria to a gas giant world like K2-18b.
Why?
Because there was no water detected on K2-18b.
Again, this doesn't mean "no life." But it's not a good candidate for water-based life as we understand it. And we absolutely cannot make the jump from "possible simple life" to "Klingons."
In other words, it’s not entirely implausible that maybe, just maybe, this is a mini-Neptune version of a water-rich Hycean world, and maybe there really is some sort of extremely exotic form of life that exists on a world like this. After all, the JWST spectrum shows a (weak) indication of dimethyl sulfide, which we know here on Earth is produced biologically. Could that truly be what’s happening here?
Short answer: no. Longer answer: probably not.
Look, finding extraterrestrial life, even if it's just an archaeum or its equivalent, would be a Big Fucking Deal. I genuinely hope it happens in my lifetime (so hurry it up, already). But jumping to conclusions helps no one, and is especially difficult on a high-gravity planet.
Let's also not forget that "habitable zone" isn't the last word on where life might exist. Venus and Mars are (barely) within what's considered the Sun's habitable zone, and despite some flurries a few years back, neither has shown any definitive signs of harboring life. On the other hand, we're looking at moons of Jupiter and Saturn as possible places for life to have gotten a foothold, and those moons are absolutely outside the habitable zone.
Meanwhile, though, K2-18b would make a good name for a Star Wars droid. |
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