2202: Earth-Like Exoplanet

Explain xkcd: It's 'cause you're dumb.
Jump to: navigation, search
Earth-Like Exoplanet
Fire is actually a potential biosignature, since it means something is filling the atmosphere with an unstable gas like oxygen. If we find a planet covered in flames, it might be an indicator that it supports life—or used to, anyway, before the fire.
Title text: Fire is actually a potential biosignature, since it means something is filling the atmosphere with an unstable gas like oxygen. If we find a planet covered in flames, it might be an indicator that it supports life—or used to, anyway, before the fire.


This comic is a reference to the recent discovery of water vapor in the atmosphere of exoplanet K2-18b. The planet was discovered already in 2015 by the Kepler Space Observatory, orbiting the red dwarf star K2-18. Water on exoplanets is considered a biosignature, meaning it's an indicator that there could be life there. However, as Megan reveals the planet's other characteristics, it becomes clear that it is unlikely to actually support life, and in fact is actually a horrible hellscape. The question of habitability by higher forms of life is profoundly different from the way astrobiologists use the term for microbes. Even a "survivable zone" can't mitigate the description of just how inhospitable this new wet planet would be to life as we know it, save possibly for extremophile organisms. In the comic 1231: Habitable Zone, this zone was the subject.

The planet being tidally locked indicates that the same side would face the planet's star year-round, meaning half of the planet would be in constant day and the other half would be in constant night. It is believed that K2-18b is tidally locked. Based on our (admittedly limited) understanding of life, abiogenesis can only occur in environments with liquid water; however, the day hemisphere would likely be so hot that all water found there would be in a gaseous state, and all water found in the night hemisphere would likely be frozen due to the intense cold. If life were to be found on this exoplanet, it would be in the twilight strip, a thin ring around the edge separating the two hemispheres where sunlight can reach but is refracted by the atmosphere. The environment in the twilight strip would thus experience something akin to an eternal sunset, and temperatures there would be moderate enough to allow life to come about.

Unfortunately, the other characteristics of the exoplanet severely undermine our chances of finding life even in its twilight strip, as there are many problems with the habitability of red dwarf systems.

  • Stellar flares are ejections of radiation and plasma from a star, and a planet being blasted with these searing-hot flares probably wouldn't readily support life. These are common for red dwarfs, which can often be of the flare star type.
  • Meteors are chunks of material that enter a planet's atmosphere, and if the planet is "blasted" by them it is likely that many of them are impacting the surface, thus becoming meteorites. As we know from the extinction of the dinosaurs, meteorites can have a sharply negative effect on a planet's habitability. There seems, however, to be no reason to believe this is a particular problem for this type of star system. This is where the comic starts to veer into absurdity.
  • Strong acids are present in some planetary atmospheres, including sulfuric acid in Venus's, and their hypothetical presence in the exoplanet's atmosphere would make life there even less likely. While life that evolves in a highly acidic environment might be able to withstand it, most life on Earth reacts poorly to strong acids. There is no reason to believe that the atmosphere of K2-18b is acidic. Apart from water the atmosphere mainly consists of hydrogen and helium. However, there is also reason to believe the planet has no solid surface.

Finally Megan describes the planet as being covered in "swinging blades." This could be a metaphorical allusion comparing the planet's dire straits to the Edgar Allan Poe poem "The Pit and the Pendulum," where the titular pendulum was a large blade swinging back and forth slowly. Due to the fame of Poe's work and the number of allusions made to it over the years, swinging blades have become a common feature in fictional deathtraps, and were used as an analogy to illustrate that the planet is chronically inhospitable to life. It could also be literal, a punchline to demonstrate the planet is so utterly inhospitable all life forms on the planet would have to squeeze between giant chopping razors.

"Biosignatures in the form of screaming" suggests that any life that had developed on the planet would be in continuous pain or fear due to their hazardous surroundings. In addition, this suggests that the screaming of these organisms would cause ripples in the atmosphere which we should be able to detect light-years away through the vacuum of space and that it would be more noticeable than other signs of life (such as the spectra from the ash produced by burning organic material.)

The title text mentions that fire could indicate the presence of life. This is because fire requires both fuel and oxygen (or some other similar, reactive gas). The occurrence of fire suggests that those things are both being continuously produced by some process. The most likely processes we know for producing oxygen are biological. The irony, of course, is that fire is also very dangerous, and almost universally lethal to organisms that are exposed to it for long enough. Munroe points out that oxygen reliably indicates that there was life, before the fire, with the implication that the fire may have killed everything.


[Side view of Megan standing behind a lectern, speaking to an off-panel audience in front of her. Two people from the audience react to her statement.]
Megan: We've discovered the most earth-like exoplanet yet!
Off-panel voices: Yay!!
[Front view of Megan behind lectern:]
Megan: Well, it's in the habitable zone. Habitable-ish. "Habitable."
Megan: The survivable zone.
[In a frameless panel with the same setting as before, Megan holds her left hand out with palm up.]
Megan: It's tidally locked. And blasted with stellar flares. And probably meteors. And bathed in acid.
[Closeup side view of Megan, now holding up a finger on her left hand. Again an unseen audience member replies.]
Megan: But we've detected water vapor! In between all the swinging blades.
Off-panel voice: I see.
Megan: We're hoping to find biosignatures in the form of screaming.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


I'm assuming this is in reference to exoplanet K2-18b? 18:30, 13 September 2019 (UTC)

I was thinking the same thing. Ianrbibtitlht (talk) 18:41, 13 September 2019 (UTC)
Note that K2-18b was actually "discovered" way back in 2015 by the Kepler Space Observatory. The recent news was the detection of water vapor in the atmosphere of the planet. Ianrbibtitlht (talk) 04:36, 14 September 2019 (UTC)

I'm seeing the actual comic alt-text as "Fire is actually a potential biosignature, since it means something is filling the atmosphere with an unstable gas like oxygen. If we find a planet covered in flames, it might be an indicator that it supports life—or used to, anyway, before the fire." Note the tab before "actually" and the odd characters after "life". But that's not what it has on this site. Is that difference intentional? 19:07, 13 September 2019 (UTC)

I've noticed a similar difference on other pages. For me, there are glitches in the title text on many XKCD pages, but here they appear as I assume they should. DanTheTransManWithoutAPlan (talk) 19:23, 13 September 2019 (UTC)
AFAIS the XKCD-webserver claims incorrectly that the charset of the page is windows-1252. --DaB. (talk) 19:49, 13 September 2019 (UTC)
Yes. If you change your browser's encoding to Unicode, it shows up properly -- though the tab before "actually" is still there. --Aaron of Mpls (talk) 00:24, 14 September 2019 (UTC)

A non-tidally-locked planet (like ours) needs to be firmly in a habitable zone (like ours) to allow the daily and seasonal cycles (like ours) to not send every square foot of the planet well outside any 'reasonable' range of conditions so that there's no possible adaptation possible by life (like ours).

OTOH, a tidally-locked planet probably sustains a belt of habitability upon it somewhere between the most sun-scorched face-centre and the most astronomy(-if-not-astronomer)-friendly area of the farside, and it may even let the lifeforms survive more extreme stellar 'seasons' than a swirling planet could, so long as that belt doesn't move so far as to 'lift off' either face, if there exist effective migration paths available for the mobile life and hibernation/aestivation states and hidey-holes for those that are forced/choose to be immobile.

There's the argument about a constant hurricane-force surface wind passing between hot and cold hemispheres, but that assumes a reverse upper flow in atmospheric cells (or a phase-cycle of liquid?) which would promote and reinforce elements of turbulance that might interact with 'surface' features (perhaps subsurface, in waterworld environment) to create areas that are lucuna in the chaos, 'islands' of calm.

Though with many theories of abiogenesis and evolution requiring some form of cycling conditions to filter out the unadaptable and promote the adaptable, so the actual 'interesting' zones are probably in habitable-edges surrounding the habital spots of constancy within the habitable belt upon the habitable-zone planet.

It's a bit moot how all this would work, though, given our knowledge based upon post-facto knowledge of a sample of one life-bearing planet. Hard to know how little or much Earth is typical compared with everyone else. At least until my people come back to rescue me, when I'll have to remember to catch up on the basic classes I've obviously missed. 22:01, 13 September 2019 (UTC)

I thought "between the swinging blades" was just a metaphor - 05:41, 14 September 2019 (UTC)

It is and should be changed in the explanation. It is all the things mentioned by Megan that are the swinging blades--Kynde (talk) 07:42, 14 September 2019 (UTC)
But anyone who can give some examples where this sentense is used in the real world? I could not find much using google. Would like it in the explanation, better than what I have done so far. --Kynde (talk) 21:25, 14 September 2019 (UTC)
What came to mind for me is the blade-on-a-pendulum from Poe's short story "The Pit and the Pendulum", except that there were multiple ones that a life form would have to avoid being hit by. Do a google image search on "Pit and the Pendulum" to see what I am talking about. Redbelly98 (talk) 22:30, 14 September 2019 (UTC)
That's what came to my mind as well. Not necessarily from this short story, but just a general depiction of a hellish place with fire and deadly (torture) devices. Bischoff (talk) 07:10, 15 September 2019 (UTC)
I was reminded of the climax of Umberto Eco's novel "Foucault's Pendulum"... considering the previous comic, Randall may have been thinking the same.
I am not sure it's a metaphor. I thought it was Randall's first (of two) instances of something too ridiculous to actually be observed, the other being the screaming. The other stuff mentioned before the blades are all plausible observations. Redbelly98 (talk) 22:30, 14 September 2019 (UTC)
Mentions of swinging blades is just a trope of an absurd life threatening hazard. Anyway, mentions of them always remind me of this 19 year old joke article from the onion.-- 07:32, 15 September 2019 (UTC)
I'm sure it is not meant as something they have detected, but the things she just mentioned are the swinging blades. She just tells the audience they found a Earth-like planet, but then she mentions four thing that doesn't look like earth, only that it is near the habitablezone and that there are water in the atmosphere. Someone put some silly video game comments in as if this was the plot and i t is not and I will revert it now. --Kynde (talk) 20:36, 15 September 2019 (UTC)
It sounds as if this planet was badly written. These Are Not The Comments You Are Looking For (talk) 06:26, 16 September 2019 (UTC)

Interestingly, there are plants on Earth that use fire as part of their lifecycle; see Fynbos for an example. Of course, the optimal cycle there is about one big fire a decade; truly continual flames would eventually wipe out fynbos as well (but then what fuel is that fire burning? A fuel that lasts forever would revolutionise the energy industry...) - 07:35, 15 September 2019 (UTC)