Editing 2202: Earth-Like Exoplanet

{{comic
| number    = 2202
| date      = September 13, 2019
| title     = Earth-Like Exoplanet
| image     = earth_like_exoplanet.png
| titletext = 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.
}}

==Explanation==

This comic is a reference to the {{w|K2-18b#Discovery_of_water|recent discovery}} of water vapor in the atmosphere of exoplanet {{w|K2-18b}}. The planet was discovered already in 2015 by the {{w|Kepler (spacecraft)|Kepler Space Observatory}}, orbiting the {{w|red dwarf}} star {{w|K2-18}}. {{w|Extraterrestrial liquid water|Water on exoplanets}} is considered a {{w|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 {{w|astrobiology|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 {{w|extremophile}} organisms. In the comic [[1231: Habitable Zone]], this zone was the subject.

The planet being {{w|Tidal locking|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 {{w|K2-18b#Physical_characteristics|K2-18b is tidally locked}}. Based on our (admittedly limited) understanding of life, {{w|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 {{w|habitability of red dwarf systems}}.
* {{w|Stellar flare|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 {{w|Habitability_of_red_dwarf_systems#Variability|common}} for red dwarfs, which can often be of the {{w|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 {{w|Cretaceous–Paleogene extinction event|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, {{w|Atmosphere_of_Venus#Composition|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 {{w|K2-18b#Discovery_of_water|atmosphere mainly consists}} of hydrogen and helium. However, there is also reason to believe the planet has {{w|K2-18b#Discovery_of_water|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 "{{w|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.

==Transcript==
:[Megan is standing behind a lectern, speaking to an off-panel audience to the right, some of which reacts to her statement. At least two reacts as shown by two lines from off panel ending in the same spoken word.]
:Megan: We've discovered the most earth-like exoplanet yet!
:Off-panel voices: ''Yay!!''

:[Frontal 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 seting 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 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.

{{comic discussion}}

[[Category:Comics featuring Megan]]
[[Category:Exoplanets]]
[[Category:Public speaking]]