Earth-Like Exoplanet
Earth-Like Exoplanet
[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.

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 .
The comic uses swinging blades as a metaphor to succinctly describe the planet's perilous conditions. That is, as far we know, there are no actual swinging blades on K2-18b. [ citation needed ] Swinging blades made their first famous appearance in 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.
"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.
