xkcd.WTF!?

Explanation ➲

Exoplanets are planets outside the Sun's Solar System. As astronomers discover more and more of these, they are finding all kinds of weird and unexpected examples, often with unusual and interesting physical properties, necessitating the introduction of new categorisations to describe them. Here, Randall presents a depiction of a hypothetical star system containing many exoplanets of different exotic types. Most of these are entirely imaginary, and some are outright nonsensical.

A number of the planet descriptions reference the potential habitability. This attracts a lot of attention, particularly in the media and popular imagination, both because it suggests a higher potential for finding extraterrestrial life of a form we might more easily recognise, and because such planets might potentially be examined as candidates for future space colonization. Unfortunately, for many hopefuls, there are a number of specific conditions required for Earth life to survive, and there are many possible ways for space to be inhospitable to life. As a result, most exoplanets that have been discovered have conditions that make it nearly impossible for humans to survive, and difficult for any life form as we know it to exist. The examples here illustrate (in an exaggerated fashion) some of the many frustrations astronomers face when analyzing planets and getting their hopes up, only to discover the planets they found are, sadly, nothing like Earth.

There are a total of 19 planets in the comic. Here are the explanations for each planet, in order of how far they are from the star:

Giant planet orbiting so close that it's actually rolling on the star's surface

Giant planets are observed to be gravitationally pulled close to their star (see Hot Jupiter, below). Here, such a process has progressed until the planet is literally touching the star, which would likely put it inside the star's Roche limit and cause the planet to disintegrate. Even if the planet stayed intact, the star has no solid surface to roll on.

Hot Jupiter

A Hot Jupiter is typical terminology used in analyzing exoplanets, generally depicting a gas giant (of a size similar to our Jupiter or Saturn) which orbits in a much closer/hotter orbit than our own. Hot Jupiters are easier to detect than many other types of exoplanets, due to both their gravitational effect on their stars (creating larger and/or more frequent wobbles, compared to any smaller and/or more distant planets) and their vastly more significant dimming effect on their host star's light (obscuring a greater quantity, if they happen to pass in front of it, than would any smaller body).

We know of our own two 'not-hot Jupiters', and various other planets, despite their relative undetectability across interstellar space. Most were observed directly (and a majority of those were first detected by the Mark-I Eyeball, over millennia), with coincidental celestial alignments and their gravitational effects upon the Sun being far less obvious. Studies suggest that planets may change their orbits over time, on timescales too long to easily observe directly, so we can somewhat account for the existance and prevalence of Hot Jupiters, given that the stars currently with more tightly orbiting Jupiter-mass planets are more likely to be confirmed as such. Whether they are anything like as common and significant as initial observational counts suggest, and thus what the 'typical temperature' of any given Jupiter-like might be (including whether our own system's distribution of planetary sizes and distances is common or rare) is therefore a question that remains open for the time-being, our more local assessment of gas-giant warmth aside.

Planet that may actually be in the habitable zone, according to a very optimistic modeling paper by some desperate postdocs

The habitable zone of a star is the range at which water is liquid. Notably, planets in the habitable zone are seen as options for colonization by humanity, which would mean greater funding for research. As such, researchers will go to great lengths to determine as many habitable planets as they can.

There's a pulsar here but it's probably fine

A pulsar is a neutron star which emits beams of radiation while spinning very fast. This means that this system is technically a binary star. The mass of a neutron star, this close to the visible star, means that any planets could not be simply in orbit around the latter. In addition to the gravitational instability this would produce, the intensity of the radiation and excess heat from the pulsar would make it extremely difficult for any form of life to exist in the system. Also, neutron stars are remnants of a Supernova explosion, an event which is likely to sterilize, eject, or outright destroy any planets exposed to it. All of this lends a substantial level of irony to the "probably fine" comment.

A waterworld paradise with beautiful oceans and warm— wait, no, we just got new measurements, it's a hellish steam oven

In any scientific field, new information may turn previously established knowledge on its head. Exoplanet research is no different, and a planet that at first seems to be habitable might turn out to be an incredibly deadly steam world. An example of this can be found in our own solar system with Venus, which was known to have clouds as early as the 1700s and was speculated to be habitable — but later, in the 1960s, those clouds were found to be made of steaming hot sulfuric acid.

Planet that could be habitable, if there's a form of life that hates water but loves acid and being on fire

Scientists remain open to the possibility that life might form from different conditions than those found on Earth. That said, the speculation that life on this planet must "love acid and being on fire" is more than a little sarcastic. Possibly to a reference to the what if? article Interplanetary Cessna, in which Randall points out that the atmosphere on Venus is pretty survivable at 55km, except for the sulfuric acid, and way too hot at the surface.

Mini Neptune

As with "Jupiter" being used for the family of larger gas-giants, "Neptune" is often used to describe those with rough size/mass categories similar to our own not-quite-so-giant examples of Neptune and Uranus — sometimes instead termed "ice giants", though superficially they are still as gaseous. To call something a mini-Neptune would make it significantly smaller than Neptune, possibly small enough to not properly be any kind of 'giant' at all (perhaps instead transitioning into the nominal super-Earth class, but still with a substantial atmosphere), making the comparison to it even less accurate.

Lukewarm Jupiter

A humorous interpolation of the "Hot Jupiter" vs. 'normal'-Jupiter scale, producing a finer distinction between 'types' of Jupiter. The French Wikipedia has an article on "warm Jupiters", indicating that it isn't an entirely unknown concept. "Lukewarm", however, still stands as vague, ill-defined and unrigorous as a description, as is often deliberately invoked for xkcd.

Planet whose atmosphere is confirmed to contain atoms

With interstellar distances, it is inevitable that some planets will be hard to get a read on. Here, the astronomers can only confirm the planet has an atmosphere, not what it is made of, or perhaps even how extensive it is. If it has an atmosphere at all, there are very few options except for it to be composed of atoms.

Possibly in reference to headlines about exoplanets whose atmospheres contain molecules that may indicate biological life, but extrapolated to comedic levels of vagueness. A similar 'too cautious' approach to the data can be seen in 2359: Evidence of Alien Life.

Earthlike data artifact

An artifact in this context is any error where it looks like something exists when it actually doesn't. This may be caused by faults in the equipment, or by other activity that looks similar to a planet signal. In this case the astronomers thought they detected an Earth-like planet, only to discover it was a data artifact. There have been several such cases already, for example Gliese 581g, which was considered to be the most Earthlike planet discovered at the time, before more detailed analysis concluded it didn't exist. Either that, or it has turned out that Earth itself is a data artifact, which would raise epistemological questions about the whole endeavour of studying the universe.

Cold Jupiter

Implicitly the opposite of a "Hot Jupiter", as described above. Used here as another extension of the "<temperature> Jupiter" running gag, "Cold Jupiter" is occasionally used in real astronomy (usually to describe our own type of 'non-hot' Jupiter, rather than an explicitly colder-still version), but is fairly informal and used mainly to directly deliniate against the Hot variety. The French Wikipedia also has an article on cold Jupiters.

Potentially habitable void

It seems that the part of this system that would be ideal for habitable planetary conditions does not actually contain any planets. This would make it much harder to actually inhabit (the notional inhabitants would have to build a world to live on, and would have nowhere to easily locate themselves while the building was in progress), to the implied frustration of the astronomers. Confusingly, the illustration of the void is as a patch that crosses several of the planetary orbits. Since it cannot remain a void if a planet passes through it, this suggests it is an object (or rather a lack of objects) that orbits in its own right. This is not normally how habitable zones work — they are usually a span of potential orbits, forming a ring (or shell) around the star. Additionally, this region is positioned right between Cold Jupiter and Hot Mars, implying that it is in between hot and cold, and between gaseous and rocky, being closer to Earth conditions.

Hot Mars

Continuing the joke regarding Hot Jupiters. This assumes that if there's hot Jupiters, there must be a "hot" variant of every planet, including Mars.

Faint dust cloud that will cause several papers to be retracted

This is in reference to the astronomical transit method of discovering planets by measuring periodic dips in brightness of the central star, done by missions like the Kepler space telescope. It turns out that not all variations in brightness are caused by planets, much to the disappointment of overeager data analysts and science news reporters; see, for example, Fomalhaut b, a former proposed exoplanet that turned out to be a dust cloud, or Tabby's Star, a star with odd irregular dimming pattern likely due to a dust cloud which was briefly thought by some to be an alien megastructure.

Either a gas giant or a fist-sized rock, depending upon which calibration method you use

Scientific instruments, including telescopes, must be calibrated in various ways, and different calibration methods can lead to different corrections applied to raw data and hence different values for calibrated data. This is exaggerated in the comic to make a raw observation emerge from the calibration correction process as two objects of vastly different size - a range of uncertainty between tens of thousands of kilometers and a few centimeters, perhaps nine orders of magnitude. This is not particularly precise, even for a cosmologist. Also, a "fist-sized rock" would be impossible to detect around a distant star using current technology; it would be difficult to spot something of this size around our own star.

Mini Pluto

In line with the Mini Neptune, above. Pluto is already significantly smaller than all other acknowledged planets (and even seven moons). Thus, in part, its redesignation as a "dwarf planet" — of which, it is not even the most massive known. A "mini Pluto" suggests an exoplanet that shares most of Pluto's features, but is somehow even smaller. But presumably bigger than the above "fist-sized rock".

Wet Saturn

A further spin on the "<adjective> <planet>" theme. May be a reference to the 'fact' that "Saturn would float in water" due to its overall density. The difficulty of finding a practical way to test this notwithstanding, perhaps someone managed it with this planet. Alternatively, this is a Saturn-like planet that somehow has liquid surface water rather than predominantly atmospheric, or features a higher than expected proportion of continually recondensing water vapour.

Planet whose surface may host conditions suitable for rocks

As with the above planet whose atmosphere "contains atoms," this planet is too difficult to get a read on, and the measurements are still so vague that it's unknown whether this planet is a rocky planet or a gas/ice giant. This is also a play on the holy grail of exo-planet research — finding a planet whose surface hosts conditions suitable for life. It's not impossible that that would involve living rocks, but that would impose a different set of environmental restrictions from those for the carbon-based life we're familiar with.

Somehow this whole system is smaller than the orbit of Mercury?!

No planet, or anything other than an orbital path, shown. But apparently an indicator that all the rest of the given orbits (for Jupiter-likes, Mars-likes, dust clouds, etc., and even semi-inconvenient pulsars) exist within a planetary system that is extremely compact, fitting into a volume of space the size of that between our Sun and the orbit of its nearest planet, Mercury.

This is likely a reference to the fact that many exoplanets located so far have been in even tighter orbits around their star than Mercury is with the Sun. It is worth noting, however, that this is likely to be observational bias, as large and tightly orbiting planets have a significantly larger (and hence easier to identify) effect on their parent star. There are likely many small and more distantly orbiting exoplanets that we are simply unable to observe effectively at this time.

This is also likely a reference to the fact that the majority of stars in our galaxy are red dwarf stars, which are much cooler and dimmer than our Sun. This means that the habitable zone, and a suitably wide range of solar irradiance, can be found within a smaller radius of the star.

A popular subject of xkcd, the Firefly/Serenity series has a fictional setting that is also purportedly compact (by Earth's system's standards), featuring many worlds (and multiple stars, though none of them pulsars) within 'easy' in-system travelling distances for narrative purposes. Some minor inspiration may have been derived from this, though no overtly direct references appear.

Title text

The title text mentions a planet within an accretion disk, which means that the planet is passing through material inspiraling into some significantly more massive object and is likely inspiraling itself from resistive effects of passing through said material. Nevertheless, the researcher speaking assures us that the planet is in the disk's habitable zone, implying it is a worthwhile option for colonization. This is despite the observed location being likely temporary as the planet's orbit inspirals closer and closer to the central object, to say nothing of the likely constant bombardment of debris and potential radiation depending on how massive the central object is and how dense the accretion disk is.

Alternatively, the accretion disk in question might be a very compact and hot one, such as those found around some black holes, which glows brightly enough to illuminate its surroundings like a star and has a habitable zone safely outside the disk itself.