Possible Undiscovered Planets
Possible Undiscovered Planets
[Caption above the chart.] Possible Undiscovered Planets in our Solar System By size and distance (from me)
[A chart of possible undiscovered planets with a log-log plot, with the objects diameter on the y-axis and the distance from “me” (Randall) on the X-axis. Both axes are labeled and have several ticks most of which also have labels. A region to the right, with possible new planets including Planet 9 with a “?”, is shaded light red, and a small rectangle at the top left with the same color tells what this color means. The region, of undiscovered dwarf planets, is shaded pink, also to indicate that here may be more of these, but the lighter color indicate that these will not be new “planets”. The eight known planets are marked with a black dot, and also this is explained with a dot under the colored rectangle. The Moon is indicated with a similar dot, but in gray, and the name is in brackets. The chart itself is divided into several labeled regions, the smallest with the label outside and an arrow pointing in. In one case a label breaks a border, and in two regions there are more labels, although these clearly belong to different regions within these regions, with different sizes and/or distances.]
[Y-axis, with a label written to the left, from bottom and up, with an arrow pointing up, and 15 ticks with a label each:] Diameter 1 mm 1 cm 10 cm 1 m 10 m 100 m 1 km 10 km 100 km 1,000 km 10,000 km 100,000 km 10 6 km 10 7 km 1 AU
[X-axis, with a label written below, with an arrow pointing right, and 17 ticks but only 11 labels as the ticks at 100 km, between 1000 and 10 6 km, 10 7 km as well as 10 and 1000 AU is not labeled:] Distance from me 10 cm 1 m 10 m 100 m 1 km 10 km 1000 km 10 6 km 1 AU 100 AU 10,000 AU
[At the top left of the chart is the light-red rectangle and the black dot labeled:] Possible undiscovered planets Known planets
[Going down and anti-clockwise from these two labels, the rest of the chart is transcribed:] Planets ruled out because I would be inside them Earth Planets ruled out because I would have noticed them above my house Planets ruled out because they wouldn’t fit through my door Birds that got into my house Skin flora Bugs (Not planets) Giant bugs Planets which are actually birds Airplanes (Fool’s planets’) Space junk Comets and asteroids Oort cloud Satellites Stuff we can see through telescopes Planets we can see at night (Moon) Dwarf planets Planet Nine? ? Planets ruled out by the WISE survey Planets ruled out because we would see them during the day


This comic is about Planet Nine , a possible Neptune -sized planet far beyond the farthest planet, Neptune . Astronomers Mike Brown and Konstantin Batygin published a paper on 2016-01-20, only two days before the release of this comic. The paper is called Evidence for a Distant Giant Planet in the Solar System and shows indirect evidence that such a planet may exist, inferred from an otherwise unlikely correlation between the unusual orbits of several dwarf planets . See for instance also explanations, for the layman, of the results here: Astronomers say a Neptune-sized planet lurks beyond Pluto and Planet X Discovered??
As this paper came out on the day of the previous comic's release ( 1632: Palindrome ), this comic's release day was the first release day after the news came out. This explains why this comic was released in the late afternoon rather than just past midnight as is often the case with normal releases. 
Once Randall heard this news, he had to decide to do this comic instead of the scheduled comic, and then invent and draw a completely new and actually very complicated comic (resulting in several position errors ) about " Planet X " before he could release this comic.
Now this planet could be called Planet IX (and is labeled Planet nine? on the chart), as Pluto , the previously "planet 9" has been degraded to a dwarf planet. The "X" did, however, not only refer to the roman numeral! Note that Mike Brown is the astronomer that killed Pluto , or at least reduced Pluto to a dwarf planet, something that has been a subject in xkcd before; see for instance 473: Still Raw .
This was the first of two times within a month where a new astronomical announcement (of something discovered months before the actual announcement) resulted in a related comic. The second being 1642: Gravitational Waves . But in that case Randall seemed to know about it in advance, as he even changed the normal release schedule to post the comic on the day of the announcement, unlike here, where he seems to have been forced to make a new comic up on the fly.
Stating the obvious, this log–log plot shows that for an object to be an unknown planet it has to be very far away, since planets are big, to explain why we haven't seen it yet. With the log scale it is possible to go from a diameter of less than 1 mm to an astronomical unit (AU) on the Y-axis and from a distance of just 1 cm up to thousands of AU on the X-axis.
Randall's chart is somewhat humorously parochial (if not downright egocentric ) in that it purports to measure distance not "from Earth" or from an arbitrary observer, but specifically from himself ("from me"). There is also more detail (e.g. a differentiation between bird, bugs, and skin flora) in the zone closest to the observer, somewhat reminiscent of the classic New Yorker cover illustration View of the World from 9th Avenue .
The distance calculations are somewhat problematic (see more on this below). Close distances seem to be measured from the surface of Randall's body (skin/eyes) rather than from his center of mass. All the planets (and moon) that are marked on the chart are so far away that it will not matter if the distance is measured from Randall's surface, his center of mass, or by the way anywhere on Earth . Also, the planets' diameters are so much smaller than the distance from Earth that their real size would hardly take up any space in the chart due to the log-scale . The dots marking these 7 planets are thus not drawn to scale that should represent their actual size compared with the other planets. But their distance from Earth (and Randall) is not constant even on the log-scale, especially not for the nearest planets, as they can be on either side of the Sun compared to Earth.
The chart correctly states that if there was a planet that was at a distance from him smaller than its diameter, he would be inside it (although at the bottom of that region, it's more like the planet that would be inside him, as this line goes down to a diameter of 1 cm). If the distance is to the planets center, this would also fit if he was only a radius away from the planet. As Randall is not inside the Earth but really close to it, Earth is correctly positioned on this line. However, for Earth, which is marked with the largest of the dots, he seems to have put himself a full Earth diameter away from Earth. Even using the center of mass of Earth as it's position he should only have been 6,350 km away from it, but now he places the measuring point of his distance to Earth on the opposite side of the Earth so his distance to it is equal to its diameter (which would make a choosing a distance of 0 km just as correct). Earth is just left of the 10,000 km line on both axis, and Earth has a diameter of 12,700 km, which will fit fine with the center of the dot, but not with the distance which should have been the maximum distance Randall could be from it (0 or 6350 km depending on the definition of distance from Randall).
The IAU definition of planet requires a solar orbit, gravitational rounding, and "clearing the neighborhood", a controversial (at the time of its introduction) calculation of relative size that excludes Kuiper Belt Objects such as Pluto. The calculation regarding Planet Nine would make it large enough to meet the IAU definition. Using this definition the chart quickly rules out birds and bugs, although at a glance they could be mistaken for planets, something that is especially the case for planes (at night) which are even called fool’s planets’ in the chart, a reference to fool's Gold . Note that anything that is actually on Earth is positioned within 60 km from Randall. This is because if it is further away he cannot see them due to the curvature of the Earth. This does not mean that he intends to indicate that they cannot be further away from him than that.
The already known planets are prominently marked on the chart. They are the solid black dots. Besides Earth and Planet Nine? , the bottom row of three small dots are Mercury , Venus and Mars . The top row of four larger dots (but smaller than the dot that marks Earth) are (from left to right) Jupiter and Saturn (visible to the naked eye) and Uranus and Neptune (visible through a telescope).
It is unclear how Randall is calculating these distances, especially to the three terrestrial planets , since neither of these options work: closest approach, average, mean, current or maximum distance. In the comic 482: Height Randall shows (among other) the distance from the Earths surface to all the planets. For especially Venus and Mars he shows that their distance changes a lot based on theirs and Earth's orbital positions. But he has neither used these loops to base the dot size or position, as these loops clearly go closer than 1 AU and only one of the planets are drawn closer than that. It is thus unclear which of the three represents which planet, but in the table below it has been reasoned that the dot situated at the largest diameter of the three inner planets planets should represent Venus (12,000 km) as it is almost twice as big as Mars (6,700 km), which on the other hand is more similar to Mercury (5,000 km) and given that the two dots furthest out are almost the same size, it would make most sense if they represent Mercury and Mars. Since the outer dot never comes closer than 2.5 AU and Mercury never gets further away than 1.5 AU it makes most sense to place Mercury as the middle of the three and Mars as the outer of the three dots.
Below some objects are mentioned that are not on the chart, and also other errors in position (probably due the hasty creation of such a complex comic.) Many of these objects as well as the planets with the errors mentioned clearly revealed can be see in this modified image , which is also inserted and explained in the trivia section below.
Pluto, no longer considered a planet (it was the ninth until 2006), is not marked on the chart, but it would be below Neptune just outside the pink region (2,300 km diameter and 30-50 AU away). This makes sense since that region is for dwarf planets not yet discovered and any one as big and close as Pluto would have been discovered by now. There are thus also other dwarf planets that would not belong in the pink region, one of them is even much much closer and is easily visible with a telescope: Ceres , which would appear roughly below Mars and Jupiter. (950 km diameter and 1.5-4 AU away from Earth). But this pink region is there to show where there could (and most likely will) still be undiscovered dwarf planets.
The Moon is also marked on the chart, with a gray dot (almost as large as the gas giants dots). The name is written in brackets since it's not a planet (because Earth is clogging up its neighborhood). Randall has messed up the positioning and the diameter of the Moon as it is clearly positioned past a million km, and it is only up to 400,000 km away from the Earth.
The Sun is not marked at all, even though it is extremely prominent, but as it is clearly not a planet it is left out. It would per definition have been at a distance of 1 AU, and with a diameter of 1.4x10 6 km it would be well inside the region of things that we can see during the day. Note that objects this big will always be shining, already a large planet such as Jupiter is brighter than if it could reflect 100% of Sun's light. In general, planets ruled out because we would see them during the day refers to objects big enough to be stars or brown dwarfs , but the only star system , other than Sun, that would fit on the chart is Alpha Centauri , which at 4.37 light-years (ly) is well within the right boundary that falls at 5.68 ly, just before the distance to the next nearest star Barnard's Star at 5.96 ly from the Sun. A light year is 63,241 AU, and with the 10,000 AU mark far from the right edge of this log-log plot, it is clear that also 100,000 AU and thus a light year is within the chart. And this also goes for 5 ly.
"Planets ruled out by the WISE survey" refers to the Wide-field Infrared Survey Explorer (WISE), a space telescope designed to look for warm objects such as brown dwarfs, which generate heat at their centers. It was capable of detecting Saturn-sized or larger planets in the outer reaches of our solar system, but did not find any. WISE would not have detected "Planet Nine" (even if it exists) because it is too small and thus too cold to be detected. There is a chance that it can be seen in some more temperature sensitive measurements. But these have not been checked for such a planet yet, (see here ).
The word satellites is written on the border of two regions indicating that these can be in both regions. Some are small enough (10 cm) to be comparable to the space junk below, (see cubesats ), others are much bigger and would fit in the region above: Stuff we can see through telescopes . Although it may not be called a satellite in daily talk, the International Space Station is in fact a satellite, which is over 100 m in the longest direction. It would thus be on the border to the Planets ruled out because I would have noticed them above my house region just above the "A" in satellites (400 km above the surface). Some satellites can be seen without a telescope, like the space station.
The title text explains why some people confuse Superman for a bird or a plane , since Superman often flies at the limit between the two categories in the diagram. This is though not really true as can be seen in the bottom of the table below. (This was later referenced in Bird/Plane/Superman .)
[Caption above the chart.] Possible Undiscovered Planets in our Solar System By size and distance (from me)
[A chart of possible undiscovered planets with a log-log plot, with the objects diameter on the y-axis and the distance from “me” (Randall) on the X-axis. Both axes are labeled and have several ticks most of which also have labels. A region to the right, with possible new planets including Planet 9 with a “?”, is shaded light red, and a small rectangle at the top left with the same color tells what this color means. The region, of undiscovered dwarf planets, is shaded pink, also to indicate that here may be more of these, but the lighter color indicate that these will not be new “planets”. The eight known planets are marked with a black dot, and also this is explained with a dot under the colored rectangle. The Moon is indicated with a similar dot, but in gray, and the name is in brackets. The chart itself is divided into several labeled regions, the smallest with the label outside and an arrow pointing in. In one case a label breaks a border, and in two regions there are more labels, although these clearly belong to different regions within these regions, with different sizes and/or distances.]
[Y-axis, with a label written to the left, from bottom and up, with an arrow pointing up, and 15 ticks with a label each:] Diameter 1 mm 1 cm 10 cm 1 m 10 m 100 m 1 km 10 km 100 km 1,000 km 10,000 km 100,000 km 10 6 km 10 7 km 1 AU
[X-axis, with a label written below, with an arrow pointing right, and 17 ticks but only 11 labels as the ticks at 100 km, between 1000 and 10 6 km, 10 7 km as well as 10 and 1000 AU is not labeled:] Distance from me 10 cm 1 m 10 m 100 m 1 km 10 km 1000 km 10 6 km 1 AU 100 AU 10,000 AU
[At the top left of the chart is the light-red rectangle and the black dot labeled:] Possible undiscovered planets Known planets
[Going down and anti-clockwise from these two labels, the rest of the chart is transcribed:] Planets ruled out because I would be inside them Earth Planets ruled out because I would have noticed them above my house Planets ruled out because they wouldn’t fit through my door Birds that got into my house Skin flora Bugs (Not planets) Giant bugs Planets which are actually birds Airplanes (Fool’s planets’) Space junk Comets and asteroids Oort cloud Satellites Stuff we can see through telescopes Planets we can see at night (Moon) Dwarf planets Planet Nine? ? Planets ruled out by the WISE survey Planets ruled out because we would see them during the day

