Title text: Planets are turning out to be so common that to show all the planets in our galaxy, this chart would have to be nested in itself—with each planet replaced by a copy of the chart—at least three levels deep.
A larger version of the comic is at xkcd.com/1071/large.
An exoplanet is a planet outside of our solar system, orbiting a different sun. 786 exoplanets were known in mid-2012; since then astronomers have found thousands more. In the comic, our Solar System's eight planets are depicted in the small square above the central text. From this we find that the largest dots (red) and second largest dots (dark brown) indicate planets larger than Jupiter, light brown is roughly Jupiter or Saturn-sized, blue is roughly Uranus or Neptune-sized, and the tiny dots are small terrestrial planets (like Earth).
We only have a few ways of finding exoplanets. Astronomers initially used doppler spectroscopy, which detects minute changes in a star's movement towards or away from us to infer the presence of large gas giants or brown dwarfs. Currently the most successful method is to notice when a star seems to briefly get dimmer on a repeating cycle. This may indicate that a body of matter has passed between that star and us, blocking some of the light. The Kepler space telescope was designed for this purpose, and has made the vast majority of exoplanet discoveries.
Most of Kepler's discoveries are between the sizes of Earth and Neptune, but it's sensitive enough to detect planets smaller than Mercury (if the orbital plane is aligned with us). Kepler is only able to observe relatively close stars in a narrow field of view. The great number of nearby planets implies there should be billions of planets in our galaxy, assuming our local arm is not uniquely abundant.
The title text refers to this by saying that to show them all each dot on the chart, should hold another chart with the same amount of dots; each of these dots should then also have a similar chart, and then do this one more time for a three level deep chart. This chart would have space for 786^4 planets (786*786*786*786 = 382 billions). This may be more room than needed? But if the chart were only two levels deep there would "only" be room for 786^3 = 0.5 billion planets.
This comic's design is similar to the Ishihara Color Test, a series of circular pictures made of colored dots, used to detect red-green color blindness. However, Randall's picture probably does not contain a hidden number like it did in 1213: Combination Vision Test.
Two different xkcd comics have the title "Exoplanets". The first was number 786, and this one was drawn at a time when 786 exoplanets had been found. Probably not a coincidence when it comes to Randall.
- [An enormous diagram of dots, mostly of varying shades of brown and greenish yellow, with a number of smaller blue dots and larger red dots.]
- All 786 known planets (as of June 2012) to scale.
- (Some planet sizes estimated based on mass)
- This [indicating a small section of 8 planets out of the several hundreds] is our solar system. The rest of these orbit other stars and were only discovered recently. Most of them are huge because those are the kind we learned to detect first, but now we're finding that small ones are actually more common. We know nothing about what's on any of them. With better telescopes, that could change. This is an exciting time.