Exoplanets
Exoplanets
[An large diagram of dots, mostly of varying shades of brown and greenish yellow, with a number of smaller blue dots, tiny green dots and some larger red dots. At the top of the circle are five lines of text in very different font size.] All 786 known planets (as of June 2012) to scale (Some planet sizes estimated based on mass.)
[Below this text is a small section of 8 planets which are framed in a light gray frame with lighter gray background . It is situated right below the above text with only a few planets in between the text and the frame. These planets include two large yellow, two smaller blue two small green and two tiny green planets. A line goes between this frame to another frame with the first word in the text below, that is in a similar frame. The rest of the text follows to the right and then below this first word covering the central part of the circle from just around the center of the circle and a bit below.] This  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.

An exoplanet is a planet outside of our solar system, orbiting a different sun. 786 planets were known in mid-2012: 778 exoplanets and the rest in our Solar System.
Since then, astronomers have found thousands more. In the comic, our Solar System 's eight planets are depicted in the small rectangle 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). Our Milky Way contains about 100-400 billion stars. 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 786: Exoplanets , and this one was drawn at a time when 786 exoplanets had been found. Probably not a coincidence when it comes to Randall . This is the first time Randall released a comic with the exact same name as a previous comic. Since then he has done so a few times . When this comic was released it caused problems on xkcd as the title of the image files were the same for the two comics. This was resolved by renaming the original image adding the year 2010, the year when it was released, two years before this one was released.
See also Category:Exoplanets and this list of lists of exoplanets .
[An large diagram of dots, mostly of varying shades of brown and greenish yellow, with a number of smaller blue dots, tiny green dots and some larger red dots. At the top of the circle are five lines of text in very different font size.] All 786 known planets (as of June 2012) to scale (Some planet sizes estimated based on mass.)
[Below this text is a small section of 8 planets which are framed in a light gray frame with lighter gray background . It is situated right below the above text with only a few planets in between the text and the frame. These planets include two large yellow, two smaller blue two small green and two tiny green planets. A line goes between this frame to another frame with the first word in the text below, that is in a similar frame. The rest of the text follows to the right and then below this first word covering the central part of the circle from just around the center of the circle and a bit below.] This  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.
