Editing 1071: Exoplanets

{{comic
| number    = 1071
| date      = June 20, 2012
| title     = Exoplanets
| image     = exoplanets.png
| titletext = 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 [http://xkcd.com/1071/large/ larger version] of this image can be found by clicking the image at xkcd.com - the comic's page can also be accessed by clicking on the comic number above.

==Explanation==
An {{w|exoplanet}} is a planet outside of our solar system, orbiting a different star. [[786: Exoplanets|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 {{w|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 {{w|Jupiter}} or {{W|Saturn}}-sized, blue is roughly {{w|Uranus}} or {{w|Neptune}}-sized, and the tiny dots are small {{w|terrestrial planets}} (like {{w|Earth}}).

We only have a few ways of {{w|Discoveries of exoplanets|finding exoplanets}}. Astronomers initially used {{w|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 {{w|brown dwarf}}s. 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 {{w|Kepler (spacecraft)|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 {{w|File:LombergA1024.jpg|narrow field of view}}. The great number of nearby planets implies there should be {{w|Carl Sagan|billions}} of planets in our galaxy, [[1339|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 billion). Our {{w|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 {{w|color perception test|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]].

See also [[:Category:Exoplanets]] and this {{w|lists of planets#Orbiting other stars|list of lists of exoplanets}}.

==Transcript==
:[A 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.]
:<big>All 786 known</big>
:<big><big><big>planets</big></big></big>
:<small>(as of June 2012)</small>
:<big>to scale</big>
:<small>'''(Some planet sizes estimated based on mass.)'''</small>

:[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.'''

==Trivia==
*This was the first time Randall released a comic with the exact same name as a previous comic, in this case [[786: Exoplanets]], released on August 30, 2010. Since then, he has done so [[:Category:Comics sharing name|a few times]]. When this comic was released, it caused problems on xkcd as the title of the image file (<code>explanets.png</code>) was the same for the two comics. This was resolved by renaming the old comic's image, adding the year of its release to the title: <code>explanets_2010.png</code>.

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Large drawings]]
[[Category:Astronomy]]
[[Category:Exoplanets]]
[[Category:Comics sharing name|Exoplanets02]]