1071: Exoplanets

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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.
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.


Ambox notice.png This explanation may be incomplete or incorrect: Should have a description of each planet in the comic. Since that's a lot, maybe just the more well known/closer to earth ones.
If you can address this issue, please edit the page! Thanks.

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.

See also Category:Exoplanets.


[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.

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Hmm... this comic and 786 have the same title. Is that a mistake? Jimmy C (talk) 01:07, 24 October 2012 (UTC)

It may very well have been on xkcd itself; there was a bit of a snafu when Randall posted the image. That's part of the reason why we decided on number+name here, to ensure that that sort of naming collision couldn't be repeated. -- IronyChef (talk) 04:39, 24 October 2012 (UTC)
It's also worth mentioning that 786 is both the number of the other strip, and the number of planets in this one. 22:38, 16 April 2013 (UTC)

The image isn't appearing for me. I think it's a problem with the thumbnail system. Bugefun (talk) 18:15, 27 December 2012 (UTC)

Same here. Using Chrome. -- St.nerol (talk) 19:20, 27 December 2012 (UTC)

Same on ipad. DruidDriver (talk) 07:12, 17 January 2013 (UTC)

And on Firefox. -- 01:01, 27 January 2013 (UTC)

Not showing up in Chrome. Alpha (talk) 23:14, 2 February 2013 (UTC)

As a side note, the pace at which we're discovering exoplanets is accelerating. The first confirmed planet-sized mass outside our solar system was discovered in 1992, and it was ten years until we could celebrate the discovery of the 100th exoplanet. In the fifteen months since this comic was posted, another 156 exoplanets have been discovered (source: Extrasolar Planets Encyclopaedia, which lists 942 exoplanets as of 2 Sep 2013). Frijole (talk) 22:41, 10 September 2013 (UTC)

There are 786 exoplanets listed in the comic, And the previous comic about exoplanets is comic 786..... Coincidence? 08:58, 3 December 2013 (UTC)

I think it's possible that he was waiting for the count to increase to that number to create some sort of meta-pun. With Randall, you never know, but the odds of that happening independently seems unfathomable to me. 16:12, 4 December 2013 (UTC)
Additional comment: I believe the original filename for 768 was just "exoplanets.png" before being changed to "exoplanets_2010.png" when this comic was released. Any website that hotlinked the first comic would have their image replaced with the newest one. (talk) (please sign your comments with ~~~~)

Does anybody else see this and think colorblindness test? 22:43, 23 January 2014 (UTC)

Absolutely. Saw this from across the room (sometimes my machine takes a while to load) and thought it was a colourblindness statement rather than a planetary one. 20:41, 22 December 2020 (UTC)
Three levels deep.. Remind anyone of Inception?A2658742 (talk) 08:36, 25 October 2014 (UTC)

This comic is referenced on exoplanet.eu, a professional site for exoplanet scientists, as the first link on a page titled "General professional Web sites relevant to extrasolar planets". The actual link goes to an interactive version of the page, but the link is at http://exoplanet.eu/sites/ labeled "Exoplanets: an interactive version of XKCD 1071". The actual interactive page is http://codementum.org/exoplanets/ . N. Kalanaga 13:58 (UTC-4) 10 April 2016

Was it actually 786 exoplanets known back then, or 786 planets including both the exoplanets and our own solar system? I would read the caption the latter way. This would make the number of exoplanets 778, like they also count it here and here, while this explanation here mentions 786 exoplanets several times. --YMS (talk) 09:25, 9 December 2017 (UTC)

I could count them, but... I don't really feel like spending ~4 minutes straight on counting dots. 10:26, 29 March 2018 (UTC)