(Every day until December 25, I’m posting a science-related image and description.)
As of December 2, 2013, astronomers have identified 1049 planets orbiting around other stars, with over 2000 more candidates awaiting follow-up observations. We’re able now to draw statistical inferences about how many exoplanets are out there, what types of stars they orbit, and the range of sizes.
In fact, the statistical approach is necessary, since our detection methods are heavily biased toward large planets orbiting close in to their host stars.  We only know of a handful of Earth-sized planets, and only a few exoplanets have been detected orbiting in the habitable zones of Sun-like stars. However, astronomers have a large enough sample now that they can extrapolate, estimating how many planets of range — super-Jupiter, Jupiter-class, Neptune-class, super-Earth, Earth-class, sub-Earth — there should be out there.
That’s what the xkcd graphic above is: a visualization of these statistical estimates. For simplicity, the graphic makes no attempt to depict distance from Earth or any other location information, other than limiting the sample to 60 light-years. Only planets orbiting within the habitable zone are included — the region where liquid water could exist on the surface — so the majority of known exoplanets aren’t in the picture. The size of the circle represents the approximate physical size of each exoplanet. Planets orbiting stars like our Sun are shaded with the red and brown tones, while those orbiting other types of stars (mostly small red stars) are gray.
So, the dark red and blue-gray circles are Earth-size planets orbiting within their host star’s habitable zone. You’ll notice that there are a lot of planets that aren’t Earth-like or that don’t orbit Sun-like stars…yet there are still a fair number within 60 light-years of the Solar System! This says nothing about whether life is likely on these planets, since we honestly don’t know, but it does tell us something about how common Earth-size worlds could be.
- Another more subtle bias is that we currently can only see star systems relatively close to the Solar System. It’s a pretty reasonable assumption that our galactic neighborhood is typical, but we’re extrapolating when we want to estimate the total number of Earth-like worlds in the entire Milky Way.
- In this case, Sun-like refers to a general match in mass, which dictates other stuff like surface temperature. See my earlier post on spectral classification.