In short: Each line is an orbit. The bigger the circle, the heavier the gravity. Green is similar to Earth in temperature and gravity.
Description: A diagrammatic representation of exo-planet data with the color green representing something resembling Earth. The comparison is very much an approximation of how Earth-like these planets are as only their basic mechanics are known at this point. The image itself is diagrammatic and bordering on an … infographic. However there is a point that is contrary to the usual infographic approach. Exo-planets tend to be presented as an alternative habitat for us to live in, whereas the practicalities involved with upping sticks and moving are usually skimmed over in infographics and media releases. This image conveys that there are many exo-planets that could be habitable, and even if a perfect candidate is not known now, it is only a matter of time before a very Earth-like one is discovered. Finding one might grab a headline and provide some juicy click-bait, but it is not really the problem. Knowing what’s there and managing the distances involved are the problems. This image is designed to overwhelm us with the choices. The distances involved is something I plan on returning to.
Point of interest: There appears to be a cluster of similar exo-planets that can be seen in both the grouping of the orbit lines and when looking at the lump in the spiral of discs representing the planets. Two visual clues to the same thing. As the Universe is often very random, wouldn’t we expect to see them more evenly spaced? This might well be down to the techniques being more sensitive to certain combinations of exo-planet characteristics, in which case there is likely a lot more of the other kinds out there that we just haven’t seen yet. Or not.
Technical: Data for all discovered exo-planets currently available at http://exoplanets.org as of end of 2016. Planets were filtered for surface temperatures between double and half that of Earth, and for gravities +/- 10% of earth. Gravity is estimated from the mass and radius of each exo-planet. Surface temperature is estimated from the luminosity of the host star and the semi-major axis. The orbits are plotted to include eccentricity. The size of the disc is proportional to gravity and is plotted diagrammatically in order of semi-major axis. The disc is heavily weighted to green if the gravity should be within 3% of Earth. For the orbit to be weighted to green the “surface temperature” should be within 5% of Earth. Otherwise the colors scale with the property.