Abel Mendez / Planetary Habitability Lab / UPR-Arecibo

Every day, it seems like there’s a new exoplanet out there—and often, claims of habitability. But it’s easy to get lost in the mess of numbers. We have discovered more than 3,700 exoplanets, but they range in size from smaller than the moon to bigger than Jupiter, bordering on the size of failed stars.

Just today, astronomers announced a new exoplanet, Ross 128 b, which is nearby (11 light-years) and possibly in a slightly chilly habitable zone.

Professor Abel Mendez of the Planetary Habitability Lab at the University of Puerto Rico-Arecibo put together this spectacular chart to show exoplanets organized by size and temperature. It reveals a few surprising facts. First, that temperate, Earth-size planets are thus far fairly rare. We’ve discovered a total of 21 exoplanets that are Earth-like in size and estimated temperature (though we know nothing about their atmospheres, so Earth-like is a loose term). In addition, a few dozen bigger rocky planets could be habitable, and perhaps one smaller rocky world.

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Here is the Periodic Table of Exoplanets with over 3700 confirmed worlds. @PlanetaryHabLab #habworlds17 pic.twitter.com/thgjtGABZv — Prof. Abel Méndez (@ProfAbelMendez) November 15, 2017

As far as candidates as habitable worlds, that’s it. There are roughly 52 planets to work with out of 3,700, and that’s given a range of sizes, temperatures, and star types. A planet around a sun-like star receives more light and less flare activity than one around a small, red star like Proxima Centauri (the closest star to the sun), and thus have a better chance at true habitability.

But there are many more fascinating exo-worlds to explore. There are the warm Neptunes and warm Jupiters. Life in a gas giant is unlikely, though possible, but if our own solar system is any indication, these worlds could have watery moons. Therefore the gas giants beyond the solar system are still worth looking at in case they have Mercury-sized moons with liquid oceans, like Jupiter and Saturn do.

On Mendez's exoplanet periodic table, there are also cool facts in the margins. The top right shows how many stars have a specific known number of exoplanets in orbit. Three stars are known to have seven planets, approaching the solar system's... eight to however many.

The chart also has percentages in the bottom right corner of each planetary category. Rocky planets bigger than Earth with scorching hot temperatures are the most common, accounting for more than 25 percent of all known exoplanets. This runs contrary to previous assumptions that Jupiter-sized planets dominate the galaxy. Overall, however, we can see that the majority of known exoplanets are burning hot.

All this information is based off only the exoplanets we have discovered, however. Exoplanet finding techniques can have a bias for certain kinds of worlds, like planets around small stars with short orbits, or large planets with a significant gravitational tug on their star. Finding an Earth-sized planet in a 300-some day orbit is shockingly rare, so far.

There are 14 known planets that likely have Earth-like temperatures, but only a few of those are around sun-size stars. Just four have orbits more than 100 days, leaving the majority of these planets in orbits that likely make one side face the star at all times—an equilibrium called tidal locking that leaves one half of the planet hot and one half cold at all times, reducing the likelihood of life.

Future planet-finding space missions like the Transiting Exoplanet Sky Survey and telescopes like the James Webb Space Telescope may help find planets in longer orbits. Future telescopes could also provide more clues about these planets' true habitability.

In the meantime, we have this handy chart of exoplanets to remind us how vast the galaxy truly is, filled with strange worlds the likes of which we can only imagine.

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