To become a candidate for a potentially habitable exoplanet is a rigorous process, since life as we know it requires very specific conditions to thrive. This visualization serves as an example of how quickly planets are thinned when we apply the criteria for a planet to be habitable. Boundaries for habitation in the attributes plotted are shown with gray lines. Anything within the plotted rectangle is suitable for habitation by conservative estimates. Optimistic estimates would yield a larger area of suitability. Read more on these values here.

We first plot all exoplanets by radius and mass, where both attributes are measured in Earth Units (i.e. the respective radius and mass values for Earth). A habitable planet should have a reasonable radius (between 0.8 and 2 Earth radii) and mass (between 0.5 and 5 Earth masses). As planets fall out of these boundaries, the existence of liquid water becomes less and less likely.

These first 2 criteria clearly cut down our prospects by quite a bit! However, there are plenty more planets to sift through before we find possible new Earths.

You may notice that there appears to be a trendline in the points. This is due to the large number of radius and mass values that had to be approximated due to lack of data. For many exoplanets, quite a few attributes have to be approximated due to the difficulty in attaining accurate readings. These approximations will not throw off our final results though, so do not worry.

Next, we need to assess the suitability of the climate on the exoplanets. Most life as we know it can only exist within a relatively constrained boundary of temperatures: those where liquid water can exist in at least some areas of the planet. The two values we filter next by are mean solar flux and mean surface temperature.

Mean solar flux is the mean solar electromagnetic radiation that a planet receives from its sun. Too much solar radiation and water will vaporize on the planet, too little and water will be frozen. Mean equilibrium temperature of a planet is strongly tied to solar flux, and helps establish if there is enough of an atmosphere to retain the heat from the planet's sun. Too thin or dense of an atmosphere will leave the planet uninhabitable.

Now we have signficantly limited our prospects of habitable exoplanets, however those that remain are very good candidates.

You may have noticed that Earth is on the upper edge of both our solar flux boundary and temperature boundary. That's not a mistake, Earth is hugging the inside of the habitable zone of our solar system. If we were just a tad closer to the sun, Earth would experience a runaway greenhouse effect from too much solar radiation, and all of the water on the planet would vaporize!

The last two attributes of habitability we graph by are Habitable Zone Distance (HZD) and Habitable Zone Atmosphere (HZA). They are two measuring tools established by the Planetary Habitability Laboratory that combine multiple factors into a measure of habitability.

Anything outside of the HZD boundaries is either too close to the sun or too far away. Anything outside of the HZA boundaries has either too thin of an atmosphere to too dense. As you can see, all of our candidates fall well within the boundaries, which means they are possible candidates for habitation!

We are left with 12 candidates for potential new Earths. All candidates are terran-type planets like Earth, although interestingly none of them have stars like our sun (G-Type). The majority of these final exoplanets have red dwarfs for a sun (M-type). This requires the planet to orbit much closer to the star to be within the habitable zone. Check out these planets in more detail below!