Today the National Geographic released an extremely interesting infographic on exoplanets. It shows all 1000 confirmed planets and 700 validated Kepler Candidates and their vital statitistics. The x-axis gives the amount of light from it’s star, the y-axis has mass, each planet’s radius can be seen from the size of each point and the clickable version even displays each planet’s name. Most prominent on the diagram is the Goldilocks square containing a handful of exoplanets “just right” for life along with Earth and Mars.
As an infographic it is a beautiful and succinct way of showing what we know about planets around other stars. It makes two key facts about exoplanet detection plainly obvious: that most of the planets we currently know are big and hot; and that despite these limitations we are gradually pushing towards the detection of habitable, Earth-like planets. The position of newly-discovered Kepler 186f, a centimetre or so to the right of Earth, is testament to that.
But how does the science itself hold up? Well, as any good science teacher will say, always label your axes and use error bars. But we can let that slide as it is an infographic and not an undergraduate project.
How about the position of each point though? Well, the graph uses planetary mass as y-axis parameter. However for almost all of the low-mass planets displayed (ie. the Kepler candidates) the mass is almost completely unknown. All is known is the radius, and this can be used to give a rough estimate of the mass. And when I say rough, I mean extremely rough. For each radius value selected, the range of potential masses varies by more than 3 Earth masses even for Earth-sized planets! That could push a planet currently within the “Just right” square such as Kepler 283c into the ‘too large’ area and vice-versa.
Even that box should not be taken as given. The idea of a habitable zone varying with its distance from a star makes sense: too hot and water begins to boil away. Too cold and it freezes. But there are a huge number of things that could change those limits including tidal locking, atmospheric composition, surface reflectivity, atmospheric density, etc. To account for all of these is almost impossible, and to plot them all on a 2D plot certainly is. Current models (and the vertical lines you see here) get around this by assuming almost every parameter is Earth-like. For different sized planets, or those with unusual atmospheres, that assumption could break down (although work is certainly being done).
The habitable limits of planetary mass are even more like guesswork. Certainly, gas giants and tiny asteroids would appear less habitable than Earth mass planets, but the position of the limits at 0.1 and 10Me are arbitrary. There is no real reason why a large super-Earth or small sub-Mars could not support life, and certainly very little science has so far been done on this area so far.
So, despite displaying the main information well, this infographic gives the impression that we know a lot more than we actually do about both the limits of life and the characteristics of the planets that could hold it.
But, as was pointed out on twitter, this is not a scientifically published figure, but an infographic. It is something designed to spread knowledge in its simplest, uncomplicated form. And for that, it is fantastic.