Reflections on the Planets

Here is a chart from Wilkinson, illustrating the bubble plot method, where a third variable is encoded by the size of the marker. Unfortunately, planets are not a good data set for demonstrating bubble plots; we automatically assume these differently-sized circles are representations of the planets to scale. We’re also not very good at discriminating between the sizes of small circles: is Earth 0.4 or 0.5? Is Mercury 0.2 or 0.1?


The units are not very friendly, either. Albedo is just the percentage of electromagnetic radiation reflected by a planet, and AU are units equal to the Earth’s distance from the sun. Why temperature was chosen as a third variable is unclear (and why use degrees Kelvin?). Sure, it varies linearly with distance from the sun on a log–log scale, but that’s no surprise. The only exception is Venus, whose temperature is a consequence of a carbon dioxide atmosphere, not albedo; if anything, its cloud layer lowers the temperature by reflecting sunlight. So the graph is not really telling a coherent story.

What could be improved? We can start by just plotting albedo against distance, using more intuitive units for both. Now some of the variation that was masked by the bubble plot begins to emerge:


This is a fairly basic chart, but it still isn’t telling a story. To flesh it out, I added relative sizes, changed the brightness of the planets to match albedo, and annotated some of the outliers. The graph has become a little too cluttered, because the pattern of data points is being swamped by supplementary information, but at least now there’s some sort of narrative going on. And it prompts questions, like why does the Earth have such a high albedo when it’s mostly ocean?


A little more research, and I realized Earth’s albedo is mostly determined by cloud cover. That was the key to understanding why Venus and the gas giants were so reflective, and dry balls of rock like Mercury and Mars weren’t. So I resimplified the chart to make that point, stripping off some of the irrelevant information.


Now the graph has a point to make, and you can just tell it’s happier.

References: The original chart is from Leland Wilkinson’s The Grammar of Graphics(Springer 1999). What piqued my interest was a fascinating discussion of what color the planets really are. Planetary albedo data are taken from part of NASA’s site, Wikipedia supplied the Earth albedo data, which merely lists “Edward Walker” as its reference (as does every other site on the internet, blithely copying Wikipedia of course.) The old Wikipedia page cites, gulp, “Walker, E., 1987: Pictures of Preschoolers Out in the Snow. Dishwasher Picture Publishing, Volume 26, 151–1103.” So you may want to take those figures with a grain of salt.