Tomorrow is a rather big day for the exploration of the Solar System. A world that has long represented our ignorance about the outer solar system will come into splendid view, as revealed by the New Horizons spacecraft. Pluto, a dwarf planet discovered in 1930, has always been that question-mark at the end of the book about the Solar System.
I tend not to cover solar system exploration on this blog because it’s dedicated to extrasolar planet science, but I think there are some interesting parallels one can make between Pluto and extrasolar planets. Until recently, Pluto has just been an unresolved dot in the sky. Indirect methods had allowed for the mapping of crude surface features — we came to learn that Pluto has significant albedo variation across its surface, with patches of bright and dark, but other than that, we knew nothing about their composition or even what caused them. To a large extent, we still don’t. We now know (as of the past couple days) that Pluto’s north pole is covered in an ice cap dominated by nitrogen and methane, and that the dark regions are comparably methane-poor, but we don’t really understand a lot of what’s going on yet. But the advance in our knowledge of Pluto over the past month has been truly revolutionary.
In the not-too-distant future, indirect methods will begin to yield crude albedo maps of extrasolar planets. These maps may have a similar quality to those acquired for Pluto. It will be worth remembering, however, that there’s so much more about those planets that we won’t be able to see simply because we lack the ability to send a probe of some sort to those extreme distances.
The image above shows a comparison between our “best map” of Pluto before New Horizons, and what is currently our limit of knowledge (with full credit to Bjorn Johnson). Quite a difference a spacecraft visit makes! The top map is an average of five separate maps acquired by HST and ground photometry, so an important caveat here is that not only is the wavelength coverage different than New Horizons’ LORRI camera made to use the bottom map, but the mapping technique is different, too, and prone to different biases. That all being said, there is a decent amount of similarity between the two.
We may live to see an exoplanet’s surface resolved to an extent similar to Pluto’s before New Horizons, however it will be several generations before we are able to map an exoplanet with the same level of precision as the bottom map in the image above.