Far away, in the outer reaches of the Solar System lies the dwarf planet Makemake. Discovered in March 2005, it is about two thirds the diameter of the more famous dwarf planet Pluto. Like all dwarf planets, it is so far away and so small that we know very little about it. Astronomers have had to make a number of assumptions, one of which is that all these objects are similar in composition. It’s a reasonable thing to because those objects in the Solar System that we’ve studied are organised by type: All four inner planets are rocky. All four outer planets are gas giants. So perhaps all the distant outer planets are the same as well?
The problem is that we just don’t know. These little worlds are so small, and so far away, that even the very best telescopes can’t do much more than measure their brightness and distance. But on 23 April 2011, Makemake passed in front of the faint star NOMAD 1181-0235723. A team of astronomers took the rare opportunity to spread out across South America and observe the occultation together. By timing how long the light from the star was blotted out, they could measure the diameter of the little world very accurately. Knowing the planets size allows astronomers to calculate the albedo of its surface, which is a measure of its reflectiveness and that gives clues to its composition. An icy world is white and highly reflective, and has a high albedo. Rock is much darker, and has a lower albedo.
But occultations can reveal more about a world than just its size. If there is an atmosphere, astonomers will observe the light from the star briefly fade before it winks out – the atmosphere blocks out some of the light, and this dimming increases as the starlight has to pass closer to the surface, when the atmosphere is thicker. By measuring exactly how the light fades, we can see how thick the atmosphere is and make educated guess about it’s composition. It had been assumed that Makemake would have a similar atmosphere to Pluto, and the team making the observations hoped to confirm whether this was in fact true or not.
But the light did not fade at all: “As Makemake passed in front of the star and blocked it out, the star disappeared and reappeared very abruptly, rather than fading and brightening gradually. This means that the little dwarf planet has no significant atmosphere,” says team leader José Luis Ortiz of the Instituto de Astrofísica de Andalucía in Spain. “It was thought that Makemake had a good chance of having developed an atmosphere — that it has no sign of one at all shows just how much we have yet to learn about these mysterious bodies. Finding out about Makemake’s properties for the first time is a big step forward in our study of the select club of icy dwarf planets.”
Occultations are particularly uncommon in the case of Makemake, because it moves in an area of the sky with relatively few stars. Accurately predicting and detecting these rare events is extremely difficult and the successful observation by a coordinated observing team, scattered at many sites across South America, ranks as a major achievement.
This research was presented in a paper “Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation” to appear in the 22 November 2012 issue of the journal Nature.