The outer reaches of the Solar System are home to many small bodies, of which Pluto is the most famous. Due to their distance from Earth and relatively small sizes, these trans-Neptunian objects are somewhat difficult to study: even our most powerful telescopes can't image their surfaces in any detail, leaving astronomers either waiting impatiently for the New Horizons space probe to reach Pluto or resorting to more indirect methods. One such method is occultation, where one of these bodies briefly blocks (or occults) the light of a star.

A group of astronomers used 7 different telescopes in South America to track the occultation of a star by the dwarf planet Makemake (pronounced MAHkayMAHkay) and measured many of its properties for the first time. They concluded Makemake is noticeably non-spherical and may consist of two distinct types of terrain to explain the surface brightness.

When a Solar System object passes directly in front of a star, astronomers can measure both how much light that object blocks and how quickly that blocking occurs. And it's possible to combine data obtained by different telescopes in different locations on Earth (or ideally between multiple occultation events). Combined, these make it possible to reconstruct both the size—via the average length of the light blocking—and the shape of the body. In the absence of direct imaging, this is often the best we can do for a small, distant body.

Dwarf planets

The "official" dwarf planets designated by the International Astronomical Union are Ceres (the largest body in the Asteroid Belt), Eris, Haumea, Makemake, and Pluto (the four biggest objects in the region beyond Neptune's orbit). Of those, Eris is the most massive, though it and Pluto are nearly the same size. This hints at some significant variation in the composition of the trans-Neptunian objects, as does the contrast between Pluto's thin atmosphere and Eris' lack thereof. Makemake is roughly 2/3 the diameter of Pluto The "official" dwarf planets designated by the International Astronomical Union are Ceres (the largest body in the Asteroid Belt), Eris, Haumea, Makemake, and Pluto (the four biggest objects in the region beyond Neptune's orbit). Of those, Eris is the most massive, though it and Pluto are nearly the same size. This hints at some significant variation in the composition of the trans-Neptunian objects, as does the contrast between Pluto's thin atmosphere and Eris' lack thereof. Makemake is roughly 2/3 the diameter of Pluto The author of this article would like it to be known that his favorite trans-Neptunian object is Quaoar, which hasn't yet been recognized as a dwarf planet by the IAU.

Additionally, the abruptness of the occultation can reveal the presence of an atmosphere. If the eclipse occurs gradually, with a gentle decrease in the light from the background star, then the foreground body has an atmosphere. (What astronomers are observing is light passing through ever-thicker layers of gas until the surface of the body passes in front of the star.) In this case, researchers can measure the extent and density of the atmosphere, and spectral analysis may be able to determine its chemical composition as well.

On the other hand, if the star vanishes like switching off a light, then the foreground body has little or no atmosphere. The rocky or icy edge of the object blocks all the star's light effectively in an instant.

Makemake potentially occults three stars in a typical year, though not all of these are useful, due to the faintness of the background star. The current study involved an occultation visible from South America on April 23, 2011. Just as eclipses may be partial or total, the "shadow" of Makemake passed over a swath of the continent, allowing telescopes in various locations to measure the passage of the star behind different parts of the dwarf planet. The researchers tried to obtain data from 16 telescopes, but only 7 of those returned successful measurements.

Each telescope saw a clear, sharp drop in the background star's light, a strong indicator that Makemake has no substantial atmosphere. The astronomers also failed to see a second occultation that would reveal the presence of a moon, though they can't rule out a satellite smaller than 200 kilometers in diameter. (Previous observations have not seen a moon either.)

The different telescopes also saw the occultation last noticeably different lengths of time. The researchers modeled several possibilities and concluded the most likely one is that Makemake is not spherical. While it's nowhere nearly as elongated as the egg-shaped Haumea, if this model is correct, it is roughly 5 percent larger along one direction than another, making an ellipsoid shape roughly 1430km along one axis and 1500km along another. (Obviously this picture isn't complete, since we lack the data on the third dimension that was along our line of sight.)

The final bit of intriguing information the researchers collected involved Makemake's albedo, the fraction of light it reflects back into space. (Albedo is relative to the wavelength of light: an object may be highly reflective in visible light, but less so in other colors.) A completely reflective surface has an albedo of 1, while a completely absorbing surface would have an albedo of 0.

(Eris is highly reflective, with a visible-light albedo of 0.96; Pluto is much darker with 0.52 albedo. By contrast, the Moon's albedo is 0.12, about the same as a classroom blackboard.)

Makemake's average albedo is 0.77, but the observations revealed significant variation. The researchers argued these fluctuations could be reconciled if the dwarf planet has two types of terrain: highly reflective regions resembling Eris mixed with darker surface features like Pluto.

The astronomers suggested that this might be related to Makemake's lack of atmosphere. Eris' extremely bright surface is thought to have been created when its atmosphere froze and made a shiny frost across the globe. This same effect may have been responsible for the bright terrain on Makemake.

In this scheme, the atmosphere partially froze, leaving some traces over the darker, unfrosted regions that were too thin to be spotted in the occultation event. This model would place Makemake in a transitional region between Pluto, with its dark surface and measurable atmosphere, and Eris, with its high albedo but no air. With the limited data and few objects observed so far, it's hard to say whether this model is promising or not, but the suggestion is nevertheless intriguing.

Nature, 2012. DOI: 10.1038/nature11597 (About DOIs).