Sometimes, a blink is more informative than a steady look. This is certainly the case when it comes to stellar occultations, events that occur when a planetary body hides a star as it moves across the sky. More than being a mere curiosity, such events provide a wealth of information about the occulting body — more so than direct images of it. For instance, stellar occultations have proved a powerful tool in discovering planetary rings, probing remote atmospheres at microbar pressure levels and measuring sizes at kilometric accuracies for bodies located hundreds of millions of kilometres from Earth. On page 897 of this issue, Elliot and colleagues1 report the first detection of a stellar occultation by a small object orbiting beyond the giant planet Neptune. The object is 2002 TX300 (also known as KBO 55636), which lurks at a distance of more than 6 billion kilometres from our planet.

The 'trans-neptunian objects', also known as Edgeworth–Kuiper belt objects (or simply KBOs), constitute a population of small planetary bodies orbiting beyond Neptune, in a vast region extending to the outskirts of our Solar System2. These objects are observed at distances as far as 50–70 astronomical units (AU) from the Sun (1 AU is the average distance from Earth to the Sun). The dwarf planet Pluto was discovered in 1930 and now ranks among the largest known KBOs. But it hasn't been a smooth ride for KBO hunters, because these objects are extremely faint and so are difficult to detect. It was only with the advent of modern charge-coupled-device light detectors, some 60 years after Pluto was spotted, that other KBOs could be discovered3. Now, after almost two decades of intense observations, more than 1,000 KBOs have been identified.

The Edgeworth–Kuiper belt is thought to be the relic of the primordial proto-planetary disk from which the planets emerged. Far from being a quiet, remote place, this region exhibits a surprising dynamic complexity2, betraying a perturbed history — one that involved gravitational stirring from the giant planets during the early ages of the Solar System4. Although a general picture of the Edgeworth–Kuiper belt has slowly been emerging, many questions have remained unanswered. The size distribution of the KBOs remains uncertain, and knowledge of basic information about their surface properties, bulk density and internal structure is poor. Yet these physical parameters are essential for assessing the present mass of the belt, and retrieving its history.

The stellar occultation described by Elliot et al.1 provides constraints on the size of KBO 55636. The two positive detections of this event, made on 9 October 2009 with telescopes in Hawaii (Fig. 1), indicate that, if spherical, the object must have a radius of 143 ± 5 km. If other shapes — ellipsoids, for example — are considered, smaller surface areas are obtained than would be produced by a sphere of such a radius. But all estimated surface areas, regardless of the shape considered, are significantly smaller than that made by a previous, indirect estimation5 that combined visible and infrared measurements and yielded an upper limit of 210 km for the radius of the body.

Figure 1: Observing a stellar occultation by KBO 55636.
figure 1

The band shows the path of the shadow of trans-neptunian object KBO 55636 that swept across Earth's surface during the stellar occultation of 9 October 2009, reported by Elliot and colleagues1. It took about five minutes for the shadow to cross the Pacific Ocean from right to left. Only the two stations denoted in black, located in the Hawaiian Islands and inside this band, successfully detected the stellar occultation. The stations denoted in blue made successful observations but did not detect the occultation, and the stations represented in red could not make observations because of weather conditions.

Full size image

For a given brightness, the smaller an object is, the higher its geometric albedo — a quantity used to measure, roughly speaking, the percentage of incoming solar light reflected by the object. In the case of KBO 55636, the authors deduced an albedo ranging from 82% to more than 100%, making it one of the most reflective objects in the Solar System. This is a surprising result. KBO 55636 is thought to be part of the trans-neptunian Haumea collisional family6; members of a collisional family are thought to be produced during a common catastrophic collision. Estimates for the age of this family7,8 indicate that KBO 55636 is the aftermath of a collision that occurred more than one billion years ago. But if that is the case, the expectation would be for the object to have a lower albedo than that deduced by Elliot and colleagues, because space weathering, which includes processes such as bombardment by cosmic rays, can darken KBO surfaces in less than a tenth of that time. Physical processes such as cryovolcanism, condensation of fresh ices caused by a putative atmosphere, or recent collisions can keep a surface shiny. But none of these processes is very appealing as an explanation for the estimated high albedo of KBO 55636, owing to its small size and to the fact that collisions are rare today among KBOs. The implication is that our understanding of space weathering or of the Edgeworth–Kuiper belt's dynamic history — or both — must be revised at some point.

Elliot and colleagues' observation1 represents the beginning of a long journey, because a dozen among the largest KBOs known are now listed as candidates for stellar occultations. Predictions of such events are difficult: the angular diameter spanned by KBO 55636 on the sky is equivalent to looking at a €1 coin from a distance of 500 km. Thus, the success of observations depends on accurate predictions made by experienced teams. The merit of this observation1 is to show that such predictions are now possible. But scientific returns may go well beyond size determinations. For instance, atmospheres with pressures as low as a few nanobars can be detected using stellar occultations. Furthermore, for KBOs with satellites, the objects' masses can be derived from the law of gravity. In those cases, accurate size determination also means accurate density derivation, an important parameter for constraining internal structure.

Finally, it should be noted that the observations presented by Elliot et al. were made using modest telescopes (with main mirrors of diameter 2.0 and 0.34 metres), and by a team of both professional and amateur astronomers. Being at the right place at the right moment is therefore more important than using big instruments — an unusual situation in physics, where the race for larger and larger detectors has become the rule.