Massachusetts Institute of Technology | 16 June 2010
Occultation provided enough data to determine the KBO’s size and albedo
Until now, astronomers have used telescopes to find Kuiper Belt objects (KBOs), moon-sized bodies, and obtain their spectra to determine what types of ices are on their surface. They have also used thermal-imaging techniques to get a rough idea of the size of KBOs, but other details have been difficult to glean. While astronomers think there are about 70,000 KBOs that are larger than 100 kilometers in diameter, the objects’ relatively small size and location make it hard to study them in detail.
One method that has been has been proposed for studying KBOs is to observe one as it passes briefly in front of a bright star; such events, known as stellar occultations, have yielded useful information about other planets in the solar system. By monitoring the changes in starlight that occur during an occultation, astronomers can determine the object’s size and temperature, whether it has any companion objects and if it has an atmosphere.
The trick is to know enough about the orbit of a KBO to be able to predict its path and observe it as it passes in front of a star. This was done successfully for the first time last October when a team of 18 astronomy groups led by James Elliot, a professor of planetary astronomy in MIT’s Department of Earth, Atmospheric and Planetary Sciences, observed an occultation by an object named “KBO 55636.”
As Elliot and his colleagues report in a paper published June 17 in Nature, the occultation provided enough data to determine the KBO’s size and albedo, or how strongly it reflects light. The surface of 55636 turns out to be as reflective as snow and ice, which surprised the researchers because ancient objects in space usually have weathered, dull surfaces. The high albedo suggests that the KBO’s surface is made of reflective water-ice particles, and that would support a theory about how the KBO formed. Many researchers believe there was a collision that occurred one billion years ago between a dwarf planet in the Kuiper Belt known as Haumea and another object that caused Haumea’s icy mantle to break into a dozen or so smaller bodies, including 55636.
More importantly, the research demonstrates that astronomers can predict occultations accurately enough to contribute to a new NASA mission known as the Stratospheric Observatory For Infrared Astronomy (SOFIA) that completed its first in-flight observations in May. A Boeing 747SP aircraft that has a large telescope mounted onto its rear fuselage, SOFIA can record infrared measurements of celestial objects that are not possible from the ground. Elliot hopes his research will help guide future flights of SOFIA to observe stellar occultations in detail.
A second look at KBO 55636
Nature 465 (17 June 2010) Editor's Summary
Kuiper belt objects (KBOs), small icy bodies orbiting in the Solar System beyond Neptune, are difficult observational targets. Occasionally one is spotted causing a stellar occultation, when data relevant to its structure can be obtained and a few properties deduced. But the stellar occultation of 9 October 2009, involving KBO 55636, a member of the water-ice rich Haumea collisional family, was a landmark event taking the observational astronomy of KBOs to a new level. Following several years of tracking of bright KBOs, it was possible to predict an imminent occultation, and to train a number of telescopes in its direction. Despite the vagaries of weather and other observational hazards, more than one (in fact two) separate telescopes were able to make the observation. With two different angles of view to work with, more accurate figures can be calculated: KBO 55636's mean radius of 143 ± 5 km, and its geometric albedo of about 0.9 in the V band, reveal it to be smaller than previously thought and, like its parent body, highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water ice can persist for billions of years in the outer reaches of the Solar System.
Solar system: Blink from a remote world
- Nature 465, 878-879 (17 June 2010) | doi: 10.1038/465878a
The use of stellar occultations to disclose unknown aspects of our Solar System is not new. But the latest such event to be reported involves an object that lies beyond the orbit of Neptune — and is a first.
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.
Size and albedo of Kuiper belt object 55636 from a stellar occultation
- Nature 465, 897-900 (17 June 2010) | doi: 10.1038/nature09109