Centaurs: The Solar system’s Most Mysterious Objects


An interesting interpretation an depiction from NASA

Out beyond the orbit of Jupiter, centaurs roam – small objects that share could be planets, comets or asteroids. Or maybe a little of each. In this article Jane McKinney on these weird little guys!

As astronomers puzzle over ways to classify these distant inhabitants of the Solar System, recent data suggests that the centaurs are complex creatures indeed.  Named for the half human, half horse beings of Greek mythology, centaurs dwell in the inner Kuiper belt, between the orbits of Jupiter and Neptune. Though they’re very diverse in appearance and behavior, they all share one key characteristic: their orbits cross the major axis of Neptune’s orbit, in periods that can range from decades to thousands of years.

Though the first centaur was observed in 1920, it wasn’t until the discovery of 2060 Chiron in 1977 that astronomers realized that these objects constituted a distinct population. Since that time, institutions such as the International Astronomical Union’s Minor Planet Center and JPL’s Deep Ecliptic Survey have catalogued 474 of them out of an estimated 44,000 such objects.

The white-dotted donut represents the many main belt asteroids that Jupiter prevented from forming into a planet

Only a few centaurs are large enough to be observed directly. The largest is 10199 Chariklo, about 250 km in diameter. Chiron follows at around 220 km. Much of what astronomers know about centaurs is based on spectral analysis and occultation – observing what happens when a centaur passes in front of a star, dimming it briefly. Those observations have led the Minor Planet Centro to classify centaurs as both minor planets and asteroids, but even that dual label doesn’t entirely explain their mystery.

Beyond Jupiter, the regions of the Kuiper Belt, the Scattered Disc and the Oort Cloud contain remnants of the Solar System’s protoplanetary disc, material from the system’s birth, pushed ever farther outward by the expansion of the inner planets. It’s the home of comets, both short and long period, as well as asteroids and the true minor planets such as Pluto.   Some of these objects have relatively stable, circular orbits, but others are pushed into eccentric elliptical ones that take them into the inner Solar System. Centaurs have features of all of these.

 Planets, Comets, Asteroids – Or All the Above?

Centaurs can be classified as planets because some centaurs, in some ways, behave as planets do. Many of them circle the sun in relatively stable orbits – and at least two, Chariklo and Chiron, appear to have rings. So far, rings have been observed only around planets. Though Saturn is the best known in our Solar System, Jupiter, Uranus and Neptune have them too.

The color and composition of centaurs suggest a very different identity, though. Known centaurs are either reddish or blue-to-blue gray in color. Those color differences, and what they meant, remained unclear until NASA’s Wide Field Infrared Survey Explorer concluded its mission in 2013. This project provided detailed data on centaurs, asteroids and other objects in the outer Solar System, and gave astronomers the clearest picture yet of the true nature of centaurs.

Wide-field Infrared Survey Explorer

Data from WISE reveal that red-hued centaurs probably have an organic surface layer and a solid core, similar to asteroids. But blue or bluish gray centaurs are very different. They seem to be covered with a dark, sooty outer layer over a surface that’s probably covered in ice.

Comets are typically dark objects, with an icy nucleus covered by dark surface dust. They come from the Kuiper Belt and beyond, pushed into long elliptical orbits by the perturbations of nearby planets such as Jupiter. Because blue centaurs share so many characteristics with comets, astronomers working with the data from WISE theorize that blue centaurs are cometary – they have been, or have the potential to become, comets, most likely from the Jupiter Family of short period comets.

But although recent research has revealed ways to describe what centaurs may be, they still remain a complicated group that defies a single, simple classification. As researchers at NASA, JPL and the Minor Planet Center work to catalogue the constantly expanding family of centaurs, new discoveries about these mysterious objects may provide clues to the origins of the Solar System itself.

Article written and supplied by: Jean McKinney. Jean is a respected US-based science and health writer. Visit her science writing sit: http://blackmoonjournal.com.

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