Centaurs
History In 1943, in the
Journal of the British Astronomical Association,
Kenneth Edgeworth hypothesised that, in the region beyond
Neptune, the material within the primordial
solar nebula was too widely spaced to condense into planets, and so rather condensed into a myriad of smaller bodies. From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies"
Subsequent hypotheses In 1987, astronomer
David Jewitt, then at
MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System."
Discovery Astronomers sometimes use alternative name
Edgeworth-Kuiper belt to credit Edgeworth, and KBOs are occasionally referred to as EKOs. However,
Brian Marsden claims neither deserve true credit; "Neither Edgeworth or Kuiper wrote about anything remotely like what we are now seeing, but
Fred Whipple did." The term
trans-Neptunian object (TNO) is recommended for objects in the belt by several scientific groups because the term is less controversial than all others — it is not a
synonym though, as TNOs include all objects orbiting the Sun at the outer edge of the solar system, not just those in the Kuiper belt.
Name Main article: List of the brightest KBOs Largest KBOs The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size,
Pluto was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.
Though Pluto is the largest KBO, a number of objects outside the Kuiper belt which may have begun their lives as KBOs are larger. Eris is the most obvious example, but Neptune's moon
Triton, which, as explained above, is probably a captured KBO, is also larger than Pluto.
Pluto The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of the
Pan-STARRS survey telescope, which should reveal many currently unknown KBOs, to determine more about this. the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
Structure Main article: Classical Kuiper belt object Classical belt Main article: Resonant trans-Neptunian object Resonances The 1:2 resonance appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the Classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.
The "Kuiper cliff" Main articles: Scattered disc and Centaur (planetoid) Scattered objects Main article: Triton (moon) Triton Satellites are markedly common among trans-Neptunian objects. Of the four largest TNOs, three (Eris, Pluto, and
2003 EL61) possess satellites, and two have more than one. This ratio is higher than that of Kuiper belt satellites as a whole, suggesting that a different formation mechanism was responsible.
Satellites Main article: Comet Comets Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as
methane),
ammonia, and water
ice, a composition they share with
comets.
Mass and size distribution As of 2006, nine stars other than the Sun are known to be circled by Kuiper belt-like structures. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (like our own Kuiper belt) with diameters of between 20 and 30 AU and relatively sharp boundaries. Most debris discs around other stars are fairly young, but the two imaged at right, taken by the Hubble Space Telescope in January, 2006, are old enough (roughly 300 million years) to have settled into stable configurations. The left image is a "top view" of a wide belt, and the right image is an "edge view" of a narrow belt. The black central circle is produced by the camera's
coronagraph which hides the central star to allow the much fainter disks to be seen.
No comments:
Post a Comment