1992

Kuiper Belt Objects

Kenneth Edgeworth (1880–1972), Gerard P. Kuiper (1905–1973)

After the discovery of Pluto in 1930, many astronomers began to wonder if the solar system really ended just beyond Neptune’s orbit. In 1943 the Irish astronomer Kenneth Edgeworth hypothesized that Pluto might be one of many small trans-Neptunian bodies that failed to grow very large because of the large separation between early-growing planetesimals (kilometer-size clumps of dust and ice) and the resulting lower impact rate in the outer solar system. In the 1950s, the Dutch-American planetary scientist Gerard P. Kuiper studied planet formation in the outer solar system and similarly hypothesized the presence of a large disk of small bodies beyond Pluto. If Pluto was an Earth-size body, however (as was thought at the time), then Kuiper speculated that this disk would have been cleared out and scattered away by Pluto’s gravitational influence.

The existence of what astronomers came to call the Edgeworth-Kuiper Belt (or just the Kuiper Belt) remained a subject of speculation for decades. With the advent of giant telescopes and super-sensitive charge-coupled device (CCD) detectors in the 1990s, however, it became possible to search for and ultimately detect small, faint, asteroid-like bodies at distances beyond Neptune’s orbit. The first member of the Kuiper Belt (besides Pluto, Charon, and possibly former members Triton and Phoebe) was discovered in 1992. Named 1992 QB1, the object orbits between 40 and 46 astronomical units (AU; Neptune is at 30 AU) and is probably about 100 miles (160 kilometers) in diameter.

Since then, more than 1,000 Kuiper Belt Objects (KBOs) have been discovered by astronomers. Some, such as 136199 Eris, 136472 Makemake, and 136108 Haumea, are comparable in size to Pluto. In fact, Eris may be larger than Pluto, partly precipitating a decision in 1996 by the International Astronomical Union to demote Pluto from planet to dwarf planet status. About 10 percent of known Kuiper Belt Objects have been discovered to have satellites (as Pluto does).

KBOs are primitive bodies containing comet-like mixtures of water, methane, and ammonia ices. The New Horizons probe will give us our first look at KBOs (Pluto and Charon) in 2015, and will go on to encounter more in the 2020s.

Painting by space artist Michael Carroll depicting a heavily impacted, icy, trans-Neptunian object (TNO) or Kuiper Belt Object (KBO) far beyond the orbit of Pluto. Like about 10 percent of known KBOs, this one is part of a binary pair, with its companion just above and to the left of the faint, distant Sun.