NASA's Hubble Space Telescope has detected a long-sought population of comets dwelling at the icy fringe of the solar system. The observation, which is the astronomical equivalent to finding the proverbial needle-in-haystack, bolsters proof for a primordial comet reservoir just beyond Neptune, currently the farthest planet from the Sun.
Based on the Hubble observations, a team of astronomers consisting of Anita Cochran of the University of Texas, Austin, TX, Hal Levison and Alan Stern of Southwest Research Institute, San Antonio, TX branch office in Boulder, CO, and Martin Duncan of Queen's University, Ontario, Canada, estimate the belt contains at least 200 million comets, which have remained essentially unchanged since the birth of the solar system 4.5 billion years ago.
"For the first time, we have a direct handle on the population of comets in this outer region. The solar system just got a lot more interesting," said Cochran. "We now know where these short-period comets formed, and we now have a context for their role in the solar system's evolution."
The existence of a comet-belt encircling our solar system like the rings which wrap around Saturn was first hypothesized more than 40 years ago by astronomer Gerard Kuiper. The so-named Kuiper Belt remained theory and conjecture until 1992, when ground-based telescopes began detecting about 20 large icy objects ranging from 60 to 200 miles in diameter. The planet Pluto is considered by astronomers to be the largest member of the Kuiper Belt region. However, researchers had to wait for Hubble Space Telescope's high spatial resolution and sensitivity before they could search for an underlying population of much smaller bodies assumed to be present just as there are more pebbles on the beach than boulders.
"This is a striking example of what Hubble can do well," said Cochran. "We can at last identify small comet-sized objects that are just a few miles across, about the size of New York's Manhattan Island. "Cochran discussed her team's findings at a 11:00 a.m. news conference June 14, at the 186th meeting of the American Astronomical Society in Pittsburgh, PA.
The team believes this apparently closes the mystery of the source of the short period comets, that orbit the Sun in less than 200 years, including such members as comet Encke, Giacobini-Zinner, and the infamous comet Shoemaker-Levy 9 that collided with the planet Jupiter in July, 1994. The comet-disk lies just beyond Neptune and might stretch 500 times farther from the Sun than Earth. This is 100 times closer to Earth than the hypothesized Oort cloud, commonly thought to be a vast repository of comets that were tossed out of the early solar system. Despite their close proximity, the Kuiper belt comets don't pose any greater threat of colliding with Earth than comets that come from much farther out, said experts.
The comet nuclei are the primordial building blocks that condensed out of the cloud of gas, dust and ices that collapsed to form the Sun. "Knowing where comets come from will help constrain models for the formation of the solar system and tells us something new about where we came from," Cochran emphasized.
"The Kuiper Belt is the best laboratory in the solar system for studying how planets formed," said Levison. "We believe we are seeing a region of the solar system where the accumulation of planets fizzled out."
The icy nuclei are too far away to have the characteristic shell (coma) and tail of gasses and dust that are a comet's trademarks, when it swings close enough to the Sun to warm up and sublimate. Detecting these bodies in their "deep-freeze" state, at the dim horizon of the solar system, pushed Hubble Space Telescope to its performance limits. "Imagine trying to see something the size of a mountain, draped in black velvet, located four billion miles away," said Stern.
The team used Hubble's Wide Field Planetary Camera 2 (WFPC 2) to observe a selected region of the sky in the constellation Taurus, that had few faint stars and galaxies that would confuse the search. The detection is based purely on a statistical approach, because the objects being discovered are so faint.
The team plans to continue searching for more objects. They have already collected more images with Hubble. These additional images allow them to better quantify the number and sizes of comets in the Kuiper belt. They also will apply for more Hubble observing time in the future to probe the structure of the Kuiper belt.
Space Telescope Science Institute, Baltimore, MD
University of Texas, Austin, TX
Hal Levison and Alan Stern
Southwest Research Institute, Boulder, CO
Queen's University, Ontario, Canada