On the prowl, NASA’s NEOWISE telescope.
Credit: NASA/JPL


It’s crowded out there…

With origins in the Oort Cloud – a group of icy bodies beginning roughly 300 billion kilometers away from the Sun – have periods of thousands or even millions of years.

A team of astronomers report that large, distant comets are more common than previously thought. In fact, the researchers found that about seven times more long-period comets measuring at least one kilometer across exist than previously thought.


Using data from NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) space telescope mission, the elusive long period comet population has been studied.

Using NEOWISE, an eight month survey indicated that the number of long-period comets passing within 1.5 AU (1.5 times the distance from the Sun to Earth) is a factor of several higher than previous estimates, while Jupiter family comets are within the previous range of estimates of a few thousand down to sizes near 1.3 km in diameter.

Illustration shows how scientists used data from NASA’s WISE spacecraft to determine the nucleus sizes of comets. They subtracted a model of how dust and gas behave in comets in order to obtain the core size.
Credit: NASA/JPL-Caltech

Higher than expected

“The number of long-period comets seen in the NEOWISE was higher than expected from previous estimates, which means that there are seven times more Oort Cloud objects around our solar system than Dutch astronomer Jan Oort predicted in 1950,” said study participant, Tommy Grav of the Tucson, Arizona-based Planetary Science Institute in a press statement.

The Oort Cloud is thought to be a population of small icy bodies spherically distributed on the outermost edge of our Solar System. They are too distant to be observed by current telescopes.

These icy bodies can be disturbed by passing stars, galactic tides, or collisions, causing them to be perturbed inwards where they appear as long-period comets.

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