Long-period comets visit inner solar system more frequently than thought

Originating in the distant Oort Cloud, these comets tend to be larger and more pristine.
By Laurel Kornfeld | Jul 26, 2017
Data collected by NASA's Wide Field Infrared Survey Explorer (WISE), an infared wavelength space telescope that scanned the sky between December 2009 and February 2011 searching for the origins of asteroids and comets, indicates long-period comets visit the inner solar system more frequently than scientists thought.

Long-period comets are those that take 200 years or more to complete a single orbit around the Sun. Originating in the Oort Cloud, a sphere of icy bodies beginning around 186 billion miles (300 billion km) from the Sun, they can take thousands or even millions of years to complete one solar orbit.

Given their long orbital periods, these comets are difficult to study since many never come near the inner solar system during a human lifetime.

In contrast, Jupiter family comets, whose orbits are shaped by Jupiter's gravity, are easier to study, as their orbits around the Sun take at most 20 years.

Over a period of eight months, WISE detected three to five times as many long-period comets than predicted and seven times more long period comets with diameters of 0.6 miles (one km) than expected.

The spacecraft also measured the long-period comets to be on average twice the size of Jupiter-family comets.

In 2010, WISE studied 95 Jupiter family comets and 56 long-period comets.

Before WISE, measuring the sizes of long-period comets posed a challenge to scientists because their nuclei are obscured by hazy clouds of gas and dust known as acoma.

Because WISE observed in the infrared, scientists were able to subtract the infrared glow of the coma from the total size of the comet and make an educated estimate as to the size of each nucleus.

James Bauer of the University of Maryland at College Park, lead author of a study on the findings published in The Astronomical Journal, confirmed comets that pass by the Sun more frequently tend to be smaller since their volatile compounds, including water, sublimate every time the comet is subject to the Sun's heat.

In contrast, comets that spend most of their time far away from the Sun are more pristine and closely resemble the makeup of the early solar system.

"Our results mean there's an evolutionary difference between Jupiter family and long-period comets," Bauer said.

"The number of comets speaks to the amount of material left over from the solar system's formation. We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought."

The higher number of long-period comets could mean the solar system has experienced more impacts between comets and planets than previously thought, with the comets having delivered icy materials, including water ice, from the Oort Cloud.

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