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Large, Distant Comets More Common Than Previously Thought

Aug. 25, 2017

Tucson, Ariz. -- Researchers found that about seven times more long-period comets measuring at least one kilometer across exist than previously thought. 

A team of astronomers including Planetary Science Institute Senior Scientist Tommy Grav used data from NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) space telescope mission to study the elusive long period comet population. With origins in the Oort Cloud – a group of icy bodies beginning roughly 300 billion kilometers away from the Sun – they can have periods of thousands or even millions of years. 

“Debiasing the NEOWISE Cryogenic Mission Comet Populations,” which appeared in The Astronomical Journal, says that over the course of the eight months of the survey, results indicate that the number of long-period comets passing within 1.5 AU (1.5 times the distance from the Sun to Earth) are 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. 

“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,” Grav said.

University of Maryland professor James Bauer is lead author, and PSI’s Grav is second author of the paper, which featured scientists from eight other institutions. 

"Our study is a rare look at objects perturbed out of the Oort Cloud," said Amy Mainzer, a co-author of the study who is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. and principal investigator of the NEOWISE mission. "They are the most pristine examples of what the Solar System was like when it formed." 

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 bodies can be disturbed by passing stars, galactic tides, or collisions, causing them to be perturbed inwards where they appear as long-period comets. The long-period comets observed in 2010 by the NEOWISE mission were probably expelled out of the Oort cloud millions of years ago.  

The study is based on 164 observed comets, including 95 short-period and 56 long-period comets. It also showed that the sizes of the long-period comets were generally larger than the short-period comets, which orbit the Sun with periods less than 200 years. "Our results mean there's an evolutionary difference between Jupiter family and long-period comets," lead author Bauer said. 

Grav’s work was funded by a subcontract from the Jet Propulsion Laboratory. 

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