Large, Distant Comets More Common Than Previously Thought

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. 

Above, this 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