Large, distant comets more common than previously thought

July 25, 2017
A new study suggests that distant "long-period" comets -- which take more than 200 years to orbit the sun -- are more common than previously thought. This illustration shows how the researchers used data from NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft to determine the nucleus sizes of several of these distant comets. They subtracted a model of how dust and gas behave in comets in order to obtain the core size. Credit: NASA/JPL-Caltech

Comets that take more than 200 years to make one revolution around the sun are notoriously difficult to study. Because they spend most of their time far from our area of the solar system, many "long-period comets" will never approach the sun in a person's lifetime. In fact, those that travel inward from the Oort Cloud—a group of icy bodies beginning roughly 300 billion kilometers away from the sun—can have periods of thousands or even millions of years.

NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft has delivered new insights about these distant wanderers. A team of astronomers led by James Bauer, a research professor of astronomy at the University of Maryland, found that there are about seven times more long-period comets measuring at least 1 kilometer across than previously predicted.

The researchers also found that long-period comets are, on average, nearly twice as large as "Jupiter family" comets, whose orbits are shaped by Jupiter's gravity and have periods of less than 20 years. The findings were published July 14, 2017, in The Astronomical Journal.

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

The Oort Cloud is too distant to be seen by current telescopes, but is thought to be a spherical distribution of small at the outermost edge of the solar system. The density of comets within it is low, so the odds of comets colliding within it are low. Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago. The observations were carried out in 2010 during the spacecraft's primary mission, before it was renamed NEOWISE and reactivated to target near-Earth objects (NEOs) in 2013.

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

Astronomers already had broader estimates of how many long-period and Jupiter family comets are in our solar system, but had no good way of measuring the sizes of long-period comets. This is because the cloud of gas and dust that surrounds each comet—known as a coma—appears hazy in images and obscures the comet's nucleus.

By using WISE data that shows the infrared glow of the coma, the scientists were able to "subtract" the coma from each comet and estimate the size of the nucleus. The data came from WISE observations of 164 cometary bodies—including 95 Jupiter family comets and 56 long-period comets.

The results reinforce the idea that comets that pass by the sun more frequently tend to be smaller than those spending much more time away from the sun. That is because Jupiter family comets get more heat exposure, which causes volatile substances like water to sublimate and drag away other material from the 's surface as well.

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

The existence of so many more long-period comets than predicted suggests that more of them have likely impacted planets, delivering icy materials from the outer reaches of the solar system.

Researchers also found clustered orbits among the long-period comets they studied, suggesting there could have been larger bodies that broke apart to form these groups.

The results will be important for assessing the likelihood of comets impacting our solar system's planets, including Earth.

"Comets travel much faster than asteroids, and some of them are very big," Mainzer said. "Studies like this will help us define what kind of hazard long-period comets may pose."

NASA's Jet Propulsion Laboratory in Pasadena, California, managed and operated WISE for NASA's Science Mission Directorate in Washington, D.C. The NEOWISE project is funded by the Near-Earth Object Observation Program, now part of NASA's Planetary Defense Coordination Office. The spacecraft was put into hibernation mode in 2011 after twice scanning the entire sky, thereby completing its main objectives. In September 2013, WISE was reactivated, renamed NEOWISE and assigned a new mission to assist NASA's efforts to identify potentially hazardous near-Earth objects.

Explore further: NASA's asteroid-hunting spacecraft a discovery machine

More information: James M. Bauer et al. Debiasing the NEOWISE Cryogenic Mission Comet Populations, The Astronomical Journal (2017). DOI: 10.3847/1538-3881/aa72df

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dfjohnsonphd
2.3 / 5 (3) Jul 25, 2017
Oort Cloud is highly theoretical with no solid data to support it. Some adherents claim it extends out as far as 2 light years, or almost half way to the closest star, Alpha Centuari. Seems like a stretch. Hard to imagine all that material so far away locked around our puny yellow dwarf. Long period comets likely come from the Kuiper Belt, a well documented region beyond Neptune that is loaded with icy bodies. Hale-Bopp has estimated period of 2500 years, making it the longest period of any comet to date. That doesn't mean it came from Oort Cloud since its period could have been increased by sling shot from the Sun, or Jupiter, etc. Long period comets represent the greatest threat to earth, after humans. Since they are only discovered a few months before passing through the inner solar system, what to do, what to do? However, humans are killing everything off right now, the next mass extinction from space may not be so dramatic.Won't have much to kill off after we are done.......
Nik_2213
not rated yet Jul 26, 2017
How far must the known Kuiper Belt extend before it becomes the hypothesised Oort Cloud ??
Dingbone
Jul 27, 2017
This comment has been removed by a moderator.
dfjohnsonphd
1 / 5 (1) Jul 28, 2017
Presumably there is a large gap between the Kuiper belt and the Oort cloud, with the latter starting at around 0.03 and 0.08 light years, and extending out to as far as several light years depending on predictions. Its presence was postulated by Jan Oort who saw a need to explain where long period comets came from since they would never have formed in their highly elliptical orbits around the sun, but must have formed well beyond the solar radiation which eats them away. Dingbone brings up the important aspect of larger bodies in the Kuiper Belt, which is well established and may very well harbor a planet 9, much larger than Pluto. This could easily explain the origin of long period comets, with these large bodies perturbing icy chunks to rain down into the inner solar system. As a large chunk approaches the Sun, its velocity would increase dramatically and then if it survives without impacting, could get thrown out a great distance, resulting in long period comets. Sling-shotting.
krzychu01230
1 / 5 (1) Aug 18, 2017
Why double lobe / elongated geometry of comet nucleus [and asteroids] is so common and how nebular hypothesis could explain it?

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