Massive, Long-Period Comets Are Way More Common Than We Thought


During the formation of the solar system, when the planets were molten messes and asteroid collisions (or “mudball” collisions, possibly) were commonplace, chunks of icy debris were flung away from the chaos surrounding our messy young star and relegated to a lifetime of solitude in the furthest-most reaches of the sun’s gravitational influence. This debris eventually settled and formed what is known as the Oort Cloud, a mysterious spherical shell of countless mountain-sized objects located nearly 200 billion miles away.

As the Oort Cloud is so distant, and there are no telescopes on Earth (or off-Earth) that can resolve these objects, we can only guess at how many icy lumps are out there lurking in the dark. But should a passing star cause a gravitational wobble in that region, a few of those ancient objects may be knocked off their delicate gravitational perches and they take the plunge back toward the sun, becoming what we humans call “long-period comets.” Only when we see these comets can we get a hint of the population of the Oort Cloud and the nature of long-period comets. But, as many of these deep space vagabonds have orbital periods of hundreds to millions of years, they are notoriously difficult to track.

A long period comet may appear in the sky tomorrow, but it may not return in Earth’s skies until the age of humanity is long gone and intelligent cockroaches roam the planet. It’s hard to keep track of comets with orbital periods longer than our lifespans, let alone the lifespan of our civilization.

So it may not come as a surprise that astronomers have woefully underestimated the number of long-period comets, according to a new study using observations from NASA’s Wide-field Infrared Survey Explorer, or WISE, mission. But not only that, these things are a lot bigger than we thought.

The study, which has been published in The Astrophysical Journal, found that WISE had detected three to five times more long-period comets pass the sun over an eight-month period than expected and revealed that there are seven-times more long-period comets at least 1 kilometer across.

“The number of comets speaks to the amount of material left over from the solar system’s formation,” said lead author James Bauer, of the University of Maryland, College Park, in a NASA statement. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”

WISE completed its primary mission in 2011, but has now embarked on a new mission to look out for dim asteroids and comets that stray close to Earth, called NEOWISE (NEO is for “Near-Earth objects”). During its primary mission, WISE was tasked to observe the universe in infrared wavelengths — revealing the otherwise hidden secrets of distant galaxies and the faint glow of mysterious objects traveling through the solar system. Among these objects were a surprising number of long-period comets, objects that WISE was uniquely qualified to characterize.

When comets approach the sun, their ices sublimate, dust is blasted into space and they form their trademark coma (a gaseous “atmosphere”) and tails around their nuclei. These factors obscure the main mass of the comet; astronomers cannot directly see the icy nucleus through the gas and dust — astronomers therefore have a hard time estimating the size of the comet.

To gauge the size of a comet’s nucleus, WISE precisely measures the size of the comet’s coma and subtracts those measurements from dust models to reveal the nucleus’ size (NASA/JPL-Caltech)

But studying WISE’s precision infrared measurements of the comets’ comas, the researchers were able to deduce the actual nuclei sizes by subtracting observational data from theoretical models of the behavior of dust around a comet. In all, 56 long-period comets were studied and compared with observations of 95 “Jupiter family comets” — comets that have short orbital periods around the sun and are gravitationally influenced by Jupiter. This comparison between the two families of comets revealed that long-period comets aren’t only bigger than we expected, these monsters are up to twice the size of Jupiter family comets.

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

The difference in comet sizes may not come as a surprise — Jupiter family comets have orbital periods less than 20 years and therefore spend much more time being heated by the sun. They lose mass through ice sublimation that, in turn, dislodges dust and other material, ultimately shedding mass. Long-period comets on the other hand are pristine having spend most of their lives in the deep space deep freeze, so they hold onto the material they were born with billions of years ago. Long-period comets are the epitome of primordial.

Naturally, no comet research would be complete without an Existential Reality Check™ and, as you may have guessed, this new research has a dark side.

“Comets travel much faster than asteroids, and some of them are very big,” said co-author Amy Mainzer, principal investigator of the NEOWISE mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Studies like this will help us define what kind of hazard long-period comets may pose.”