In the fall of 2014, a “Manx” comet hurtled past Earth on an 860-year journey around the solar system. Its trajectory told astronomers that the object came from the far-distant Oort cloud, a vast shell of icy objects that orbit the Sun up to thousands of astronomical units away (1 astronomical unit is the distance between the Earth and the Sun). But its severely diminished tail—hence its nickname after the Manx breed of tailless cat—told astronomers it differed from the usual ice-rich visitors from those parts, which typically sport long, bright tails.
In a recent paper in Science Advances, the comet’s discoverers take a close look at the object, which revealed that it’s made up mainly of minerals found on Earth and contains little ice—the source of a comet’s tail. The findings indicate that scientists have confirmed for the first time the existence of a rocky, asteroid-like body from the Oort cloud. Moreover, the passing chunk of rock likely constitutes a long-lost and largely pristine sample of what made up the inner solar system as it formed billions of years ago.
If astronomers can find more rocky Oort cloud visitors, they may be able to calculate the ratio of icy to rocky denizens of that vast cloud—a missing but essential clue to how our solar system took shape long ago.
How did the newfound comet, labeled C/2014 S3, get tossed into the Oort cloud when it originated in our very own neighborhood, possibly from the very material that built Earth? According to Karen Meech, an astronomer at the University of Hawai’i at Mānoa in Honolulu and lead author on the 29 April paper, the object started off as a chunk of rock drifting around the inner solar system while Earth and the other interior planets were forming. Then, gravitational forces, perhaps from migrations of giant gas planets such as Jupiter, might have flung it from its place of birth.
“It got kicked out and stored in the Oort cloud over the 4.5 billion years of our solar system,” explained Meech. “And now, we’re seeing it thrown inwards, so we’re getting a look at something that was kind of in a deep freeze, preserved from that time.”
She and her colleagues first spotted C/2014 S3 in September 2014 using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii. After examining the object’s “colors”—the spectrum of wavelengths it emits—the researchers realized they weren’t looking at any old comet. Usually, comets emit more toward the red end of the spectrum than the blue, Meech said. But this comet didn’t look red, so the researchers rushed to gain access to Chile’s Very Large Telescope in the Atacama Desert to look more closely at the spectrum of light the comet reflected. They were able to obtain observations of the comet within a month—just 2 weeks later, and the comet would have been too far from the Sun to be visible, Meech said.
The spectral data showed that this comet absorbed infrared wavelengths, which is characteristic of asteroids composed of minerals like olivine and pyroxene. However, the comet still has ice, whereas our inner solar system asteroids generally don’t—a further indication that “we’re looking at stuff that could be relatively fresh because it’s been kept in cold storage at the edge of the solar system,” explained Meech.
Testing Solar System Formation Models
Although a number of models of how the solar system formed can account for its structure today, those models clash on the relative numbers of icy versus rocky bodies the Oort cloud should contain.
One model, called Grand Tack, suggests that Jupiter or the other gas giant planets migrated inward in the early days of the solar system, flung debris aside with gravitational force, and then migrated back outward to their current positions, Meech said. Grand Tack calls for a ratio of icy bodies to rocky ones in the Oort cloud of 500 or 1000 to 1.
Grand Tack and other similar models require large chunks in the protoplanetary disk to smack into each other over and over to build the inner planets. In other models, rocky planets formed via “pebble accretion,” whereby tinier objects caked onto each other over and over.
Pebble accretion not only helps to explain why Mars is only 10% the mass of Earth but also suggests that few materials from the inner solar system would be left to get flung to the Oort cloud, explained Andrew Shannon, an astronomer at the University of Cambridge in the United Kingdom who was not involved in the research. Such pebble accretion models peg the ratio of icy bodies to rocky ones at around 1000 to 1.
If it turns out that rocky material is common in the Oort cloud, “then this will rule out pebble accretion models as they are now described,” Meech said. “But if the material is very rare, it might rule out Grand Tack.”
For now, observers need to find at least 50 more inner–solar system rocky objects in the Oort cloud before they can assess which models are more likely. Telescopes spot about 15 candidates per year, and six have already been found, although their data have yet to be processed. Researchers await telescope time to observe another six candidates.
“Of course, discovering one object—showing [rocky comets] exist and it’s possible to find them—is the first step to getting the telescope time you need to go out and find a statistically significant population,” Shannon said.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2016), Comet with stunted tail hints at how solar system formed, Eos, 97, doi:10.1029/2016EO052535. Published on 18 May 2016.