Comet K2, one of the most distant “active” comets ever discovered, swings past our planet this week as it makes its first journey from the far ends of the solar system toward the sun. Astronomers watch the mysterious ball of ice in awe, making new and often surprising discoveries at every step.
“It’s like being able to touch something from the beginning of the solar system,” David Jewitt, an astronomer at the University of California, Los Angeles, told Space.com. “It’s probably the most primitive thing in the inner solar system right now.”
Jewitt has published several articles on Comet K2 and was one of the first astronomers to study the object after it was discovered by the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) in Hawaii in May 2017 comet was about 17 times farther from the sun than Earth. Scientists call The average distance of the earth from the sun an astronomical unit; an AU measures about 93 million miles (150 million kilometers).
Even at that distance, between the orbits of Saturn and Uranus, the comet emitted what scientists call a coma: a fuzzy halo of gas that stretched 130,000 kilometers into space. This coma forms when frozen material on the comet sublimes, or goes directly from a solid to a gas.
“As [comets] As you get closer to the Sun, they get warmer and warmer,” Jewitt said. “And then the water ice can sublimate and create a whooshing atmosphere of gas that creates the coma and the comet’s tail.”
Related: Why is Comet K2 so hard to see given its enormous size?
The halo that shouldn’t be
It was this halo that surprised astronomers. So deep in the solar system that SunThe rays from are too weak to sublimate the comet’s frozen material. Typically, this process only begins about Jupiter‘s orbit, at five AU.
Astronomers’ surprise only increased when they found this halo in images taken before the official discovery, when the comet was 23 AU from the Sun, beyond the orbit of the Solar System’s outermost planet. Neptune.
Jewitt then determined that K2 must have been active for several years when this first image was taken. Backward modeling showed that the comet must then have been ejecting gas about 35 AU from the Sun, deep in the interior Kuiper beltthe disc of debris, space rocks and comets beyond Neptune’s orbit.
“At 35 AU from the Sun, the temperature is probably about 40 degrees above absolute zero,” Jewitt said. (Absolute zero is minus 460 degrees Fahrenheit or minus 273 degrees Celsius. At this temperature, the natural movement of atoms stops). “So we know that water is as solid as rock there. It cannot account for the activity we have seen at these great distances.”
A source of surprises
Since then, Comet K2 has been a source of fascinating discoveries, surprising scientists with unexpected behavior. But Jewitt thinks what astronomers see K2 doing is not unique at all. In fact, the comet’s behavior is probably typical of comets making their first journey towards the Sun – we just haven’t observed it before.
“What’s special about this comet is that it was discovered early,” Jewitt said. “We were able to track how the comet changes with distance from the Sun over a much wider range than ever before.”
Comet K2 comes even further away than the Kuiper Belt, Jewitt said. The original home of the comet was most likely the Oort cloud, the collection of comets and planetary fragments that extends from 2,000 to 200,000 AU from the Sun. There, surrounded by billions of other frozen snowballs and space rocks, K2 spent billions of years asleep until it received an unexpected gravitational jolt, likely from a star whizzing past the outer edges of the solar system. This kick propelled K2 on the journey that we can now observe in real time.
Untouched or not
The fact that the comet is on its first journey through the solar system is what makes it so scientifically valuable. Since K2 has had so little exposure to heat in its past, K2 should contain trapped chemicals that have long since faded from more typical comets.
“Temperatures in the Oort cloud are really only a few degrees above absolute zero,” Jewitt said. “Any material that was introduced into the comet’s nucleus during its formation has since been frozen solid. So the idea is that we study a small piece of the protoplanetary disk from the beginning of the solar system that has remained completely unchanged.”
Comets are often viewed as time capsules carrying messages from the earliest days of mankind solar system Formation. However, what astronomers are observing on K2 suggests that its chemistry may have changed much more than previously thought as the ice spheres began their journey toward the sun.
“If the activity really does start as far from the Sun as we’re seeing with this comet, then what we’re seeing with other comets closer to the Sun isn’t necessarily as pristine,” Jewitt said. “Comet K2 helps us understand how pristine or pristine are the comets we see in the inner solar system.”
Starting from the earliest observations, Jewitt wanted to explain what constitutes K2’s mesmerizing coma. Because it couldn’t be water ice sublimating from the surface, he and his colleagues knew that there must be other chemical compounds that were gasified at such low temperatures.
“The first thing we singled out was carbon monoxide,” Jewitt said. “Carbon monoxide has been detected in other closer comets, albeit in much smaller amounts than water. Carbon monoxide is very volatile and difficult to freeze into a solid.”
Jewitt and his colleagues theorized that the carbon monoxide ice near its surface turned into a gas that rushed like a constant wind into the vacuum of interplanetary space as the comet warmed slightly while descending on the planetary region of the solar system. This wind kicked up dust particles from the comet’s surface, creating the round, ball-like coma.
But it took several failed attempts to measure the comet’s chemical composition to confirm the theory, Jewitt said. Last year, the team finally succeeded when the comet entered Jupiter’s orbit and scientists eventually discovered carbon monoxide at the comet’s heart.
As astronomers watched K2 on its way to the inner solar system, they expected a brightening. They also waited for the coma to develop the characteristic tail-like shape typical of passing comets Earth. But nothing changed at K2 for years, raising both questions and theories.
“These comet tails are caused by the pressure of sunlight pushing the dust particles [that come out of the comet] away from the sun,” Jewitt said. “This comet had none until recently.”
For Jewitt, this means the dust blown out of the comet by ice turning into gas must be larger than scientists anticipated. In fact, much larger: About a million times more massive.
“We were able to estimate the size [of those particles] be about a 10th of a millimeter (about the width of a human hair) or larger,” Jewitt said. “But the expectation was because of the initial sublimation [of the gasses] is very weak, they couldn’t possibly eject large particles. Like on a windy day, when you see dust kicking up on the road, you don’t see cars being blown down the road because they’re so heavy.”
When K2 makes its closest approach to Earth on Wednesday (July 13), it will be an overwhelming sky-gazing target. Far beyond orbit Marsthe comet will be barely visible even with binoculars or backyard telescopes.
But professional astronomers like Jewitt will keep K2 in the crosshairs of their powerful machines as the comet moves further towards its December perihelion, when it is closest to the sun. They will try to extract as much science as possible, watch if it undergoes the expected transformation, and then follow it as it goes back into the Oort Cloud.
Jewitt thinks the James Webb Space Telescope is that first pictures revealed to the public on Tuesday (July 12) may shed more light on the mysterious comet K2. The telescope’s powerful spectrometers could reveal the comet’s chemical composition in much more detail, including that of its strangely behaving dust.
“If we can get the infrared spectrum with Webb, we can figure out exactly what’s in that coma. What kind of dust do we see there?” said Jewitt.