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Wayward Comet Makes a Temporary Stop Near Jupiter’s Asteroids

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Trojan Asteroid 2019 LD2. Credit: NASA, ESA, STScI, B. Bolin IPAC/Caltech)

Comet Comes in from the Cold and Finds an Off-Ramp on Its Sunward Journey

Long road trips can be tedious and boring. That’s why many road travelers break up their arduous journey by making rest stops along the way.

Astronomers found that at least one roaming comet is doing the same thing. The wayward object made a temporary stop near giant Jupiter. The icy visitor has plenty of company: It has settled near the family of captured asteroids known as Trojans that are co-orbiting the Sun alongside Jupiter.

This is the first time a comet-like object has been spotted near the Trojan asteroid population. Hubble Space Telescope observations reveal the vagabond is showing signs of transitioning from a frigid asteroid-like body to an active comet, sprouting a long tail, outgassing jets of material, and enshrouding itself in a coma of dust and gas.

The interloper came from the frigid outskirts of our solar system, a comet nesting-ground called the Kuiper Belt. This nomad was likely snatched by Jupiter’s powerful gravity after it had a brush with the giant planet.

Jupiter’s uninvited guest probably will not hang around the planet for very long. As the “bouncer” of the solar system, the monster planet’s gravitational tug will eventually boot the comet back onto its road trip toward our Sun.

Trojan Asteroid 2019 LD2 Compass. Credit: NASA, ESA, STScI, B. Bolin IPAC/Caltech)

For the first time, a wayward comet-like object has been spotted near the family of ancient asteroids.

After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called Centaurs, they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

Visible-light snapshots by NASA’s Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA’s Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

The main asteroid belt lies between Mars and Jupiter, and Trojan asteroids both lead and follow Jupiter. Scientists now know that asteroids were the original “building blocks” of the inner planets. Those that remain are airless rocks that failed to adhere to one another to become larger bodies as the solar system was forming 4.6 billion years ago. Credit: NASA, ESA, and J. Olmsted (STScI)

“Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets,” said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

Describing the Centaur’s capture as a rare event, Bolin added, “The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter.”

The team’s paper appears in the February 11, 2021 issue of The Astronomical Journal.

The research team’s computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group’s co-orbital location, leading Jupiter by about 437 million miles.

Bucket Brigade

The nomadic object was discovered in early June 2019 by the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Japanese amateur astronomer Seiichi Yoshida tipped off the Hubble team to possible comet activity. The astronomers then scanned archival data from the Zwicky Transient Facility, a wide-field survey conducted at Palomar Observatory in California, and realized that the object was clearly active in images from April 2019.

They followed up with observations from the Apache Point Observatory in New Mexico, which also hinted at the activity. The team observed the comet using Spitzer just days before the observatory’s retirement in January 2020, and identified gas and dust around the comet nucleus. These observations convinced the team to use Hubble to take a closer look. Aided by Hubble’s sharp vision, the researchers identified the tail, coma structure, the size of the dust particles, and their ejection velocity. These images helped them confirm that the features are due to relatively new comet-like activity.

Although LD2’s location is surprising, Bolin wonders whether this pit stop could be a common pull-off for some sunward-bound comets. “This could be part of the pathway from our solar system through the Jupiter Trojans to the inner solar system,” he said.

The unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system.

“The cool thing is that you’re actually catching Jupiter flinging this object around and changing its orbital behavior and bringing it into the inner system,” said team member Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. “Jupiter controls what’s going on with comets once they get into the inner system by altering their orbits.”

The icy interloper is most likely one of the latest members of the so-called “bucket brigade” of comets to get kicked out of its frigid home in the Kuiper Belt and into the giant planet region through interactions with another Kuiper Belt object. Located beyond Neptune’s orbit, the Kuiper Belt is a haven of icy, leftover debris from our planets’ construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper Belt inward into the giant planet region.

The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, which are gravitationally pulling on them – about 5 million years – than they do crossing into the inner system where we live.

“Inner-system, ‘short-period’ comets break up about once a century,” Lisse explained. “So, in order to maintain the number of local comets we see today, we think the bucket brigade has to deliver a new short-period comet about once every 100 years.”

An Early Bloomer

Seeing outgassing activity on a comet 465 million miles away from the Sun (where the intensity of sunlight is 1/25th as strong as on Earth) surprised the researchers. “We were intrigued to see that the comet had just started to become active for the first time so far away from the Sun at distances where water ice is barely starting to sublimate,” said Bolin.

Water remains frozen on a comet until it reaches about 200 million miles from the Sun, where heat from sunlight converts water ice to gas that escapes from the nucleus in the form of jets. So the activity signals that the tail might not be made of water. In fact, observations by Spitzer indicated the presence of carbon monoxide and carbon dioxide gas, which could be driving the creation of the tail and jets seen on the Jupiter-orbiting comet. These volatiles do not need much sunlight to heat their frozen form and convert them to gas.

Once the comet gets kicked out of Jupiter’s orbit and continues its journey, it may meet up with the giant planet again. “Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate,” Lisse said. “Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”

Reference: “Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy” by Bryce T. Bolin, Yanga R. Fernandez, Carey M. Lisse, Timothy R. Holt, Zhong-Yi Lin, Josiah N. Purdum, Kunal P. Deshmukh, James M. Bauer, Eric C. Bellm, Dennis Bodewits, Kevin B. Burdge, Sean J. Carey, Chris M. Copperwheat, George Helou, Anna Y. Q. Ho, Jonathan Horner, Jan van Roestel, Varun Bhalerao, Chan-Kao Chang, Christine Chen, Chen-Yen Hsu, Wing-Huen Ip, Mansi M. Kasliwal, Frank J. Masci, Chow-Choong Ngeow, Robert Quimby, Rick Burruss, Michael Coughlin, Richard Dekany, Alexandre Delacroix, Andrew Drake, Dmitry A. Duev, Matthew Graham, David Hale, Thomas Kupfer, Russ R. Laher, Ashish Mahabal, Przemyslaw J. Mróz, James D. Neill, Reed Riddle, Hector Rodriguez, Roger M. Smith, Maayane T. Soumagnac, Richard Walters, Lin Yan and Jeffry Zolkower, 11 February 2021, The Astronomical Journal.
DOI: 10.3847/1538-3881/abd94b

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, managed the Spitzer mission for NASA’s Science Mission Directorate in Washington, D.C. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spitzer’s entire science catalogue is available via the Spitzer data archive, housed at the Infrared Science Archive at IPAC. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.



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Bad Astronomy | Comet near Jupiter is going to be ejected from the solar system

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In June 2019, the automated survey ATLAS (Asteroid Terrestrial-impact Last Alert System) found a new object moving against the background stars. Initially called 2019 LD2, it was thought to be an asteroid orbiting the Sun out near Jupiter. However, an amateur astronomer noticed it appeared to be fuzzy, not point-like, which means it was more like a comet: Icy material on the surface turning into a gas as it’s warmed by the Sun.

Checking archived images, astronomers determined it had been “active” for several months at least. The name of the object was then changed to P/2019 LD2, indicating its status as a periodic comet.

Images by other observatories confirmed this, including Hubble. When they looked at the comet in April 2020 they saw it sporting quite a grand tail, extending for about 600,000 kilometers, nearly twice the distance of the Moon from the Earth! Mind you, the nucleus — the solid part of the comet — is probably only about 4 kilometers across.

Calculations show that around that time it was losing about 80 kilograms of water ice per second. It was also shedding gases like carbon monoxide (about 50 kilos/second), carbon dioxide (7 kilos/second) and diatomic carbon (two carbon atoms bound together; at a rate of 40 grams per second).

That may sound like a lot, but it turns out it just started outgassing like this… and it won’t for very long. It’s status as a periodic comet is only temporary. Extremely temporary: Follow-up measurements to determine its orbit found it’s actually in a similar orbit as Jupiter, and there’s an excellent chance that, in the distant future, the mighty gravity of the giant planet will fling the comet out of the solar system entirely.

When that happens it will become an interstellar comet like 2I/Borisov or ‘Oumuamua, interstellar objects that both recently passed through our solar system (and which, I’ll note, are not alien spaceships).

That’s fitting, since it probably began life in the outer reaches of the solar system, too.

It’s likely that P/2019 LD2 started out as what’s called a Trans-Neptunian Object, an icy body orbiting the Sun in the Kuiper Belt out past Neptune. Over time, very gentle nudges by Neptune’s gravity urged it into a smaller orbit, closer to the Sun. Eventually it got close enough that Neptune could yank on it much harder, changing its orbit substantially, putting it in an orbit between that of Jupiter and Neptune (from about 800 million to 3 billion kilometers from the Sun). Objects on orbits like that are called Centaurs.

Centaurs are interesting. Over time, the gas giants tend to change their orbits still more. Generally, after a few million years in this part of the solar system, they get too close to one of the planets. Either they get dropped down into the inner solar system (and become what we call Jupiter Family Comets) or get thrown out of the solar system entirely. Because of that we call them transitional objects*.

What will be the fate of P/2019 LD2? And where did it originally come from?

Observations over time of an object can be used to determine its orbit, which can then be projected into the past and future. The problem is we can’t measure the orbit exactly; there’s always some uncertainty in it. The farther you try to predict its position in the future (or antedict its position in the past) the fuzzier it gets, the bigger the volume of space it might occupy. That makes this sort of prognostication dicey.

To get around this, astronomers did something clever: They simulated its orbit using what’s called a Monte Carlo technique. They take the physical characteristics of the orbit (the shape, the distance from the Sun, the tilt, and so on) and then change each one very slightly, creating a slightly different orbit. They then run that into the past and future and see what it does. They do this again and again, creating a virtual cohort of objects each with marginally different paths. This way, you get a more statistical idea of what the history and future of the object was and will be.

What they found for P/2019 LD2 is that it probably only entered Jupiter’s space about 2.5 years ago! Before that it was a standard-issue Centaur, but got nudged into its current orbit very recently.

And its future? They found it likely that it will only stay in its current orbit for 8 or 9 more years. After that it will likely drop down into the inner solar system, becoming a Jupiter Family Comet. This means is that it’s only making a pit stop near Jupiter.

Even that’s temporary. It has a 50% of being ejected from the solar system in 340,000 years, which rises to 95% in 4 million years.

It’s likely that, over the age of the solar system, billions of objects like this have been ejected. And there are billions of stars like the Sun… which is why astronomers think the galaxy is loaded with rogue interstellar iceballs like P/2019 LD2, and why it’s not so surprising that we see them passing through our solar system, too.

Will some alien scientists in the distant future see LD2 passing through their own system? What would they make of it? It’s fun, and oddly reassuring, to know that pieces of our neighborhood will be scattered among the stars, going from citizens of our solar system to citizens of the galaxy.

*Which is pretty cool it worked out that way, given that they’re named after mythical half-human/half-horse creatures.

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Why some scientists think a comet, not an asteroid, caused the dinosaurs to go extinct

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One day 66 million years ago, Earth suddenly transformed from being a verdant, dinosaur-ridden world to a soot-covered apocalyptic hellscape. The extinction event wiped out 75 percent of the world’s animal and plant species at the time, including dinosaurs.

Evidence of that calamitous day can be found in the Chicxulub crater, a heavily eroded 90-mile wide impact site located on Mexico’s Yucatán Peninsula, which is widely believed to be the impact site for whatever triggered the mass extinction event. And while there is scientific consensus that something hit the Earth that fateful day, there are different theories about exactly what. 

Indeed, for decades, geologists and geophysicists have fixated on the idea that an asteroid is to blame. Now, astrophysicists at Harvard University are theorizing that an icy comet from the Oort cloud — a theoretical shell of icy debris at the edge of the solar system— flew too close to the sun, in part due to Jupiter’s strong tidal forces, and eventually broke apart and crashed into Earth. In other words, “cometary shrapnel” from a long-period comet which pinged around our solar system could have caused the impact that led to a mass extinction, rather than an astroid.

Amir Siraj, lead researcher and undergraduate in astrophysics at Harvard University, and Avi Loeb, who is the former chair of astronomy at Harvard University, landed on this theory using statistical analysis and gravitational simulations. Their findings were published in Nature’s Scientific Reports on February 15.

In the paper, the researchers put forth their new calculations that increase by a factor of 10 the likelihood of long-period comets — meaning those which have orbital periods longer than 200 years — striking Earth. They also calculate that 20 percent of long-period comets become sungrazers, meaning comets that fly very close to the Sun and are whipped back through the terrestrial planets. The timing of these calculations would be “consistent with the age of the Chicxulub impact crater,” the researchers explained, providing a “satisfactory explanation for the origin of the impactor.”

Siraj told Salon he didn’t originally seek out to find the answer to the origins of the Chicxulub impactor, but he started to probe deeper while looking into the asteroid impact rates for Earth-like exoplanets. This led him to study cometary impact rates on those systems, which led to him creating numerical simulations to calculate long-period comets in our own solar system.

“What I ended up finding most striking was that a significant fraction of Earth-crossing comet —  Earth-crossing meaning comets 1 AU [astronomical unit] of the sun — were directly preceded by remarkably close encounters with the Sun,” Siraj said. “I found that these comets were passing so close to the Sun that they were within the Roche limit, where you can get tidal disruptions, and I dug into this point further, and what I ended up finding is that these comets were being produced by and large by interactions with Jupiter, which was essentially acting like a pinball machine.”

A common theory on the origin of the Chicxulub crater suggests that the source originated from the main belt, an area between the orbit of Jupiter and Mars populated with asteroids. The researchers say their theory provides a more realistic basis that can eventually be proved.

“Our paper provides a basis for explaining the occurrence of this event,” Loeb said in a media statement. “We are suggesting that, in fact, if you break up an object as it comes close to the sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs.”

Previously, evidence from the Chicxulub crater suggested the impact object was made of carbonaceous chondrite.

“Data in the past decade or so, and even before that, show that this composition is quite rare amongst asteroids,” Siraj said. “And we have a fairly good sense of the distribution of asteroid compositions simply as a result of having meteorites, which primarily come from asteroids.”

Yet comets, he noted, are not as well understood. Yet we know from one successful sample-return comet mission that comets do contain carbonaceous chondrite, Siraj said.

Siraj and Loeb aren’t the only ones positing the theory that a comet killed the dinosaurs. Two geoscientists advanced the theory in 2013, partially because the levels of iridium and osmium around the impact site were lower than should appear in an asteroid and more apt for a comet impact. Siraj said studying iridium will be an “important active area of research” to better understand what impactor that caused the Chicxulub crater.

Let’s say scientists eventually prove that a comet led to the extinction of dinosaurs and completely transformed Earth. Will that change how we perceive asteroids (or comets) as a threat to life on Earth?

“Asteroids are still the major short-term risk,” Siraj said. He noted that the good news about their theory is that there’s a low probability that shrapnel from a long-period comet will hit Earth in our lifetime. “We don’t have to worry about cometary impact being extremely common on very short timescales . . . however, it does change the way we think about longer term, like a million years and more — I imagine our civilization will have to have to reckon with these questions of deflecting small asteroids, which is very different from deflecting big asteroids, which is also very different from deflecting comets.”

Humanity’s need to make “contingency plans” to address planet-wide devastation events highlights the importance of future research around the dynamics of small bodies in our solar system.

“Science is really the tool that we can use to address these looming existential threats and be prepared,” Siraj said.

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Impact that killed dinosaurs was piece of comet, not asteroid: study

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About 66 million years ago, a space rock more than 6 miles wide collided with Earth, striking land that is now part of Mexico.

The impact sparked wildfires that stretched for hundreds of miles, triggered a mile-high tsunami, and released billions of tons of sulfur into the atmosphere. That gaseous haze blocked the sun, cooling the Earth and dooming the dinosaurs, along with 75% of all life on the planet.

But the origins of that dinosaur-killing rock, named Chicxulub, have remained a mystery. 

Most theories suggest Chicxulub was a massive asteroid; hundreds of thousands of these rocks sit in a donut-shaped ring between Mars and Jupiter. But in a study published Monday, two Harvard astrophysicists suggested an alternate idea: that Chicxulub wasn’t an asteroid at all, but a piece of shrapnel from an icy comet that had been pushed too close to the sun by Jupiter’s gravity.

Asteroids and comets are both classified as space rocks by NASA, but they differ in key ways: Comets form from ice and dust outside our solar system and are generally small and fast-moving, whereas rocky asteroids are larger, slower, and form closer to the sun.

“We are suggesting that, in fact, if you break up an object as it comes close to the sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs,” Avi Loeb, an astrophysicist and cosmologist at Harvard University and co-author of the new study, said in a press release. 

The solar system acts like a ‘pinball machine’ for comets

An artist’s depiction of an asteroid approaching Earth.

Vadim Sadovski/Shutterstock



Most asteroids come from the asteroid belt between the solar system’s inner and outer planets. But NASA scientists who keep tabs on space objects that pass near Earth have yet to figure out where Chicxulub came from. 

In the new study, published in the journal Scientific Reports, Loeb and his co-author, Amir Siraj, suggest Chicxulub didn’t come from the asteroid belt. Rather, they say it more likely originated outside our solar system, in an area called the Oort cloud. 

Think of the Oort cloud as ring made of 1 trillion pieces of icy debris, which sits beyond the farthest reaches of the solar system, surrounding it. It’s located at least 2,000 times farther away from the sun than Earth is. Comets that originate in the Oort cloud are known as long-period comets because they take so long to complete one orbit around the sun.

But these comets can sometimes get pulled off-course by the gravity of massive planets like Jupiter. Such a tweak to a comet’s orbit could send it hurtling on a path much closer to the sun. 

“The solar system acts as a kind of pinball machine,” Siraj said in the release.

Comets that get near the sun are called “sungrazers.” The new study calculated that about 20% of Oort cloud comets are sungrazers. As they approach our star, its gravity starts to pull them apart. Fragments of comet slough off and may careen toward nearby planets. 

This, the study authors say, is “a satisfactory explanation for the origin of the impactor” that killed the dinosaurs.

The asteroid-versus-comet argument isn’t settled



A painting depicting an asteroid slamming into tropical, shallow seas of the Yucatan Peninsula in what is today southeast Mexico. The aftermath is believed to have caused the extinction of the dinosaurs.

Donald Davis/NASA



Siraj and Loeb aren’t the only scientists who think a comet, not an asteroid, doomed the dinosaurs. A group of researchers from Dartmouth College similarly suggested in 2013 that a high-speed comet could have created the Chicxulub crater. 

Chicxulub hit Earth at a speed of 12 miles per second (43,200 mph), which is about 30 times faster than the speed of a supersonic jet. The resulting 100-mile-wide crater extended 12 miles into the depths of the Gulf of Mexico. Some scientists have estimated the asteroid’s power was equivalent to 10 billion of the atomic bombs used in World War II.

But not all researchers are convinced a comet caused that destruction.

Natalia Artemieva, a senior scientist at the Planetary Science Institute in Arizona, told The New York Times that comet fragments from a sungrazer would have been too small to create the Chicxulub crater. And Bill Bottke, a planetary scientist at the Southwest Research Institute in Colorado, suggested that the study overestimates the frequency of sungrazers — and, consequently, the amount of fragments those comets produce.

Existing evidence favors the idea that Chicxulub was an asteroid, “but it’s not conclusive,” Bottke told the Times. “There’s still wiggle room if somebody really wants it to be a comet. I just think making that case is really hard.”

Siraj and Loeb, however, said their theory is supported by a type of material found deep inside the Chicxulub crater and other craters in South Africa and Kazakhstan. That substance, carbonaceous chondrite, may have come from comets. Whereas just 10% of asteroids from the asteroid belt are composed of carbonaceous chondrites, the material “could potentially be widespread in comets,” the study authors wrote.

The only samples ever collected from a comet in space were brought back in 2006. They revealed that object, called Wild 2, was composed of carbonaceous chondrite.



Artwork depicting the icy cores of baby comets beyond Neptune at the edge of our solar system.

ESO/M. Kornmesser


Finding the correct answer in the Chicxulub debate is useful because it could help researchers figure out the likelihood of a similar impact event in the future. Only two to three comets from the Oort Cloud have hit Earth during the last 500 million years, according to one study. By contrast, according to the Planetary Society, a Chicxulub-sized asteroid impacts Earth every 100 million years or so. 

Siraj and Loeb modeled how many long-period comets get close enough to the sun to shed large fragments in the direction of Earth. Their numbers suggest 10 times more Chicxulub-sized objects hit Earth over its history than scientists previously thought.

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The space rock that doomed the dinosaurs was shrapnel from a comet that flew too close to the sun, a Harvard study suggests

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An artist’s depiction of the moment the Chicxulub asteroid struck present-day Mexico 66 million years ago. Chase Stone

About 66 million years ago, a space rock more than 6 miles wide collided with Earth, striking land that is now part of Mexico.

The impact sparked wildfires that stretched for hundreds of miles, triggered a mile-high tsunami, and released billions of tons of sulfur into the atmosphere. That gaseous haze blocked the sun, cooling the Earth and dooming the dinosaurs, along with 75% of all life on the planet.

But the origins of that dinosaur-killing rock, named Chicxulub, have remained a mystery.

Most theories suggest Chicxulub was a massive asteroid; hundreds of thousands of these rocks sit in a donut-shaped ring between Mars and Jupiter. But in a study published Monday, two Harvard astrophysicists suggested an alternate idea: that Chicxulub wasn’t an asteroid at all, but a piece of shrapnel from an icy comet that had been pushed too close to the sun by Jupiter’s gravity.

Asteroids and comets are both classified as space rocks by NASA, but they differ in key ways: Comets form from ice and dust outside our solar system and are generally small and fast-moving, whereas rocky asteroids are larger, slower, and form closer to the sun.

“We are suggesting that, in fact, if you break up an object as it comes close to the sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs,” Avi Loeb, an astrophysicist and cosmologist at Harvard University and co-author of the new study, said in a press release.

The solar system acts like a ‘pinball machine’ for comets

An artist’s depiction of an asteroid approaching Earth. Vadim Sadovski/Shutterstock

Most asteroids come from the asteroid belt between the solar system’s inner and outer planets. But NASA scientists who keep tabs on space objects that pass near Earth have yet to figure out where Chicxulub came from.

In the new study, published in the journal Scientific Reports, Loeb and his co-author, Amir Siraj, suggest Chicxulub didn’t come from the asteroid belt. Rather, they say it more likely originated outside our solar system, in an area called the Oort cloud.

Think of the Oort cloud as ring made of 1 trillion pieces of icy debris, which sits beyond the farthest reaches of the solar system, surrounding it. It’s located at least 2,000 times farther away from the sun than Earth is. Comets that originate in the Oort cloud are known as long-period comets because they take so long to complete one orbit around the sun.

But these comets can sometimes get pulled off-course by the gravity of massive planets like Jupiter. Such a tweak to a comet’s orbit could send it hurtling on a path much closer to the sun.

“The solar system acts as a kind of pinball machine,” Siraj said in the release.

Comets that get near the sun are called “sungrazers.” The new study calculated that about 20% of Oort cloud comets are sungrazers. As they approach our star, its gravity starts to pull them apart. Fragments of comet slough off and may careen toward nearby planets.

This, the study authors say, is “a satisfactory explanation for the origin of the impactor” that killed the dinosaurs.

The asteroid-versus-comet argument isn’t settled

A painting depicting an asteroid slamming into tropical, shallow seas of the Yucatan Peninsula in what is today southeast Mexico. The aftermath is believed to have caused the extinction of the dinosaurs. Donald Davis/NASA

Siraj and Loeb aren’t the only scientists who think a comet, not an asteroid, doomed the dinosaurs. A group of researchers from Dartmouth College similarly suggested in 2013 that a high-speed comet could have created the Chicxulub crater.

Chicxulub hit Earth at a speed of 12 miles per second (43,200 mph), which is about 30 times faster than the speed of a supersonic jet. The resulting 100-mile-wide crater extended 12 miles into the depths of the Gulf of Mexico. Some scientists have estimated the asteroid’s power was equivalent to 10 billion of the atomic bombs used in World War II.

But not all researchers are convinced a comet caused that destruction.

Natalia Artemieva, a senior scientist at the Planetary Science Institute in Arizona, told The New York Times that comet fragments from a sungrazer would have been too small to create the Chicxulub crater. And Bill Bottke, a planetary scientist at the Southwest Research Institute in Colorado, suggested that the study overestimates the frequency of sungrazers – and, consequently, the amount of fragments those comets produce.

Existing evidence favors the idea that Chicxulub was an asteroid, “but it’s not conclusive,” Bottke told the Times. “There’s still wiggle room if somebody really wants it to be a comet. I just think making that case is really hard.”

Siraj and Loeb, however, said their theory is supported by a type of material found deep inside the Chicxulub crater and other craters in South Africa and Kazakhstan. That substance, carbonaceous chondrite, may have come from comets. Whereas just 10% of asteroids from the asteroid belt are composed of carbonaceous chondrites, the material “could potentially be widespread in comets,” the study authors wrote.

The only samples ever collected from a comet in space were brought back in 2006. They revealed that object, called Wild 2, was composed of carbonaceous chondrite.

Artwork depicting the icy cores of baby comets beyond Neptune at the edge of our solar system. ESO/M. Kornmesser

Finding the correct answer in the Chicxulub debate is useful because it could help researchers figure out the likelihood of a similar impact event in the future. Only two to three comets from the Oort Cloud have hit Earth during the last 500 million years, according to one study. By contrast, according to the Planetary Society, a Chicxulub-sized asteroid impacts Earth every 100 million years or so.

Siraj and Loeb modeled how many long-period comets get close enough to the sun to shed large fragments in the direction of Earth. Their numbers suggest 10 times more Chicxulub-sized objects hit Earth over its history than scientists previously thought.

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Dinosaur-killing comet may have come from the edge of the solar system

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Artist’s illustration of a comet bound for Earth.


Harvard-Smithsonian Center for Astrophysics

Harvard’s most controversial astronomer has a new theory about the space rock that took out the dinosaurs. He’s saying there’s reason to believe it came from farther afield than previously assumed.

Avi Loeb has been making waves for a few years now by arguing that first ever interstellar object Oumuamua could be a wayward piece of alien technology from far beyond our solar system. But his latest paper has nothing to do with that.

Loeb and Harvard University astrophysics undergraduate student Amir Siraj suggest in a new study published Monday in Scientific Reports that the Chicxulub Impactor, which ended the rule of the thunder lizards, originated from the edge of our own solar system.

A popular theory about the demise of the dinos says the impactor likely originated from the asteroid belt between Mars and Jupiter, but Loeb and Siraj use statistical analysis and gravitational simulations to calculate that more Earth impactors actually originated from the far-off Oort cloud where most long-period comets hail from.

The pair’s calculations suggest some such comets can get knocked off track on their journey toward the inner solar system, with potentially catastrophic consequences.

“The solar system acts as a kind of pinball machine,” Siraj explained in a statement. “Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the sun.”

So-called sungrazer comets can then be torn apart by the pull of the sun’s gravity.

“And crucially, on the journey back to the Oort cloud, there’s an enhanced probability that one of these fragments hit the Earth,” Siraj said. 


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The research finds that the odds of such an impact are significantly higher than previously thought and that the new rate of impact lines up with the age of the Chicxulub impact crater in the Gulf of Mexico. A comet fragment from the Oort cloud also matches up with the unusual makeup of the impactor better than an asteroid from closer to home.

Even more important than solving the mystery of what killed off the dinosaurs, Loeb says a deeper understanding of natural traffic from deep space could also be important if a potential impactor should threaten our planet in the future.

“It must have been an amazing sight,” he said, “but we don’t want to see that again.”  

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Science

Need some space? You can now buy 7 billion-year-old stardust and pieces of the moon and Mars

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If you’re looking for an out-of-this-world gift this Valentine’s Day, an auction house is offering up rare meteorite chunks from the moon, Mars and beyond — for as little as $250.

In an online sale beginning Tuesday, February 9, Christie’s auction house is auctioning off 72 meteorites — solid pieces of debris from celestial objects like comets and asteroids that arrive on Earth as shooting stars, somehow managing to survive their journey through our atmosphere to land on the surface.  

“The weight of every known meteorite is less than the world’s annual output of gold, and this sale offers spectacular examples for every collector, available at estimates ranging from hundreds to hundreds of thousands of dollars,” the auction house wrote on its website. 

Included in the collection is a meteorite containing 7 billion-year-old stardust, space gems encased in iron and the fourth-largest slice of the moon. A large chunk of Martian rock, worth an estimated $30,000 to $50,000, holds bubbles of the planet’s atmosphere trapped inside.

“The Most Beautiful Extraterrestrial Substance Known — End piece of the Fukang Meteorite.” Estimate: $3,500 – 4,500.

Christie’s


According to Christie’s, there are a dozen samples from the moon and Mars, and another dozen previously housed by famous museums around the world. 

“Everyone has an image in mind of how a meteorite ‘should look’ – an extraterrestrial body frictionally heated while punching through Earth’s atmosphere,” James Hyslop, head of science and natural history for Christie’s, said in a statement. “Rarely do the objects survive this fiery descent look like that shared ideal seen in this meteorite. It is a wonder to behold and an honor to have been entrusted with its sale.”

One object in the collection never hit the ground — a young boy in Morocco found the meteorite in the branches of a tree a day after a meteor shower — it’s worth an estimated $15,000 to $25,000. Yet another hailed from the U.S.’ largest meteorite shower in Odessa, Texas, expecting to fetch $40,000 to $60,000. 

“If there was ever a time to be awed by the infiniteness of the night sky, we’re living in it, but if you want to inspire and see eyes widen — touch a meteorite,” said curator Darryl Pitt.

“Gibeon Meteorite — Natural exotic sculpture from outer space.” Estimate: $15,000 – $25,000.

Christie’s


The auction house said that one of the highlights of the sale is a 16-pound “highly aesthetic oriented stone meteorite,” estimated to sell for $50,000 to $80,000.

“Unlike 99% of all other meteorites, this meteorite did not tumble or invert as it plunged to Earth but maintained a stable orientation throughout its descent,” the auction house said. “The surface that faced Earth showcases elongated flight marks that radiate outwards in this compelling, extraterrestrial aerodynamic form.” 

The meteorites have been found all over the world, from the Sahara Desert to Chile to Russia. 

The “Deep Impact: Martian, Lunar and Other Rare Meteorites” auction runs until February 23, and interested buyers located in New York can see them in person, by appointment. 

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