Surprisingly simple explanation for the alien comet 'Oumuamua's weird orbit

In 2017, a mysterious comet dubbed 'Oumuamua fired the imaginations of scientists and the public alike. It was the first known visitor from outside our solar system, it had no bright coma or dust tail, like most comets, and a peculiar shape -- something between a cigar and a pancake -- and its small size more befitted an asteroid than a comet.

But the fact that it was accelerating away from the sun in a way that astronomers could not explain perplexed scientists, leading some to suggest that it was an alien spaceship.

Now, a University of California, Berkeley, astrochemist and a Cornell University astronomer argue that the comet's mysterious deviations from a hyperbolic path around the sun can be explained by a simple physical mechanism likely common among many icy comets: outgassing of hydrogen as the comet warmed up in the sunlight.

What made 'Oumuamua different from every other well-studied comet in our solar system was its size: It was so small that its gravitational deflection around the sun was slightly altered by the tiny push created when hydrogen gas spurted out of the ice.

Most comets are essentially dirty snowballs that periodically approach the sun from the outer reaches of our solar system. When warmed by sunlight, a comet ejects water and other molecules, producing a bright halo or coma around it and often tails of gas and dust. The ejected gases act like the thrusters on a spacecraft to give the comet a tiny kick that alters its trajectory slightly from the elliptical orbits typical of other solar system objects, such as asteroids and planets.

When discovered, 'Oumuamua had no coma or tail and was too small and too far from the sun to capture enough energy to eject much water, which led astronomers to speculate wildly about its composition and what was pushing it outward. Was it a hydrogen iceberg outgassing H2? A large, fluffy snowflake pushed by light pressure from the sun? A light sail created by an alien civilization? A spaceship under its own power?

Jennifer Bergner, a UC Berkeley assistant professor of chemistry who studies the chemical reactions that occur on icy rocks in the cold vacuum of space, thought there might be a simpler explanation. She broached the subject with a colleague, Darryl Seligman, now an National Science Foundation postdoctoral fellow at Cornell University, and they decided to work together to test it.

"A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result. Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?" Bergner said. "Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?"

Surprisingly, she found that experimental research published in the 1970s, '80s and '90s demonstrated that when ice is hit by high-energy particles akin to cosmic rays, molecular hydrogen (H2) is abundantly produced and trapped within the ice. In fact, cosmic rays can penetrate tens of meters into ice, converting a quarter or more of the water to hydrogen gas.

"For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn't necessarily expect that to be a detectable effect," she said. "But because 'Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration."

The comet, which was slightly reddish, is thought to have been roughly 115 by 111 by 19 meters in size. While the relative dimensions were fairly certain, however, astronomers couldn't be sure of the actual size because it was too small and distant for telescopes to resolve. The size had to be estimated from the comet's brightness and how the brightness changed as the comet tumbled. To date, all the comets observed in our solar system -- the short-period comets originating in the Kuiper belt and the long-period comets from the more distant Oort cloud have ranged from around 1 kilometer to hundreds of kilometers across.

"What's beautiful about Jenny's idea is that it's exactly what should happen to interstellar comets," Seligman said. "We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it's just the most generic explanation."

Bergner and Seligman will publish their conclusions this week in the journal Nature. Both were postdoctoral fellows at the University of Chicago when they began collaborating on the paper.

Messenger from afar

Comets are icy rocks left over from the formation of the solar system 4.5 billion years ago, so they can tell astronomers about the conditions that existed when our solar system formed. Interstellar comets can also give hints to the conditions around other stars surrounded by planet-forming disks.

"Comets preserve a snapshot of what the solar system looked like when it was in the stage of evolution that protoplanetary disks are now," Bergner said. "Studying them is a way to look back at what our solar system used to look like in the early formation stage."

Faraway planetary systems also seem to have comets, and many are likely to be ejected because of gravitational interactions with other objects in the system, which astronomers know happened over the history of our solar system. Some of these rogue comets should occasionally enter our solar system, providing an opportunity to learn about planet formation in other systems.

"The comets and asteroids in the solar system have arguably taught us more about planet formation than what we've learned from the actual planets in the solar system," Seligman said. "I think that the interstellar comets could arguably tell us more about extrasolar planets than the extrasolar planets we are trying to get measurements of today."

In the past, astronomers published numerous papers about what we can learn from the failure to observe any interstellar comets in our solar system.

Then, 'Oumuamua came along.

On Oct. 19, 2017, on the island of Maui, astronomers using the Pan-STARRS1 telescope, which is operated by the Institute for Astronomy at the University of Hawaii in Manoa, first noticed what they thought was either a comet or an asteroid. Once they realized that its tilted orbit and high speed -- 87 kilometers per second -- implied that it came from outside our solar system, they gave it the name 1I/'Oumuamua (oh MOO-uh MOO-uh), which is Hawai'ian for "a messenger from afar arriving first." It was the first interstellar object aside from dust grains ever seen in our solar system. A second, 2I/Borisov, was discovered in 2019, though it looked and behaved more like a typical comet.

As more and more telescopes focused on 'Oumuamua, the astronomers were able to chart its orbit and determine that it had already looped around the sun and was headed out of the solar system.

Because 'Oumuamua's brightness changed periodically by a factor of 12 and varied asymmetrically, it was assumed to be highly elongated and tumbling end over end. Astronomers also noticed a slight acceleration away from the sun larger than seen for asteroids and more characteristic of comets. When comets approach the sun, the water and gases ejected from the surface create a glowing, gaseous coma and release dust in the process. Typically, dust left in the comet's wake becomes visible as one tail, while vapor and dust pushed by light pressure from solar rays produces a second tail pointing away from the sun, plus a little inertial push outward. Other compounds, such as entrapped organic materials and carbon monoxide, also can be released.

Why was it accelerating?

But astronomers could detect no coma, outgassed molecules or dust around 'Oumuamua. In addition, calculations showed that the solar energy hitting the comet would be insufficient to sublimate water or organic compounds from its surface to give it the observed non-gravitational kick. Only hypervolatile gases such as H2, N2 or carbon monoxide (CO) could provide enough acceleration to match observations, given the incoming solar energy.

"We had never seen a comet in the solar system that didn't have a dust coma. So, the non-gravitational acceleration really was weird," Seligman said.

This led to much speculation about what volatile molecules could be in the comet to cause the acceleration. Seligman himself published a paper arguing that if the comet was composed of solid hydrogen -- a hydrogen iceberg -- it would outgas enough hydrogen in the heat of the sun to explain the strange acceleration. Under the right conditions, a comet composed of solid nitrogen or solid carbon monoxide would also outgas with enough force to affect the comet's orbit.

But astronomers had to stretch to explain what conditions could lead to the formation of solid bodies of hydrogen or nitrogen, which have never been observed before. And how could a solid H2 body survive for perhaps 100 million years in interstellar space?

Bergner thought that outgassing of hydrogen entrapped in ice might be sufficient to accelerate 'Oumuamua. As both an experimentalist and a theoretician, she studies the interaction of very cold ice -- chilled to 5 or 10 degrees Kelvin, the temperature of the interstellar medium (ISM) -- with the kinds of energetic particles and radiation found in the ISM.

In searching through past publications, she found many experiments demonstrating that high-energy electrons, protons and heavier atoms could convert water ice into molecular hydrogen, and that the fluffy, snowball structure of a comet could entrap the gas in bubbles within the ice. Experiments showed that when warmed, as by the heat of the sun, the ice anneals -- changes from an amorphous to a crystal structure -- and forces the bubbles out, releasing the hydrogen gas. Ice at the surface of a comet, Bergner and Seligman calculated, could emit enough gas, either in a collimated beam or fan-shaped spray, to affect the orbit of a small comet like 'Oumuamua.

"The main takeaway is that 'Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing," Bergner said. "The models we ran are consistent with what we see in the solar system from comets and asteroids. So, you could essentially start with something that looks like a comet and have this scenario work."

The idea also explains the lack of a dust coma.

"Even if there was dust in the ice matrix, you're not sublimating the ice, you're just rearranging the ice and then letting H2 get released. So, the dust isn't even going to come out," Seligman said.

'Dark' comets

Seligman said that their conclusion about the source of 'Oumuamua's acceleration should close the book on the comet. Since 2017, he, Bergner and their colleagues have identified six other small comets with no observable coma, but with small non-gravitational accelerations, suggesting that such "dark" comets are common. While H2 is not likely responsible for the accelerations of dark comets, Bergner noted, together with 'Oumuamua they reveal that there is much to be learned about the nature of small bodies in the solar system.

One of these dark comets, 1998 KY26, is the next target for Japan's Hayabusa2 mission, which recently collected samples from the asteroid Ryugu. The 1998 KY26 was thought to be an asteroid until it was identified as a dark comet in December.

"Jenny's definitely right about the entrapped hydrogen. Nobody had thought of that before," he said. "Between discovering other dark comets in the solar system and Jenny's awesome idea, I think it's got to be correct. Water is the most abundant component of comets in the solar system and likely in extrasolar systems, as well. And if you put a water rich comet in the Oort cloud or eject it into the interstellar medium, you should get amorphous ice with pockets of H2."

Because H2 should form in any ice-rich body exposed to energetic radiation, the researchers suspect that the same mechanism would be at work in sun-approaching comets from the Oort cloud at the outer reaches of the solar system, where comets are irradiated by cosmic rays, much like an interstellar comet would be. Future observations of hydrogen outgassing from long-period comets could be used to test the scenario of H2 formation and entrapment.

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Dust plumes observed being 'pushed' into interstellar space by intense starlight

The results, made using infrared images of the binary star system WR140 taken over 16 years, are reported in the journal Nature.

In a complementary study of WR140, published in Nature Astronomy, NASA's James Webb Space Telescope (JWST) was able to see much deeper to snap an image of not just a single accelerating dust plume, but almost 20 of them, nested inside each other like a giant set of onion skins.

WR140 is comprised of a huge Wolf-Rayet star and an even bigger blue supergiant star, gravitationally bound in an eight-year orbit. This binary star, in the Cygnus constellation, has been monitored for two decades with one of the world's largest optical telescopes at the Keck Observatory in Hawaii.

WR140 episodically puffs out plumes of dust stretching thousands of times the distance from the Earth to the Sun. These dust plumes, produced every eight years, give astronomers a unique opportunity to observe how starlight can affect matter.

It's known that light carries momentum, exerting a push on matter known as radiation pressure. Astronomers often witness the outcome of this phenomenon in the form of matter coasting at high speed around the cosmos, but it's been a difficult process to catch in the act. Direct recording of acceleration due to forces other than gravity is rarely witnessed, and never in a stellar environment like this.

"It's hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over," said Yinuo Han from Cambridge's Institute of Astronomy, first author of the Nature paper. "To witness acceleration at the level that it becomes measurable, the material needs to be reasonably close to the star or the source of the radiation pressure needs to be extra strong. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data."

All stars generate stellar winds, but those from Wolf-Rayet stars can be more like a stellar hurricane. Elements such as carbon in the wind condense out as soot, which remains hot enough to glow bright in the infrared. Like smoke in the wind, this gives telescopes something that can be observed.

The team used an imaging technology known as interferometry which was able to act like a zoom lens for the 10-metre Keck telescope mirror, enabling the researchers to recover sufficiently sharp images of WR140 for the study.

Han and his team found that the dust does not stream out from the star with the wind in a hazy ball. Instead, the dust forms where the winds from the two stars collide, on the surface of a cone-shaped shock front between them.

Because the orbiting binary star is in constant motion, the shock front also rotates. The sooty plume gets wrapped into a spiral, in the same way that droplets form a spiral in a garden sprinkler.

The researchers found that WR140 has other tricks up its sleeve. The two stars are not on circular but rather elliptical orbits, and dust production turns on and off as the binary nears and departs the point of closest approach. By modelling these effects into the three-dimensional geometry of the dust plume, the astronomers were able to measure to location of dust features in three-dimensional space.

"Like clockwork, this star puffs out sculpted smoke rings every eight years, with all this wonderful physics written then inflated in the wind like a banner for us to read," said co-author Professor Peter Tuthill from the University of Sydney. "Eight years later as the binary returns in its orbit, another appears the same as the one before, streaming out into space inside the bubble of the previous one, like a set of giant nested Russian dolls."

Because the dust produced by this Wolf-Rayet is so predictable and expands to such large distances, it offered the astronomers a unique laboratory to examine the acceleration zone.

"In the absence of external forces, each dust spiral should expand at a constant speed," said Han, who is also a co-author on the JWST paper. "We were puzzled at first because we could not get our model to fit the observations, until we finally realised that we were seeing something new. The data did not fit because the expansion speed wasn't constant, but rather that it was accelerating. We'd caught that for the first time on camera."

"In one sense, we always knew this must be the reason for the outflow, but I never dreamed we'd be able to see the physics at work like this," said Tuthill. "When I look at the data now, I see WR140's plume unfurling a like giant sail made of dust. When it catches the photon wind streaming from the star, like a yacht catching a gust, it makes a sudden leap forward."

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SU(N) matter is about 3 billion times colder than deep space

Japanese and U.S. physicists have used atoms about 3 billion times colder than interstellar space to open a portal to an unexplored realm of quantum magnetism.

"Unless an alien civilization is doing experiments like these right now, anytime this experiment is running at Kyoto University it is making the coldest fermions in the universe," said Rice University's Kaden Hazzard, corresponding theory author of a studypublished today in Nature Physics. "Fermions are not rare particles. They include things like electrons and are one of two types of particles that all matter is made of."

A Kyoto team led by study author Yoshiro Takahashi used lasers to cool its fermions, atoms of ytterbium, within about one-billionth of a degree of absolute zero, the unattainable temperature where all motion stops. That's about 3 billion times colder than interstellar space, which is still warmed by the afterglow from the Big Bang.

"The payoff of getting this cold is that the physics really changes," Hazzard said. "The physics starts to become more quantum mechanical, and it lets you see new phenomena."

Atoms are subject to the laws of quantum dynamics just like electrons and photons, but their quantum behaviors only become evident when they are cooled within a fraction of a degree of absolute zero. Physicists have used laser cooling to study the quantum properties of ultracold atoms for more than a quarter century. Lasers are used to both cool the atoms and restrict their movements to optical lattices, 1D, 2D or 3D channels of light that can serve as quantum simulators capable of solving complex problems beyond the reach of conventional computers.

Takahashi's lab used optical lattices to simulate a Hubbard model, an oft-used quantum model created in 1963 by theoretical physicist John Hubbard. Physicists use Hubbard models to investigate the magnetic and superconducting behavior of materials, especially those where interactions between electrons produce collective behavior, somewhat like the collective interactions of cheering sports fans who perform "the wave" in crowded stadiums.

"The thermometer they use in Kyoto is one of the important things provided by our theory," said Hazzard, associate professor of physics and astronomy and a member of the Rice Quantum Initiative. "Comparing their measurements to our calculations, we can determine the temperature. The record-setting temperature is achieved thanks to fun new physics that has to do with the very high symmetry of the system."

The Hubbard model simulated in Kyoto has special symmetry known as SU(N), where SU stands for special unitary group -- a mathematical way of describing the symmetry -- and N denotes the possible spin states of particles in the model. The greater the value of N, the greater the model's symmetry and the complexity of magnetic behaviors it describes. Ytterbium atoms have six possible spin states, and the Kyoto simulator is the first to reveal magnetic correlations in an SU(6) Hubbard model, which are impossible to calculate on a computer.

"That's the real reason to do this experiment," Hazzard said. "Because we're dying to know the physics of this SU(N) Hubbard model."

Study co-author Eduardo Ibarra-García-Padilla, a graduate student in Hazzard's research group, said the Hubbard model aims to capture the minimal ingredients to understand why solid materials become metals, insulators, magnets or superconductors.

"One of the fascinating questions that experiments can explore is the role of symmetry," Ibarra-García-Padilla said. "To have the capability to engineer it in a laboratory is extraordinary. If we can understand this, it may guide us to making real materials with new, desired properties."

Takahashi's team showed it could trap up to 300,000 atoms in its 3D lattice. Hazzard said accurately calculating the behavior of even a dozen particles in an SU(6) Hubbard model is beyond the reach of the most powerful supercomputers. The Kyoto experiments offer physicists a chance to learn how these complex quantum systems operate by watching them in action.

The results are a major step in this direction, and include the first observations of particle coordination in an SU(6) Hubbard model, Hazzard said.

"Right now this coordination is short-ranged, but as the particles are cooled even further, subtler and more exotic phases of matter can appear," he said. "One of the interesting things about some of these exotic phases is that they are not ordered in an obvious pattern, and they are also not random. There are correlations, but if you look at two atoms and ask, 'Are they correlated?' you won't see them. They are much more subtle. You can't look at two or three or even 100 atoms. You kind of have to look at the whole system."

Physicists don't yet have tools capable of measuring such behavior in the Kyoto experiment. But Hazzard said work is already underway to create the tools, and the Kyoto team's success will spur those efforts.

"These systems are pretty exotic and special, but the hope is that by studying and understanding them, we can identify the key ingredients that need to be there in real materials," he said.

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Voyager 2 reaches interstellar space

This artist's concept shows the locations of NASA's Voyager 1 and Voyager 2 spacecraft relative to the heliosphere, or the protective bubble of particles and magnetic fields created by our Sun. Both Voyagers are now outside the heliosphere, in a region known as interstellar space, or the space between stars.

Voyager 1 has a companion in the realm of the stars.

Researchers at the University of Iowa report that the spacecraft Voyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the sun. Voyager 2, thus, becomes the second human-made object to journey out of our sun's influence, following Voyager 1's solar exit in 2012.

In a new study, the researchers confirm Voyager 2's passage on Nov. 5, 2018, into the ISM by noting a definitive jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space. It's also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space.

"In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true," says Iowa's Don Gurnett, corresponding author on the study, published in the journal Nature Astronomy. "We show with Voyager 2 -- and previously with Voyager 1 -- that there's a distinct boundary out there. It's just astonishing how fluids, including plasmas, form boundaries."

Gurnett, professor emeritus in the UI Department of Physics and Astronomy, is the principal investigator on the plasma wave instrument aboard Voyager 2. He is also the principal investigator on the plasma wave instrument aboard Voyager 1 and authored the 2013 study published in Science that confirmed Voyager 1 had entered the ISM.

Voyager 2's entry into the ISM occurred at 119.7 astronomical units (AU), or more than 11 billion miles from the sun. Voyager 1 passed into the ISM at 122.6 AU. The spacecraft were launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet they crossed into the ISM at basically the same distances from the sun.

That gives valuable clues to the structure of the heliosphere -- the bubble, shaped much like a wind sock, created by the sun's wind as it extends to the boundary of the solar system.

"It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed," says Bill Kurth, University of Iowa research scientist and a co-author on the study. "That says that these two points on the surface are almost at the same distance."

"There's almost a spherical front to this," adds Gurnett. "It's like a blunt bullet."

Data from the Iowa instrument on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere and the point where the solar wind piles up against the approaching wind in interstellar space, which Gurnett likens to the effect of a snowplow on a city street.

The Iowa researchers say the heliosheath has varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause, a boundary where the solar wind and the interstellar wind are in balance and considered the crossing point to interstellar space. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere.

"It's kind of like looking at an elephant with a microscope," Kurth says. "Two people go up to an elephant with a microscope, and they come up with two different measurements. You have no idea what's going on in between. What the models do is try to take information that we have from those two points and what we've learned through the flight and put together a global model of the heliosphere that matches those observations."

The last measurement obtained from Voyager 1 was when the spacecraft was at 146 AU, or more than 13.5 billion miles from the sun. The plasma wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft now so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth.

"The two Voyagers will outlast Earth," Kurth says. "They're in their own orbits around the galaxy for five billion years or longer. And the probability of them running into anything is almost zero."

"They might look a little worn by then," Gurnett adds with a smile.

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