Interstellar Interlopers Like 'Oumuamua' and 'Borisov' Are Much More Common Than Thought

The first interstellar intruder detected passing through the solar system, 1l/'Oumuamua, approached within about 38 million kilometers of the Sun in 2017.NASA, ESA, JOSEPH OLMSTED (STSCI), FRANK SUMMERS (STSCI)

On October 17 and 18, 2017, an unusual object sped across the field of view of a large telescope located near the top of a volcano on the Hawaiian island of Maui. The Pan-STARRS1 telescope was designed to observe the sky for transient events, such as flybys of asteroids or comets. But this case was different: the object was not gravitationally bound to the Sun or any other celestial body. It had come from another place.

The mysterious object was the first visitor to interstellar space observed as it passed through the solar system. Astronomers named it 1I/'Oumuamua, borrowing a Hawaiian word that roughly translates to “first-arriving messenger from afar.” Two years later, in August 2019, amateur astronomer Gennadiy Borisov discovered the only other known interstellar intruder, now called 2I/Borisov, using a telescope he built at the MARGO observatory in Nauchnij, Crimea.

While typical asteroids and comets in the solar system orbit the Sun,

Oumuamua

and

Borisov

are celestial nomads that spend most of their time wandering through interstellar space. The existence of these intruders in the solar system was a hypothesis, but scientists expected them to be rare. “I never thought we'd see one,” says astrophysicist Susanne Pfalzner of the Jülich Supercomputing Center in Germany. At least, not during her lifetime.

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With these two discoveries, scientists now suspect that interstellar intruders are much more common. Right now, in Neptune's orbit alone, there could be about 10,000 interstellar objects the size of

Oumuamua

, estimates planetary scientist David Jewitt of UCLA, co-author of a review of current knowledge of interstellar interlopers in the

Annual Review of Astronomy and Astronomy. Astrophysics

2023.

Researchers are working to answer basic questions about these extraterrestrial objects, such as where they come from and how they end up roaming the galaxy. The intruders could also provide a new way to probe the characteristics of distant planetary systems.

But first, astronomers need to find more. “At the moment we are a little behind,” says Jewitt. “But we hope to see more.”

2I/Borisov appears as a blurry blue dot in front of a distant spiral galaxy (left) in this November 2019 image, taken by the Hubble Space Telescope, when the object was approximately 200 million miles from Earth. NASA, ESA, AND D. JEWITT (UCLA)

Alien origins

Since at least the early 18th century, astronomers have considered the possibility that interstellar objects exist. More recently, with computer models, it has been shown that the solar system long ago sent its own population of smaller bodies into the voids of interstellar space due to gravitational interactions with the giant planets.

Scientists expected most of the intruders to be exocomets composed of icy materials.

Borisov

fit this profile: it had a tail made up of gases and dust created by ice that evaporated during its close pass to the Sun. This suggests that it originated in the outer region of a planetary system where temperatures were cold enough to that gases, such as carbon monoxide, would freeze in its rocks. At some point, something threw

Borisov

, about a kilometer in diameter, out of his system.

One of the possible culprits is a stellar flyby. According to a recent study led by Pfalzner, the gravity of a passing star can eject smaller bodies, known as planetesimals, from the confines of a system. A giant planet could also eject an object from the outer regions of a planetary system, if an asteroid or comet comes close enough that the planet's gravitational pull accelerates the smaller body enough for it to escape its grasp. star. Approaches can also occur when planets migrate through their planetary systems, as Neptune is believed to have done in the early solar system.

Interstellar intruder 2I/Borisov was discovered three months before it passed by the Sun, allowing astronomers to image the object for about a year. Borisov's trajectory placed it about 290 million kilometers from Earth. The black dots mark the relative locations of Earth (blue) and Borisov (dark red) in November 2019 and January 2020.

Oumuamua

, on the other hand, is not what scientists expected. Observations suggest that it is quite elongated—perhaps 240 meters long and as narrow as 40 meters. And unlike

Borisov

, it shows no gas or dust activity, raising the possibility that it originated closer to its star, where it was too hot for ice to form. If this were the case, a stellar flyby or a giant planet probably would not have been able to remove the object from its system. Instead, it could have been ejected during its star's death throes: Pulsations of gas from a dying star could push planets and planetesimals outward, destabilizing their orbits enough to send some of them flying into interstellar space.

However, it is possible that

Oumuamua

formed in the cold confines of its system and, as it approached the Sun, it developed a tail of gas that was not detected by telescopes. One clue is that the object was accelerated more than would be expected from the solar system's gravity alone. A recent study suggests that such acceleration could be due to small amounts of hydrogen outgassing that telescopes did not detect. According to another study, several asteroids in our solar system could have received a similar boost from water vapor outgassing. Future observations from the James Webb Space Telescope and JAXA's Hayabusa2 Extended mission (which will rendezvous with one of these solar system asteroids, known as “dark comets,” in 2031) could detect low levels of outgassing.

“We'll have to wait and see, but they could be analogues of

Oumuamua

,” says planetary scientist Darryl Seligman of Cornell University, co-author with Jewitt of the review of interstellar intruders.

In search of nomads

More data, from more intruders, could help resolve some of these questions. To gather this data, scientists will need better chances of detecting the objects as they pass through the solar system. “If Pan-STARRS1 had not observed where we did that night,

Oumuamua

likely

would never have been found,” says astronomer Robert Weryk, who was a professor at the University of Hawaii and the discoverer of the interloper in the telescope data.

The Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time is expected to increase astronomers' chances of finding these fast-moving objects. Starting in 2025, the observatory's telescope will image the entire visible southern sky every few nights, and its primary mirror is nearly seven meters larger in diameter than Pan-STARRS1, allowing it to see fainter, more distant objects. Once detected, ground and space telescopes will take images of them to determine what they are made of. And if an achievable target is discovered, the European Space Agency and Japan Aerospace Exploration Agency's Comet Interceptor, scheduled for launch in 2029, could be redirected to obtain close-up images of the visitor.

The Vera C. Rubin Observatory, located in northern Chile, will be home to the Legacy Survey of Space and Time, beginning in 2025. The Observatory's 8.4-meter Simonyi Survey Telescope will collect images at a rate that will cover the entire visible sky every few nights, potentially allowing the detection of more interstellar intruders. RUBINOBS / NSF / AURA / H. STOCKEBRAND

Over time, astronomers hope to create a catalog of interstellar objects similar to the inventory of exoplanets, which has surpassed 5,500 entries since the first discovery in 1992. That future inventory could help researchers answer the age-old question of how typical they are. the Earth and the solar system. The composition of a large sample of interstellar objects could provide clues about the composition of objects in exoplanetary systems, including those that could support life.

“Planetesimals are the building blocks of exoplanets,” says astronomer Meredith Hughes of Middletown Wesleyan University in Connecticut. This means that “they can provide information about the diversity of environments, including those that could be habitable.”

Now,

Oumuamua

is located beyond the orbit of Neptune, and Comet

Borisov

is almost the same distance away. They will continue their journey back to interstellar space, where no one knows what will happen next. Maybe they spend an eternity wandering the vast voids of space, or maybe they are captured by a star. Or they could collapse into an evolving disk of gas and dust in a new planetary system and begin their journey again.

Astronomers estimate that there could be more interstellar objects in the Milky Way than stars in the observable universe. Finding more of them will offer a new way to probe the mysteries of the cosmos.

“The really cool thing,” Pfalzner says, “is that interstellar objects are coming to us.”

Theo Nicitopoulos

is a freelance journalist covering Earth and space science news.

Article translated by

Debbie Ponchner

This article originally appeared on

Knowable en español

, a nonprofit publication dedicated to making scientific knowledge available to everyone.

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