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The first interstellar interloper detected passing through the Solar System, 1l/‘Oumuamua, came within 24 million miles of the Sun in 2017. It’s difficult to know exactly what ‘Oumuamua looked like, but it was probably oddly shaped and elongated, as depicted in this illustration.
CREDIT: NASA, ESA, JOSEPH OL
MSTED (STSCI), FRANK SUMMERS (STSCI)
On October 17 and 18, 2017, an unusual object sped across the field of view of a large telescope perched near the summit of a volcano on the Hawaiian island of Maui. The Pan-STARRS1 telescope was designed to survey the sky for transient events, like asteroid or comet flybys. But this was different: The object was not gravitationally bound to the Sun, or to any other celestial body. It had arrived from somewhere else.
The mysterious object was the first visitor from interstellar space observed passing through the solar system. Astronomers named it 1I/‘Oumuamua, borrowing a Hawaiian word that roughly translates to “messenger from afar arriving first.” Two years later, in August 2019, amateur astronomer Gennadiy Borisov discovered the only other known interstellar interloper, now called 2I/Borisov, using a self-built telescope 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, spending most of their time wandering interstellar space. The existence of such interlopers in the solar system had been hypothesized, but scientists expected them to be rare. “I never thought we would see one,” says astrophysicist Susanne Pfalzner of the Jülich Supercomputing Center in Germany. At least not in her lifetime.
With these two discoveries, scientists now suspect that interstellar interlopers are much more common. Right now, within the orbit of Neptune alone, there could be around 10,000 ‘Oumuamua-size interstellar objects, estimates planetary scientist David Jewitt of UCLA, coauthor of an overview of the current understanding of interstellar interlopers in the 2023 Annual Review of Astronomy and Astrophysics.
Researchers are busy trying to answer basic questions about these alien objects, including where they come from and how they end up wandering the galaxy. Interlopers could also provide a new way to probe features of distant planetary systems.
But first, astronomers need to find more of them.
“We’re a little behind at the moment,” Jewitt says. “But we expect to see more.”
2I/Borisov appears as a fuzzy 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.
CREDIT: NASA, ESA, AND D. JEWITT (UCLA)
Alien origins
At least since the beginning of the 18th century, astronomers have considered the possibility that interstellar objects exist. More recently, computer models have shown that the solar system sent its own population of smaller bodies into the voids of interstellar space long ago due to gravitational interactions with the giant planets.
Scientists expected most interlopers to be exocomets composed of icy materials. Borisov fit this profile: It had a tail made of gases and dust created by ices that evaporated during its close passage to the Sun. This suggests that it originated in the outer region of a planetary system where temperatures were cold enough for gases like carbon monoxide to have frozen into its rocks. At some point, something tossed Borisov, roughly a kilometer across, out of its system.
One potential culprit is a stellar flyby. The gravity of a passing star can eject smaller bodies, known as planetesimals, from the outer reaches of a system, according to a recent study led by Pfalzner. A giant planet could also eject an object from the outer regions of a planetary system if an asteroid or comet gets close enough for the planet’s gravitational tug to speed up the smaller body enough for it to escape its star’s hold. Close approaches can also happen when planets migrate across their planetary systems, as Neptune is thought to have done in the early solar system.
The interstellar interloper 2I/Borisov (large black dot) was discovered three months before it passed by the Sun, allowing astronomers to capture images of the object for about a year. Borisov’s path brought it within 180 million miles of Earth (large blue dot). The relative locations of Borisov and Earth are shown for three points in time.
‘Oumuamua, on the other hand, is not what scientists expected. Observations suggest it is quite elongated — perhaps 240 meters long and as narrow as 40 meters. And unlike Borisov, it didn’t show any gas or dust activity, raising the possibility that it originated closer to its star where it was too warm for ices to form. If this was the case, a stellar flyby or giant planet probably would not have been able to pull the object out of its system. Instead, it may have been ejected during the death throes of its star: Pulses 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.
It’s possible, however, that ‘Oumuamua did form in the cold outer reaches of its system and, as it neared the Sun, developed a gas tail that was not detected by telescopes. One clue is that the object sped up more than would be expected from the gravity of the solar system alone. A recent study suggests that such a boost could have come from small amounts of hydrogen outgassing that the telescopes didn’t detect. Several asteroids in our solar system may have gotten a similar boost from outgassing of water vapor, according to another study. Future observations by the James Webb Space Telescope, and by the JAXA Hayabusa2 Extended Mission (which will rendezvous with one of these solar system asteroids, known as “dark comets,” in 2031) may detect low levels of outgassing.
“We’ll have to wait and see, but they could be analogs of ‘Oumuamua,” says planetary scientist Darryl Seligman of Cornell University, coauthor with Jewitt of the review of interstellar interlopers.
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