NASA's Hubble Space Telescope captured comet 2I/Borisov as a blue glow in dark space.

The comet 2I/Borisov is the second interstellar object ever to be observed by scientists on Earth.Credit: NASA, ESA, and D. Jewitt (UCLA)

A research team made headlines last week when it claimed to have scooped up from the sea floor fragments of a meteorite that came from beyond our Solar System1. Finding such an interstellar sample on Earth would be exciting because it might shed light on how planets and stars beyond our own form. But a number of scientists say that the evidence that the material came from another planetary system is not convincing so far.

Here, Nature explores how scientists identify when something is truly interstellar — and how they are preparing to study many such objects after a new telescope comes online next year.

Have we seen interstellar objects before?

As long ago as 1705, astronomer Edmond Halley, of Halley’s Comet fame, speculated that some comets might come from outside the Solar System. Scientists have since thought that icy bodies flung out of other planetary systems might make their way to ours. That prediction came true in 2017, when astronomers spotted an object making its way past the Sun in an orbit that, when traced backwards, would mean that the object originated outside the Solar System. Scientists have several ideas about what the object — named 1I/‘Oumuamua — could be. For instance, it could be a fragment of a comet, a hydrogen iceberg or even a shard of nitrogen ice from a far-off dwarf planet similar to Pluto.

A second interstellar object, 2I/Borisov, was spotted two years later. These are the only two confirmed interstellar objects so far. Both flew past the Sun and exited the Solar System.

How common are they, and where do they come from?

Researchers estimate that there are a quadrillion interstellar objects in the cubic parsec of space surrounding the Sun, a region that does not stretch as far as the nearest neighbouring star, Alpha Centauri. If one of these objects interacts gravitationally with another in the right way, it can be flung onto a trajectory that would send it into our Solar System. As happened with ‘Oumuamua and Borisov, it would initially look like a comet or an asteroid from our own Solar System. But its ‘hyperbolic’ orbit, which indicates that it is not bound to the Sun by gravity, would give it away as an interstellar interloper.

Particles of interstellar dust fly through the Solar System all the time in hyperbolic orbits. Researchers have captured and studied bits of that dust using high-flying aircraft and space missions. NASA’s Stardust spacecraft collected interstellar dust in 2004 and brought it back to Earth for study2.

But bigger interstellar objects — ‘Oumuamua was the size of a skyscraper — can provide much more information on the chemistry of distant planetary systems. That’s why scientists are keen to observe and study as many interstellar objects as possible.

What would the chemical make-up of an interstellar object be?

The chemical composition of an interstellar object reflects the gas and dust that were present in the system where it formed.

In the latest claim, posted on the arXiv preprint server ahead of peer review1, Avi Loeb, an astrophysicist at Harvard University in Cambridge, Massachusetts, announced that his team had found hundreds of tiny metallic spheres on the sea floor off the coast of Papua New Guinea. The scientists say that the spheres came from a meteorite that hit Earth in 2014, which they say arrived from interstellar space. Other specialists disagree, contending that the flash of light observed when an object hit Earth’s atmosphere in 2014 cannot be proved to have come from an interstellar object3, and that very little of any meteorite would have survived the force of re-entry in any case. Confirming an interstellar origin for any meteorite requires precisely measuring all aspects of the incoming fireball4, which has not been done accurately enough for the 2014 flash, says Mária Hajduková, an astronomer at the Slovak Academy of Sciences in Bratislava.

Loeb’s team says that several of the spheres are unusually rich in the elements beryllium, lanthanum and uranium, which they claim could come from a magma ocean on an iron-rich celestial body. But abundances of these trace elements aren’t the right place to be looking for a “smoking gun” that indicates interstellar origin, says Larry Nittler, a cosmochemist at Arizona State University in Tempe. A better way, Nittler says, would be to analyse oxygen isotopes in the spheres. These isotopes are very similar in objects from the Solar System but different in those from other planetary systems. An oxygen analysis was not reported in the preprint, but Loeb tells Nature that his team is planning more analyses, including an evaluation of oxygen isotopes.

What’s next for interstellar studies?

Vera C. Rubin Observatory, a Program of NSF’s NOIRLab pictured under the Milky Way.

The Vera C. Rubin Observatory in Chile will probably discover more interstellar objects passing through our Solar System after it comes online.Credit: RubinObs/NOIRLab/NSF/AURA/B. Quint

Astronomers are keen to observe more interstellar objects and work out what they are made of. “Observers are now highly motivated to get [the] chemistry of any future interstellar objects,” says Steven Desch, an astrophysicist at Arizona State University.

Discoveries are expected to ramp up dramatically after the new Vera C. Rubin Observatory in Chile comes online as early as next year. The Rubin telescope will survey the entire southern sky every three nights, and will be able to spot fainter objects than those detected in earlier sky surveys. The telescope might discover dozens of interstellar objects during its ten-year survey5.

It might soon become possible to study the entire history of the Milky Way through interstellar objects, says Michele Bannister, a planetary astronomer at the University of Canterbury in Christchurch, New Zealand. These fragments from other planetary systems form a cosmic sea of clues to galactic evolution6.

“This is the true promise of interstellar objects,” Bannister says. “They don’t just tell us about the history of our local neighbourhood — they potentially tell us about the history of our Galaxy.”