Where do 3I/ATLAS and other interstellar visitors come from?

Roughly one trillion billion million objects reside in interstellar space — that’s a 1 followed by 27 zeros — but only three have ever been discovered passing through the Solar System. These are our sole known visitors from other stars.

The first, called ‘Oumuamua, was also the most mysterious. After it was discovered in 2017, some were quick to claim ‘Oumuamua was an alien spaceship — complete with a solar sail — because of its seemingly strange orbit and elongated shape. But there was no good evidence for this, and scientists argue that ‘Oumuamua is far more likely to be debris from another star, like a chunk of ice or giant clod of dust. 

Then, researchers discovered two more interstellar objects in relatively quick succession: the comets 2I/Borisov and 3I/ATLAS, each with a chemical makeup unlike anything else in the Solar System.

Already, these three “interstellar interlopers” have started to draw back the curtain on long-standing astronomical mysteries. Discovering more will help reveal how planets form, evolve, and (occasionally) get thrown out into interstellar space. In the not-so-distant future, a mission to an interstellar object could even be our best bet at exploring other stars.

But where exactly are these visitors coming from?  

A galaxy of flotsam

The short answer is: everywhere. 

An average star system in the Milky Way throws off roughly 10 quadrillion objects into interstellar space throughout its lifetime. Most of these bits of dust, rock, and ice get ejected soon after a star is born, when planets are still forming and settling into stable orbits. Some debris gets slung out as these orbits rearrange, while other bits get pulled away by the gravitational influence of passing stars.

This flotsam spreads out into streams as it travels through the Milky Way. There are millions of streams like this in our galaxy, all tangled up in each other like ribbons. 

That makes it much harder to tell where any single interstellar visitor originally came from. Scientists’ best guess about ‘Oumuamua, for example, is that it likely came from one of the millions of stars within roughly 3,000 light-years of Earth.

There is an exception to this rule, though. Once we’ve discovered hundreds of interstellar visitors, we’ll likely start to see “siblings”: two visitors that originated from the same star. That could allow us to pinpoint exactly where both came from.

The first three of many?

With only three interstellar interlopers discovered to date, finding hundreds might sound like a stretch — but it could actually happen in the very near future.

The trick isn’t waiting for more interstellar visitors, but getting better at spotting those that are already here. Astronomers estimate that there are around 60 interstellar objects passing within Jupiter’s orbit around the Sun at any given time, plus an additional eight or so that are actually here to stay. Some of the more peculiar asteroids and comets we’ve discovered could actually be debris from other stars, hiding in plain sight.

Most interlopers are small and dim, though, and can only be detected if they pass near enough to Earth. Then, scientists can track their movements and reconstruct their interstellar origins. 

Fortunately, two new observatories will be powerful tools for exactly this kind of search. The Vera C. Rubin Observatory in Chile, which saw first light earlier this year, is designed to hunt asteroids as one of its main goals. So is NASA’s NEO Surveyor space telescope, slated for launch in 2027. Combined, these two observatories could discover up to 70 interstellar interlopers per year. 

But some scientists warn these searches could also turn up far fewer. It all depends how well interstellar objects tend to reflect light, and that remains unclear.

Windows to alien stars

Discovering more interstellar visitors would not just up our chances of finally tracing one back to its home. For every new interloper we discover, we gain a window into planetary systems throughout the galaxy.

Scientists can study a visitor’s shape and chemical makeup to try to pinpoint where exactly it came from around its host star — and how it was ejected. That could tell us more about how planets are born, how they rearrange,  and how they tend to settle down over time. 

Interstellar objects themselves may be important ingredients for making new planets. When something like ‘Oumuamua arrives at a newborn star, it might potentially form a core for other material to glom onto and grow into the larger building block of a world. Some scientists think this could be how many planets form, if not most of them.

Interstellar objects can have even more unexpected effects. Some researchers believe that fast radio bursts, a mysterious kind of celestial explosion, could be partially explained by interstellar debris exploding against neutron stars. This might sound far-fetched, but interstellar debris is common throughout the galaxy. The Sun itself is thought to get smashed into by an interstellar visitor once every 60 years or so. 

Visiting the visitors

Intercepting one of these visitors is our best bet at exploring another star system for the foreseeable future. With current technology, a mission to the Sun’s closest neighboring star would take thousands of years. But with a mission to an interstellar object, we could examine a piece of a star system that’s thousands of times farther away — and do it within decades.

Scientists have proposed a number of different missions to intercept a future visitor as it flies by the Sun. The key would be acting fast, because interstellar objects tend to exit the inner Solar System only months after they’re discovered.

One idea is to build a spacecraft, keep it on alert, and then launch almost as soon as we discover a new object on a suitable trajectory. That could work, but it would be expensive to keep a probe flight-ready for such a long period. And it’d be challenging to put together a successful launch in a matter of weeks. 

Alternatively, a mission could launch a small probe into a holding pattern around the Sun or Earth. Once a good target is discovered, the probe would spring into action, accelerating to speeds on the order of 100,000 kilometers per hour (62,000 miles per hour). The spacecraft could then fly through the visitor’s cometary tail, tap against it to see what it’s made of, or possibly land and collect samples to bring back to Earth. 

The European Space Agency plans to launch a mission like this in 2029, but to catch up with a “regular” comet instead. If an interstellar object happens to fly by at the right time and on the right path, though, there is a chance that the spacecraft could pay a visit. 

Many of these mission ideas depend, in part, on work that The Planetary Society has done. Our LightSail missions demonstrated a milestone in solar sailing, successfully using the force of the Sun’s radiation to propel a small spacecraft. This technology could be key to getting a small, nimble probe to intercept an interstellar object. 

If we ever do explore a piece of another stellar system, members of The Planetary Society might have helped pave the way.