Suggested titles:
Interstellar comet 3I/Atlas reveals the Solar System is filled with alien objects
New book claims some planets might be alien


While 3I/Atlas, the third confirmed interstellar visitor, is temporarily out of view behind the Sun, it raises the question of how many more unconfirmed alien visitors might exist within the Solar System.

According to "The Living Universe" by Brian G Turner, which shows how the latest research is rewriting what we know about space, planets, and the origins of life, a number of other potential alien objects have already been identified.

"The Solar System has traditionally been treated as a closed system, with no influence from outside affecting it. The discovery of the first interstellar visitor in 2017, 'Oumuamua, forced us to question that assumption. Since then we've come to realize there are probably many more alien objects in the Solar System, either moving at slower speeds, or already having been captured."

One of the first contenders are the centaurs, which exhibit the properties of both comets and asteroids, and exist between Jupiter and Neptune. Many have orbits that researchers say show they couldn't have formed in the Solar System, and therefore must be interstellar visitors which were captured by the gravity of the Sun [https://phys.org/news/2020-04-discovery-asteroid-population-solar.html]. However, that's not all, as some asteroids also have orbits that suggest they must have been captured [https://phys.org/news/2018-05-interstellar-immigrant-solar.html] [also https://www.nationalgeographic.com/science/article/interstellar-asteroid-jupiter-bz509-astronomy-space-science].

Researchers now suggest that it must be routine to capture material from stars passing close by, with one simulation suggesting the Solar System could already contain over a million objects the size of the Statue of Liberty from our nearest stellar neighbour, the Alpha Centauri system [https://www.livescience.com/space/astronomy/1-million-interstellar-objects-each-larger-than-the-statue-of-liberty-may-lurk-in-the-outer-solar-system].


SUBTITLE: Alien planets

Turner, however, says we can go further than that and suggest other possibilities. "The second confirmed interstellar visitor, 2I/Borisov, looked like an ordinary comet. In fact, chemically it was very similar to some of the existing comets in the Solar System, including Halley's Comet. Although Borisov was moving at high speed, we know Halley's Comet used to move much faster, but was slowed by interactions with the planets. Additionally, Halley's Comet orbits the Sun backward, which shouldn't be the case if it formed in the Solar System.

"In other words, Halley's comet could also have once been an interstellar visitor captured by the Sun's gravity. That would mean meteor showers associated with Halley's Comet - the Orionids in October, and the Eta Aquarids in May [https://phys.org/news/2025-10-orionid-meteor-shower-debris-halley.html] - could actually be showers of alien dust.

"It's not just comets, though, that hint at having been captured. There are a lot of minor moons around the gas giant planets that have irregular orbits that make little sense unless they were also captured by the Solar System. For example, Phoebe, one of the small moons around Saturn, has a chemical signature that's unique in the Solar System [https://phys.org/news/2018-12-saturn-satellites-earth-moon-phoebe.html]. Not only that, but it also looks more like the remains of a dwarf planet, which means it might not just be a captured interstellar object, but also a captured dwarf planet."

In fact, Turner even makes the claim that Pluto, now regarded as a dwarf planet, might also be a captured alien visitor. "We think that the Solar System was formed by a rotating disk of material, which means any planets that formed from that disk would inherit the same spin and orbit and move in the same plane. Pluto, however, is very different, in that it has a very irregular orbit that goes above and below the plane of the other planets, and it also spins almost upside down by comparison to them. This suggests Pluto might not have formed here."

On that point Turner makes the biggest claim yet in his book: that some of the major planets in the Solar System might also be alien in origin. "Pluto isn't the only planet to have an irregular orbit different to the plane of the planets, as Mercury does so as well. And Pluto isn't the only planet to orbit the Sun upside down, as Venus also does and there's no explanation for that."

Does this mean Turner thinks some of our planets must be alien? "It's more that we can't assume anything about the Solar System. We need to explore every possibility if we're going to better understand its past, no matter how far-fetched some ideas might initially appear. After all, ten years ago the thought of being able to observe interstellar comets would have also seemed outrageous, but now it's proven."

The Living Universe by Brian G Turner is available in paperback, Kindle format, and audiobook, exclusively from Amazon.