Aliens may not travel in starships, but on complete but lonely planets through interstellar space.

Professor Irina Mullins comes up with this idea in the International Journal of Astrobiology on the up. “I propose that alien civilizations could also use orphan planets (planets that are not bound to a star, but wander solitary through space, ed.) as interstellar transport means to reach, explore and colonize other planetary systems.” That may seem a bit far-fetched, but there’s a fascinating line of thought behind it that also has very interesting implications. For example, for our search for extraterrestrial life.

Traveling

Aliens can have different reasons for leaving their home planets. Out of curiosity, for example: because they are curious about what can be found outside their own system. Or out of necessity: because an existential threat – disease, climate change, war, a dying mother star, etc. – forces them to flee from their familiar surroundings. However, especially in the latter scenario, (also advanced) aliens are likely to run into problems quickly, Mullins writes. “They would likely face serious or insurmountable problems if they wanted to use a spacecraft to transport large populations over interstellar distances.”

Orphan planet

It got Mullins thinking. Because how else could advanced civilizations move? “And then it occurred to me that alien civilizations could get a lift from orphaned planets passing through their planetary system,” she tells. Scientias.nl†

Piggybacking on such an orphan planet has several advantages over a starship. “Orphan planets can provide constant gravity, but also a lot of space and resources,” Mullins writes. Think of water on or just below the surface of orphaned planets: aliens can consume or use it – for example by creating an undersea habitat – to protect themselves against any dangerous radiation from space.

Find a lift

But in order to be able to hitch a ride on such an orphan planet – and to be able to make use of everything that such a planet has to offer – you first have to figure it out. This can be done in various ways, Mullins writes. For example, aliens can simply wait for such an orphan planet to travel through their planetary system. “Scientific studies suggest that the number of orphan planets may be greater than the number of stars in the Milky Way. That increases the likelihood of orphan planets traveling through the outer reaches of planetary systems.” Once aliens have colonized those outer reaches of their system, they need only travel a short distance with their spacecraft to colonize such a passing orphan planet.

Take matters into your own hands

The disadvantage of this approach is of course that you have to wait and see when such an orphan planet shows up. And that is certainly not ideal in the case of an existential threat, Mullins acknowledges. Advanced civilizations can therefore take matters into their own hands and search for a nearby orphan planet and adjust its course. “Advanced alien civilizations can also use technologies to direct passing orphan planets toward their planetary system so that their species can travel to the passing orphan planet.”

Create an orphan planet

Another possibility is that aliens travel to a (dwarf) planet in their own planetary system and then release it from the influence of the parent star, so that the planet transforms into an orphan planet. “They (the aliens, ed.) can then use advanced propulsion systems and gravitational pendulums to hurl Sedna-like objects (Sedna is a dwarf planet on the edge of our solar system, ed.) out of their own planetary systems.” However, it is an approach that is only for the more advanced alien civilizations. “With the propulsion systems required to do so, the aliens would be centuries ahead of our space technology.”

A push from the mother star

Dwarf planets can also be thrown out of their planetary systems by the parent star itself. This is when such a parent star is dying and transforming into a red giant or supernova. In that case, there is enough reason for any aliens in the heart of the planetary system to flee. In theory, they could switch relatively close to home to an object with a very eccentric (oval-shaped) orbit. That object then takes them to the outer reaches of their planetary system where they can transfer to a dwarf planet that is later ejected from the planetary system by the dying parent star and thus becomes an orphan planet.

Means of transport, no home

Some of the scenarios described above require advanced drive systems, others do not. But in all scenarios, the result is the same: the alien civilizations end up on a planet that wanders lonely through space. We should really see such a planet as an alternative to the interstellar starship and not as a new home, Miller emphasizes. Because orphaned planets are not very hospitable in the long run. “Due to decreasing heat production within their interiors, such planets will fail to maintain their liquid water-filled oceans (if any). In addition, orphan planets have fewer resources to offer than planetary systems. Therefore, extraterrestrial civilizations would not make them their permanent home either, but only use them as an interstellar means of transport to reach and colonize other planetary systems.”

Send

Once on such an orphan planet, it is of course important that the aliens eventually dock at a new home. “Aliens can send their orphan planet into interstellar space using propulsion technologies, perhaps supplemented by gravitational pendulums,” Mullins tells us. Scientias.nl† “Or they just have to wait for their orphan planet to reach another planetary system on their own.”

Acceptable scenario?

Whether aliens actually move through the universe – whether or not adjusting their orphan planet where necessary – remains a mystery, of course. But it is certainly not an unthinkable scenario for Mullins. “Scientific studies tell us it’s impossible to build ships that would allow entire civilizations to travel between the stars,” she said. Scientias.nl† “We know that stars die as they get older. We know that humans instinctively want to save their asses, and other civilizations may feel the same way. So when their parent stars are dying, advanced intelligent species will look for ways to escape. Since large starships are not an option, they will look for other methods.” And orphan planets – or dwarf planets orphaned by the aliens – are a logical option. “Orphan planets with all their resources and oceans filled with water provide everything aliens need during their interstellar journeys.”

Looking

For Mullins, aliens piggybacking on an orphaned planet are therefore a real possibility. And as for her, we’ll soon be supplementing our search for intelligent extraterrestrial life (also referred to as SETI) with a search for migratory intelligent life (Mullins calls it: SMETI). “Scientists traditionally look for technosignatures (traces of extraterrestrial technologies that hint at the presence of intelligent life, ed.) in the habitable zone of planetary systems containing a star that is similar or smaller than our sun. Those are attractive places for life. But after sixty years of searching, we still haven’t found anything. Therefore, some scientists believe that as alien civilizations become more advanced, they use their technologies to blend in with their environment and become more difficult to detect at interstellar distances.” But when such a civilization (of necessity) sets course for other planetary systems, technologies have to be used that are a lot more striking and with that it should also be a little easier to look for those aliens.

We just have to look elsewhere; not near sun-like stars, but, for example, just on the edge of planetary systems, where aliens may be busy detaching a dwarf planet from its parent star or just waiting for an orphan planet to pass by. In addition, star-trapped orphan planets could also be good places to look for traces of life. Like the environment of dying stars; aliens that have fled on a dwarf or orphan planet can just be found there, or so the idea is. It does not make the traditional search for extraterrestrial life superfluous, Mullins emphasizes, but can be seen as a supplement to it. “I would like to say that my research paper is a call to expand the SETI program.”

Finally, the research could also have implications for us humans, because Mullins presents a form of transport that could not only be of interest to possible aliens, but that could also benefit us (or rather our progeny). “Like aliens, humanity can use orphan planets to reach other planetary systems,” Mullins said. “That would be something for the distant future; when our space technology is much more advanced.”