Many scientists currently searching for traces of extraterrestrial life focus on planets orbiting within the habitable zones of their stars, where liquid water might exist. Now, a researcher has proposed a mind-boggling alternative that turns this convention on its head: What if alien civilizations learn to use free-floating “rogue” planets, which are not bound to any star, to traverse across interstellar distances? And how might we try to spot these extraterrestrials wandering across the dark patches of the galaxy, if they do exist?
This concept of “cosmic hitchhikers” is the brainchild of Irina Romanovskaya, a professor of physics and astronomy at Houston Community College who outlines how “extraterrestrial civilizations may travel from their home worlds to free-floating planets, and how they may transfer from their free-floating planets to other planetary systems” in her recent study published in the International Journal of Astrobiology.
“Almost 10 years ago, when reading about discoveries of free-floating planets, I thought of a hypothetical scenario of a free-floating planet approaching the Solar System,” Romanovskaya said in an email to Motherboard. “There are no traffic lights in the Galaxy. If the Solar System happens to be in the way of some free-floating planet, the planet will not stop at the red light. It will fly right through the Solar System. The probability of such an event would depend on how many free-floating planets exist in our Galaxy, which at that time remained to be estimated.”
Now, scientists estimate that tens of billions of planets have been catapulted from their native star systems by gravitational encounters with other objects, resulting in a large hidden population—they are generally tricky to spot because there is no nearby starlight to illuminate them—of untethered rogue planets drifting across the Milky Way. “This increases the chance that some advanced extraterrestrial civilizations, if they exist, might hitch a ride on free-floating planets,” Romanovskaya said. “Which is why I call such hypothetical civilizations Cosmic Hitchhikers.”
While rogue planets may not benefit from life-nourishing stellar energy, Romanovskaya points out that these worlds may still be habitable, especially if they contain the kind of subsurface oceans that are suspected to exist in a few bodies within our own solar system. Moreover, these planets offer many advantages as interstellar vessels, compared to the traditional vision of artificial generation ships—ark-like spacecraft intended to host many generations of a population—that must be laboriously constructed by space-faring species.
Romanovskaya notes that rogue planets can come ready-made with resources, potentially including water and valuable minerals, that could facilitate long journeys between stars. Natural habitats on these worlds could serve as effective shields from the radiation of space travel, and their gravitational heft could negate the need for artificial gravitation systems in spacecraft.
Moreover, Romanovskaya suggests that a clever alien species could implement multiple scenarios to hitch a ride on one of these worlds. Though space is very big and interstellar distances are immense, it is still not uncommon for stars and planets to pass by each other in close encounters that could provide an opportunity to jump ship—or in this case, jump off a planet. If an alien civilization, perhaps in an aging system, knew about the approach of a rogue planet, or even an entire star system, it could plan an escape from the inevitable death of its host star.
In an even wilder scenario, Romanovskaya posits that intelligent life might want to purposely wait for the dying days of their host star so that it could position itself on a distant planet in the solar system and wait for the winds of the star’s demise to propel it into the galaxy. An extraterrestrial species could also migrate deep into the outer reaches of its star system, where stellar gravitational bonds are weaker, and find some way to artificially eject a planetary body there. In this way, the cosmic hitchhikers would set out for the stars on a rogue world from their home system.
These are tantalizing thought experiments, but Romanovskaya also offers practical ways to search for intelligent aliens that might have embarked on this type of free-floating interstellar journey. Scientists might want to look for signs of unusual electromagnetic radiation around rogue planets that could indicate artificial activity. Romanovskaya speculates that a species that mastered this technique of travel might also leave similar technosignatures across the galaxy, potentially providing a trail of breadcrumbs that could be identified from Earth.
The recently-launched James Webb Telescope picks up infrared radiation, Romanovskaya said, which would be emitted by a rogue planet that’s been retrofitted into a spaceship. Unmanned probes could also join the search. “NASA has already sent spacecraft to travel to the outskirts of the Solar System and beyond,” she said. “Perhaps, in the not-so-distant future, another spacecraft will be sent to the remote regions of our planetary system to study any newly discovered objects that might be captured free-floating planets.”
Perhaps most exciting, Romanovskaya raises the possibility that rogue planets that were once used for interstellar travel might get ditched in space, and end up captured in the outer reaches of another star system—perhaps even our own. In this scenario, a spent planet, packed with alien artifacts, might await any lifeforms that expand into the dark and distant patches of their solar backyards. Romanovskaya also proposes that Earth’s neighbors could bear traces of aliens that came across our solar system in the past, perhaps in the dried-up remains of Mars’ oceans.
There’s another element to this work, which is that humans are aliens, too. And one day, we might want to hitch a ride on a rogue planet as well.
“If humankind exists long enough, then humans can witness the Sun getting old and dying. Or there may be other existential threats arising,” Romanovskaya said. “Then, the question of escape may become a very practical question. Some populations of humankind may also decide to relocate to another planetary system to expand the presence of our civilization in the Galaxy. Or else, humans may send large amounts of technologies to explore stars and interstellar space.”
“For all these scenarios, humankind could use free-floating planets as interstellar transportation,” she said, adding that not too long ago humans used horses for transportation, and that she anticipates humanity will develop propulsion powerful enough for planet-ships in the future.”