About 5 billion years from now, the Sun will leave the main sequence and become a red giant. It will expand and transform into a bright, malevolent ball, consuming and destroying Mercury, Venus, Earth, and probably Mars. Can humanity survive the red giant phase of the Sun? Extraterrestrial civilizations (ETCs) may have already faced this existential threat.
Could they have survived migrating to another star system without the use of spacecraft?
Today’s readers of the Universe are well versed in the difficulties of interstellar travel. Our nearest neighboring solar system is the Alpha Centauri system. If humanity had to flee an existential threat to our Solar System, and if we could identify a planetary home in Alpha Centauri, it would still take more than four years to reach it, if we could travel at the speed of light!
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It still takes us five years to reach orbit Jupiter in our technological stage. There is a lot of talk about generation ships, where humans could live for generations as they walk to a distant habitable planet. These ships do not need to reach anywhere near the speed of light; instead, whole generations of humans would live and die on a journey to another star that takes hundreds or thousands of years. It’s fun to think about, but pure fantasy at this point.
This is a picture of the ship of the Nauvoo generation of “The Expanse”. Generational ships are science fiction stuff, for now. Image credit: Legendary Television Distribution.
Is there another way that we, or other civilizations, could escape our doomed homes?
The author of a new research paper in the International Journal of Astrobiology says that ETCs may not need starships to escape existential threats and travel to another star system. Instead, they could use free floating planets, also known as rogue planets. The article is “Migration of extraterrestrial civilizations and interstellar colonization: implications for SETI and SETA.” The author is Irina Romanovskaya. Romanovskaya is a professor of physics and astronomy at Houston Community College.
“He proposes that extraterrestrial civilizations can use floating planets as interstellar transport to reach, explore, and colonize planetary systems,” writes Romanovskaya. And when it comes to researching other civilizations, these efforts could leave technosignatures and artifacts. “It proposes possible technosignatures and artifacts that can be produced by extraterrestrial civilizations that use free floating planets for interstellar migration and interstellar colonization, as well as strategies for finding their technosignatures and artifacts,” he said.
Rogue planets, either in the Milky Way or some of the other hundreds of billions of galaxies, may carry their own life with them in subsurface oceans that are kept warm by radiogenic decay. Then, if they come across a star and join it gravitationally, this life has effectively used a rogue planet to hopefully be transported to a more hospitable place. So why couldn’t a civilization imitate that?
We think that floating planets are dark, cold, and inhospitable. And they are unless they have warm subterranean oceans. But they also offer some benefits. “Free-floating planets can provide a constant surface gravity, large amounts of space and resources,” Romanovskaya writes. “Planets that float freely with surface and subsurface oceans can provide water as a consumable resource and to protect themselves from space radiation.”
An advanced civilization could also design the planet for an even greater advantage by directing it and developing energy sources. Romanovskaya suggests that if we are about to use controlled fusion, it is possible that advanced civilizations are already using it, which could turn a cold rogue planet into something that could support life.
The author describes four scenarios where ETCs could take advantage of rogue planets.
The first scenario involves a rogue planet passing through the home world of an ETC. The frequency with which this can happen is related to the number of rogue planets in general. So far no one was able to send in the perfect solution, which is not strange. In 2021, a team of researchers announced the discovery of between 70 and 170 rogue planets, each the size of Jupiter, in a region of the Milky Way. And by 2020, a study suggested there could be up to 50 billion in our galaxy.
Where do they all come from? Most are probably ejected from their solar systems due to gravitational events, but some can be formed by accretion as stars do.
Another source of rogue planets is the Oort cloud in our Solar System. If other systems also have a cloud of objects like this, they can be an abundant source of rogue planets ejected by stellar activity. Romanovskaya writes: “Stars with 1 to 7 times the mass of the sun that undergo evolution after the main sequence, as well as a supernova of a parent with 7 to 20 times the mass of the sun, can expel objects from the cloud. Oort of their systems so that these objects become “detached from their host stars”.
But how often can an ETC, or our civilization, wait for a rogue planet to get close enough to hitchhike? A 2015 study showed that the binary star W0720 (Scholz star) passed through the Oort cloud of our Solar System about 70,000 years ago. Although it was a star and not a planet, it shows that objects pass relatively close together. If studies predicting billions of free floating planets are correct, it is likely that some of them passed close by, or directly through the Oort cloud, long before we had the means to detect them.
The Oort cloud is far away, but a sufficiently advanced civilization could have the ability to see a rogue planet approach and go out to find it.
Distance; the inner solar system (top left), the outer solar system (top right), the Sedna orbit (bottom right) and the inner edge of the Oort cloud (bottom left) the left). Image credit: NASA
The second scenario involves using technology to direct a rogue planet closer to the home of a civilization. With enough technology, they could choose an object from their own Oort cloud, assuming they have one, and use a propulsion system to direct it to a safe orbit near their planet. With enough time, they could adapt the object to their needs, for example, by building underground shelters and other infrastructure. Perhaps, with the right technology, they could alter or create an atmosphere.
The third scenario is similar to the second. It also involves an object from the outer Solar System of civilization. Romanovskaya uses the dwarf planet Sedna in our Solar System as an example. Sedna has a very eccentric orbit that takes it from 76 AU from the Sun to 937 AU in about 11,000 years. With enough technology and delivery time, an object like Sedna could become an escape ship. The author notes that “Civilizations capable of doing so would be advanced civilizations that already have their planetary systems explored at a distance of at least 60 AU from their host stars.”
Artistic conception of Sedna, a dwarf planet in the solar system that is only 76 astronomical units (Earth-Sun distances) from our Sun. Credit: NASA / JPL-Caltech
There are many potential problems. Carrying a dwarf planet from the far reaches of the Solar System to the inner Solar System could disrupt the orbits of other planets, causing all sorts of dangers. But the dangers are mitigated if a civilization around a star after the main sequence has already migrated outward with the changing habitable zone. Romanovskaya analyzes the energy required and the time required in more detail in his article.
The fourth stage also includes objects such as Sedna. When a star leaves the main sequence and expands, there is a critical distance where objects will be ejected from the system instead of being gravitationally bound to the dying star. If an ETC could accurately determine when these objects would be ejected as rogue planets, they could prepare them in advance and leave the dying solar system. This could be extraordinarily dangerous, as periods of violent mass loss of the star create enormous danger.
Three rings of ejected gas move away from an aged star called V Hydrae, seen in this false-color radio image of the Atacama Large Millimeter / Submillimeter Array in Chile. Image credit: ALMA
In all these scenarios, the rogue planet or other body is not a permanent home; is a lifeboat. “For all of the above scenarios, floating planets may not serve as a permanent means of escaping existential threats,” the author explains. “Due to declining heat production inside them, these planets are ultimately unable to sustain oceans of liquid water (if those oceans exist).”
Floating planets are also isolated and have fewer resources than planets in a solar system. For example, there are no asteroids per mine, no free solar energy. There are no night or day stations. There are no plants, no animals, no bacteria. They are simply a means to an end. “Therefore, instead of making free floating planets their permanent homes, extraterrestrial civilizations would use free floating planets as interstellar transport to reach and colonize other planetary systems,” Romanovskaya writes.
In her article, Professor Romanovskaya speculates on where this might lead. She imagines a civilization that does this more than once, not to escape a dying star, but to spread across a galaxy and colonize it. “In this way, the parent civilization can create unique, autonomous daughter civilizations that inhabit different planets, moons, or regions of space.”
“A civilization of cosmic hitchhikers would act as a ‘father civilization’ by spreading the seeds of ‘daughter civilizations’ in the form of their colonies on the planet …