why aliens may struggle with space flight
Science fiction likes to show us aliens that explore the galaxy with ease. But the reality may for intelligent life on other worlds may be very different.
Generally, when we think of habitable worlds, we think of a planet with vast oceans, or a supersized Earth-like planet with a denser atmosphere and larger, hotter core. The logic is that water is a requirement for life, and the protection from stronger magnetic fields and denser atmospheres would give more complex life more chances to evolve into intelligent organisms and eventually create something like a civilization. We aren't completely confident about what these places may be like up close and personal, but we can do some math and basic modeling based on what we know, and the more we model and learn about these worlds, the more caveats we notice.
In case of ocean planets, water that covers too much of the surface and extends too far will smother potential sources of life by turning into a form of ice that exists only at absurdly high pressures. And a planet even a bit too massive may host intelligent life, but prevent it from ever exploring space. The latter is the conclusion of a recent paper which tries to provide yet another answer to the Fermi Paradox, the question of where are all the intelligent aliens if statistically, the galaxy should be full of them. Its answer may be that some worlds we'd think of as the most habitable places in the cosmos are less than conducive to communication technology and space travel.
On an ocean planet with relatively shallow seas, life would almost certainly seldom set foot on land since there may barely be any. Species able to think like us could use the water to communicate with each other for hundreds of kilometers, making it virtually unnecessary to create technology that does more than repeat certain vibrations, then pass them on with minor amplification. They would have no need for a working theory of electromagnetism and how to send data through the atmosphere. Without that, any possibility of reaching out and talking to other worlds may seem impossible to them in this context. They may still figure it out, but it's a lot less likely.
Meanwhile, a more massive planet will have a higher escape velocity, that is, how fast you have to go to launch your craft into orbit and beyond. Returning to such a planet would also be more complicated because there is more atmosphere to pass through, meaning that a spaceship will experience more atmospheric friction and needs much thicker and heavier heat shields. At some point, the paper posits, a planet would have such a dense atmosphere and so much gravitational pull that building spaceships will become highly unlikely as they would need to much fuel, too much shielding, and be too heavy to be practical.
But there's plenty of optimism to be found in the paper's conclusions as well. First of all, difficult or impractical doesn't mean impossible. After all, we found a way to leave our planet on a regular basis, walk on the Moon, and launch machines throughout and beyond our solar system despite the engineering and mathematical challenges. Aliens with enough determination and motivation could accomplish very unlikely things. And secondly, according to the calculations, a planet massive enough to pose a significant gravitational burden for takeoff would almost certainly be a gas or ice giant where the odds of intelligent, spacefaring life evolving would be almost nil.
Since planetary scientists think that anything more than two of three Earth masses is almost certain to become a mostly gaseous ice planet like Neptune, and at about 2.5 times the mass of our planet is when you start seeing the aforementioned problems, meaning that anything under twice our planet's heft allows for space exploration by a clever and motivated enough species. The only real caveat is that intelligent life in an ocean would have to figure out how to launch through a column of water and create hybrid propulsion systems. Then again, as we noted, a lot of complicated things are possible with enough determination...
See: RodrÃguez, E.Q., (2023) Introducing the Exoplanet Escape Factor and Fishbowl Worlds, JBIS, 76, pp. 365-368, DOI: 10.59332/jbis-076-10-0365
