Space travel is seen here on Earth as the ultimate accomplishment of an advanced civilization. However, for some hypothetical extraterrestrial species inhabiting super-earth planets, space travel may be impossibly hard due to the velocity needed to escape the gravity of their planet. These unfortunate beings would indeed have all the difficulties in the world to launch rockets from the surface of these telluric exoplanets much larger than the Earth.
At 21 light-years away, a small red dwarf star called Gliese 625 secretly kept a huge treasure until astronomers discovered it in 2017. There, in the star’s habitable zone, orbits a rocky planet 2.8 times more massive than Earth, belonging to the super-earth category. These worlds, very common in the universe, can reach up to ten times the terrestrial mass. This is the case of Kepler-20b, the largest super-earth known to date, which revolves around the Kepler-20 star, 945 light years from our planet.
For any hypothetical inhabitants, this kind of exoplanet would be a kind of paradise. The thicker atmosphere, retained by the higher gravity, blocks better the harmful radiations. Huge shallow oceans, studded with emerged lands, could cover the flattened and eroded surface, transforming the super-earth into an “archipelago-planet”. But if these extraterrestrials would one day desire to leave, they would find it very hard to escape the gravity of their giant planet, according to Michael Hippke, independent researcher affiliated to the observatory of Sonneberg in Germany.
In an article, available on arXiv, pending its publication in the International Journal of Astrobiology, the researcher stresses the limitations of space technologies that are used on earth when they are transposed on other planets, such as super-earths. Indeed, high gravity greatly complicates space travel. Thus, to take off from an exoplanet of ten earth masses, a conventional rocket with chemical propulsion will have to weigh 400,000 tons! In comparison, the Saturn V launcher developed by NASA for the Apollo missions on the Moon, by far the most powerful ever built, weighed only 3,050 tons.
While humans live on a planet of a reasonable size, compatible with space travel, “other civilizations, if they exist, might not be so lucky,” says Michael Hippke, interviewed by Space.com. To take off from a super-earth, the extraterrestrials will have to redouble their efforts. A rocket must reach the so-called liberation speed of 11.2 km/s to escape the gravitational pull of the Earth, but one a planet ten times the size of earth, a speed of 27.1 km/s is needed.
It requires a more powerful rocket, which involves more fuel, weighing down the craft. However, the mass of fuel required increases exponentially depending on the weight. Astronomical dimensions are reached very quickly, as is proved by Michael Hippke’s calculations.
For example, with a payload of 6.2 tons, which amounts to carrying the James-Webb Space Telescope, a rocket launched from Kepler-20b will have to carry 55,000 tons of fuel. For larger loads, of the order of 45 tons required during the Apollo missions to the Moon, a giant rocket of 400,000 tons will be needed. Worse, for planets beyond ten-earth masses, if they exist, the researcher believes that propelling a single rocket would consume almost all the exoplanet’s fuel.
As a result, according to the researcher, “super-earth civilizations are far less likely to explore the universe. They would rather be confined to their planet and, for example, would use lasers or radio telescopes to communicate through space instead of sending probes or spacecraft.” However, his calculations are valid for traditional rockets with chemical propulsion. Other technologies, such as nuclear-powered rockets, could be used.