The image above shows an artists’ impression of the exoplanet Kepler-452b, the first exoplanet that was found to orbit a Sun-like star. Dubbed a super-Earth, the planet is about 60% bigger than Earth, which is the main reason why we may not want to send a manned mission there.
Although the total number of known exoplanets is now approaching about 4,000 or so, most of them are known to be gas giants. However, a small percentage of exoplanets are now thought to be rocky in nature, and an even smaller sub-set of exoplanets may very well be Earth-like in the sense that they are in stable orbits in the habitable zones around their host stars, have atmospheres, and have liquid water on their surfaces. Nevertheless, to date no Earth-like exoplanets with masses comparable to that of Earth have been positively identified.
In a recently published paper by German “citizen scientist” Michael Hippke, who is affiliated with the Sonnenberg Observatory in Germany, the author uses the exoplanet Kepler-452b as an example to explain why it may not be desirable to send a manned mission to super-Earth planets that typically have masses of several times that of Earth.
Essentially, the paper describes the engineering challenges the inhabitants of super massive rocky planets would have to overcome in attempts to achieve meaningful space flight capabilities. Of principal importance is the fact that even on Earth, getting a rocket carrying a small payload into orbit is exceedingly expensive in terms of fuel and energy expenditure. For instance, in order to achieve an Earth orbit, a launch vehicle has to have a mass of between 50 and 150 times that of the payload, with most of the weight of the launch vehicle being taken up by fuel.
Using the exoplanet Kepler-452b that is 9.7 times as massive as Earth as an example, Hippke used simple, high-school level math to calculate that a rocket requiring 9,000 tons of fuel to reach a stable Earth orbit, would require at least 55,000 tons of fuel to reach a similar orbit around Kepler-452b. As a practical matter, a rocket equivalent to the Saturn V rocket that carried the Apollo missions, would require at least 400,000 tons of fuel to lift a payload similar to the weight of the lunar landing modules into an orbit around Kepler-452b. Clearly, building rockets that can routinely carry fuel loads that compare favorably to the mass of a 100-storey skyscraper is beyond anything that current Earth-based technology and engineering skills can accomplish.
Of course, Hippke concedes that Kepler-452b is an extreme example, but he also points out that less-massive rocky exoplanets will present the same, but appropriately scaled engineering challenges. Moreover, in a closely related paper published by Abraham Loeb (Harvard University), the author makes the point that it may be extremely difficult, if not impossible to leave the surface of a planet that is orbiting close to red dwarf stars with chemically fueled rockets, since the gravitational pull of the host star would have to be overcome as well. While Kepler-452b does not orbit a red dwarf, it does orbit its host star in only 3.7 days at a distance of 0.5 astronomical units, which is 50% of the average distance between Earth and the Sun, and in turn, greatly increases the energy required to leave the surface of the planet.
Of particular note is the fact that Hippke’s paper provides one possible solution to the Fermi Paradox, in the sense that we don’t see alien life in the Universe because the inhabitants of planets that host life may not be able to leave those planets.
For this reason, visiting massive exoplanets may be a very bad idea from our perspective. Even if we did manage to get there, we may not be able to leave again simply because we could never carry sufficient fuel to escape from the combined gravitational pull of the massive planet and its host star, and like the residents of the Hotel California, we would be stuck there forever.