Even If Aliens Live on Super-Earths, Science Says They May Be Stuck There
One of the major assumptions scientists and sci-fi fans have made about alien civilizations is that their journey into space is probably going to look like ours—they'll probably have their own rocket age, their own missions to explore their solar system, and maybe even interstellar spacecraft and probes, depending on how advanced they are. However, that may not be true for aliens living on a super-Earth, a planet similar in climate and composition to our, but several times our size.
A new study shows that on these massive planets, conventional rocketry (and space exploration as we know it) runs into some major problems.
The biggest issue is escape velocity, the speed something has to travel in order to escape the gravity of its planet.
For Earth, escape velocity is 11.2 kilometers per second, meaning that rockets need to go about 25,000 miles per hour (almost Mach 33) to escape Earth's gravity.
To go that fast, however, rockets need to carry a lot of powerful rocket fuel, which adds to their weight...which means they need even more fuel to lift that weight.
Eventually, the thrust-to-weight ratio balances out, and a rocket can carry enough fuel and create enough thrust to make it off the planet.
On a super-Earth like Kepler-20b, however, the escape velocity is about 2.4 times greater than here on Earth, meaning rockets would need to be several times larger, more powerful, and more expensive.
According to Michael Hippke, the author of a new study exploring the issue of spaceflight on super-Earths: "On more-massive planets, spaceflight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission or a Hubble Space Telescope."
According to Hippke's estimations, rockets would need to have a mass of about 440,000 tons on a super-Earth. For comparison, the Apollo 11 rocket weighed just 3,050 tons fully loaded.
The only thing for an alien civilization to do would be to invest in exponentially more expensive rockets...or go for the nuclear option.
A nuclear pulse propulsion system, where a series of atomic bombs propel a rocket instead of normal engines, could overcome the strong gravity of a super-Earth, but it would be a huge risk, according to Hippke:
"A launch failure, which typically happens with a 1 percent risk, could cause dramatic effects on the environment...I could only imagine that a society takes these risks in a flagship project where no other options are available, but the desire is strong - for example, one single mission to leave their planet and visit a moon."
Chalk up another possible reason we haven't found aliens: Space travel is hard.