Rocky worlds larger than Earth are commonplace in the galaxy, and a few of them may even be habitable. Which poses an interesting question: How difficult would it be for aliens living on these super-Earths to launch rockets into space, given the tremendous gravity? According to new research, it would be difficult to the point of impossibility - meaning that some aliens may be perpetually trapped on their home worlds.
Artist's depiction of Kepler-22b - a super-Earth located 910 light-years from our Sun. Illustration: NASA/Ames/JPL-Caltech
That we can launch objects into space using rockets is something we tend to take for granted, but what if our planet were heavier? Like, a lot heavier? It's a reasonable question to ask, given the prevalence of super-Earths in the Milky Way - large, rocky exoplanets with strong surface gravity.
Intelligent extraterrestrials who live on such planets may thus have a difficult time using chemical rockets to leave their world, as new research suggests. This doesn't mean space travel is impossible for super-Earthlings, it just means they'd have to find another way to do it.
For SETI enthusiasts, this is an important question to ask, as planet size may determine which aliens get to explore and colonise space and which are doomed to stay in their cradle. This issue has added relevance given that super-Earths, despite their tremendous girth, may be very capable of fostering life.
And in fact, astrobiologists have even speculated that some super-Earths may be "super-habitable"; the strong gravity of these worlds could hold thick atmospheres that protect life from harmful cosmic rays (imagine how much you'd save on sunscreen!), and their flat topography and intense surface erosion would create an "archipelago" planet with warm, shallow oceans suitable for biodiversity. If located in the habitable zone around a star, such a planet could go on to produce intelligent beings who, like us, would eventually look upwards at the universe beyond.
Here on Earth, we've solved the challenge of getting ourselves into space thanks to the happy fact that chemical propulsion is sufficient for producing the speeds required to leave our planet's gravity (Earth's escape velocity is 11km/s). But the amount of fuel required to deliver a rocket into space increases at an exponential rate as planet surface gravity increases.
In his new two-page study (published last week to the preprint arXiv server and not yet scrutinised by peer review), Michael Hippke, an independent astrophysicist affiliated with Sonneberg Observatory, sought to determine the practical limits of using conventional rocket technology to leave super-Earths, finding that chemical propulsion is highly impractical for escaping worlds 10 times heavier than Earth.
To demonstrate this, Hippke considered the hypothetical challenge of launching conventional rockets from the surface of Kepler-20b, a super-Earth located 905 light-years from our Solar System. This planet is nearly 10 times heavier than ours and has an escape velocity that's 2.4 times greater. That doesn't sound like a huge difference, but the maths tells a very different story. To launch a satellite weighing 6.6 tonnes, for example, a rocket would have to be loaded with 60,600 tonnes of fuel - which is about the weight of a large battleship. To launch the 50 tonnes of cargo used during the Apollo missions, a rocket would need 440,000 tonnes of fuel, which is roughly equal to the mass of the Great Pyramid of Giza.
Artist's depiction of Kepler-62f, a super-Earth located 1200 light-years from Earth. Illustration: NASA/Ames/JPL-Caltech
That's obviously a completely impractical feat, but even if it were possible, it would be completely unsustainable, as "a sizable fraction of the planet would need to be used up as chemical fuel per launch, limiting the total number of flights," writes Hippke in his paper. One option would be to launch rockets from the highest mountain tops, but given the flat topology of super-Earths, tall mountains would be hard to come by.
Another challenge would be the complete lack of terrestrial surface area; most super-Earths, it has been argued, are waterworlds, in which the entire planet is covered by a single, massive ocean. If we assume that human-like intelligence can emerge on a waterworld, Hippke says space exploration is not completely out of the question.
"Rockets on waterworlds could either be launched from floating pontoon-based structures, or directly out of the water. Underwater submarine rocket launches use classical explosives to flash-vaporise water into steam," he writes. "The pressure of the expanding gas drives the missile upwards in a tube. This works well for ICBMs launched from submerged submarines. These aquatic launch complications make the theory of oceanic rocket launches appear at first quite alien; presumably land-based launches seem equally human to alien rocket scientists."
More plausibly, Hippke says super-Earthlings would have to develop other ways of getting into space, including nuclear-powered rockets (dangerous!) and space elevators (which could only be built after the aliens got into space).
Anders Sandberg, a researcher at Oxford's Future of Humanity Institute, likes the new paper, but he said super-Earths are likely to be waterworlds, which means there would be "trouble getting metals to build rockets, and perhaps trouble getting life started, since that may require a wet-dry cycle for early biomolecules".
At any rate, Hippke's paper suggests there are physical conditions that would prevent eventual space settlement. For us, this doesn't seem to be the case, so it's unlikely that Hippke's paper is a good explanation for why we haven't been visited by aliens (that is, the Fermi Paradox), Sandberg said. "It might just mean that there are indeed [aliens who are] practically locked onto their worlds," he told Gizmodo.
Avi Loeb, chair of Harvard University's astronomy department, said the new paper is only half the story. "The key point is that escape from the orbit is much more challenging than escape from the surface of the planet," he told Gizmodo.
Again, it's another example of how lucky we are here on Earth; not only is it relatively easy for us to escape our planet's gravity, it's also relatively easy for us to escape the Sun's gravitational pull (escape velocity from the Sun at Earth's position is about 42 km/s). That's why we've been able to send the Voyager probes and the New Horizons spacecraft to the outer reaches of the Solar System.
But as Loeb pointed out in a paper released a couple of weeks ago (also available on arXiv and also not yet peer reviewed), some exoplanets - especially those around dwarf stars - may preclude intelligent life from venturing deeper into space and embarking on interplanetary travel. Chemical rockets, Loeb argues, wouldn't be strong enough, and those explorers would need alternative technologies such as solar sails.
Studies such as these make us realise how special our planet really is. Hopefully our civilisation can keep it together so that we'll be able to continue enjoying our special place in the cosmos.