Aliens exist but are ‘hiding in underground seas and imprisoned on super-Earths’

Where is everybody? The question, famously asked by pioneering nuclear physicist Enrico Fermi, is one that many people have wondered about at one point or another.

Our planet is teeming with life. Even in the most inhospitable environments; deep underground, in the depths of freezing Antarctic lakes, life will always find a way.

But as yet there is no hard evidence of life anywhere else in the universe.

Unless Copernicus was wrong, the Earth is not special. There are planets just like ours scattered throughout the Universe.

Even if breaking the light speed barrier is completely impossible, and there isn’t an interstellar empire thriving somewhere in our galaxy, we should be able to perceive the signs of alien megastructures and listen in on extraterrestrial communications.

Why not?

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Some theorists have suggested that there is a "Great Filter" that most habitable planets don’t make it through: either a purely biological one like the transition from single-celled to multicellular life, or a more technical one, like the difficulty of sustaining an industrial civilisation without succumbing to runaway climate change.

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UFOs, Aliens and Outer Space

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But maybe the answer to that question is simpler than that. Maybe we’re just looking in the wrong places.

We have already detected a number of ocean worlds, where the surface is either mostly or completely covered with water.

NASA’s Alan Stern thinks that water worlds would provide the kind of stable environment that intelligent life would thrive in: shielded from radiation and all but the biggest asteroid impacts, undersea life could evolve in peace.

"Impacts and solar flares, and nearby supernovae, and what orbit you're in, and whether you have a magnetosphere, and whether there's a poisonous atmosphere — none of those things matter," he told Space.com.

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But that liquid "suit of armour" is also a prison. "If they have technology, and let's say they're broadcasting, or they have city lights or whatever — we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]," he said.

Stern, who works on NASA’s long-range New Horizons space probe, adds that intelligent sea creatures would naturally have less interest in the stars than us, and their spacecraft – filled as they would be with water – would be very heavy and hard to launch.

It would be even harder to get off the ground if you lived on a Super-Earth.

Super-Earths – rocky planets with a mass between two and ten times that of Earth – appear to be very common in the Universe.

Since first being discovered in 1992, many more have been identified. Of the 1,000 or so exoplanets so far mapped, over 300 of them are Super-Earths.

Many of these are in what scientists consider to be the "Goldilocks Zone" of their parent star – not too hot and not too cold for liquid water: and therefore candidates for life.

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Not only habitable, they could be "super-habitable" with denser atmospheres providing better shielding against cosmic menaces such as asteroids and solar flares.

"On more-massive planets, spaceflight would be exponentially more expensive," says astronomer Michael Hippke. "Such civilisations would not have satellite TV, a moon mission or a Hubble Space Telescope.”

He says that humans are very lucky to live on a comparatively "lightweight" planet with a gravitational pull that’s weak enough to escape.

"Other civilisations, if they exist,” he says “might not be as lucky."

To launch a 1 tonne satellite from Kepler-20b (a Super-Earth about 1.87 times Earth's radius and 9.7 times its mass), a conventional-fuel rocket would need to be about 3 times the size of the Saturn V that carried Neil Armstrong and his crew to the Moon – a payload of around 45 tonnes.

If our imaginary Super-Earth dwellers actually wanted to stage their own equivalent of the Apollo programme, they’d need a rocket in the region of 400,000 metric tons, or roughly the size of the Great Pyramid of Giza.

In a paper published in the International Journal of Astrobiology, Dr Hippke drily notes that a pyramid-sized rocket "is probably a realistic limit for chemical rockets regarding cost constraints.”

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There are other explanations for our cosmic isolation. Perhaps we are in a simulated universe where no other intelligent species have been created. Maybe we being quarantined by super-intelligent aliens who don’t think we’re ready for contact yet.

Until we actually find extraterrestrial life we won’t know.

But as author and satellite communications pioneer Arthur C Clarke pointed out: “Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

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