Despite the vastness of the cosmos, we've yet to find any evidence that extraterrestrial intelligence exists. It's a conundrum known as the Fermi Paradox. Quite simply, the odds are strong that aliens are everywhere, and yet, nada.

Now a new theory has been proposed to explain this paradox, and it's one that's so simple that it's puzzling why no one has thought of it until now.

The theory is the brainchild of Michael Hippke, a researcher affiliated with the Sonneberg Observatory in Germany. He suggests that perhaps the reason we've never been contacted by aliens is because they're all pinned to their planets by gravity, reports

We're fairly fortunate here on Earth. We're not too hot, not too cold, so that liquid water is plentiful. Our planet's atmosphere shields us from radiation. And the Earth isn't so massive that we can't build rockets and launch things into space without too much gravity making such endeavors irrevocably impractical.

That last point, though, is one we've been taking for granted. In the search for extraterrestrial life, astronomers have been looking for exoplanets that could have many of the conditions that make Earth such a habitable place, but many of these planets that have been identified are so-called "super-Earths." In other words, they're bigger than Earth is, and therefore, ought to have a much stronger gravitational force.

"On more-massive planets, spaceflight would be exponentially more expensive," explained Hippke.

In fact, once a planet gets too big, spaceflight might even be impossible, at least regarding the type of spaceflight that we utilize here on Earth.

Rich lives, but stuck where they are

To put things in perspective, if aliens were to attempt to launch the equivalent of one of our Apollo moon missions from a super-Earth, they would require a rocket that would need to have a mass of about 440,000 tons (400,000 metric tons), due to fuel requirements, according to the study. That's a mammoth rocket on the order of the mass of the Great Pyramid of Giza in Egypt.

If super-Earths are too massive, it could take "a sizeable fraction of the planet" in fuel just to launch a single rocket. In his paper, Hippke calculates that once a planet gets up over 10 times Earth's mass, chemical rockets simply become impossible because there wouldn't be enough fuel available to mine.

Aliens on such worlds might live technologically rich lives, but they'd be stuck where they're at.

It's a theory that significantly limits the number of planets where extraterrestrial life can possibly achieve spaceflight. It therefore changes the equation about why we haven't met E.T. yet.

Of course, it's possible that intelligent aliens could come up with another solution besides chemical fuel. In fact, perhaps the fact that chemical fuel is enough for us here on Earth has placed limitations on how our rocket science has evolved; we've taken the easy route, and haven't had to develop alternative, more powerful methods for launching ourselves into space. Even so, it's certainly true that not having an easy route available makes it more difficult, and thus less likely, that civilizations on super-Earths would have taken to the stars.

"Civilizations from super-Earths are much less likely to explore the stars," Hippke said. "Instead, they would be to some extent arrested on their home planet and, for example, make more use of lasers or radio telescopes for interstellar communication instead of sending probes or spaceships."