Extraterrestrials could be stuck on their exo-planet 'home world' due to 'physical limitations,' study claims - is this why we haven't found aliens?

'Where is everyone?' It's the famous question about aliens, echoing out to our quiet galactic neighbors after Manhattan Project physicist Enrico Fermi asked it in 1950.

But perhaps even a highly advanced extraterrestrial civilization might find itself without the resources or the key information needed to escape their home world, at least according to a new study from Spain's Atlántico Medio university.

Some so-called 'Super Earths' might be within their star's habitable zone, but so massive that their gravity makes interplanetary rocket launches all but impossible.

These and other 'Fishbowl Worlds' are just one of several ideas introduced in the new paper, which hopes to help explain Enrico Fermi's infamous 1950 'Fermi Paradox.'

Fermi's paradox has perplexed astronomers for over six decades, asking in short: In a universe teeming with as many as 200 billion trillion stars, and still more planets, many capable of supporting life, why haven't Earth's scientists spotted any aliens?

Even highly advanced alien civilizations might find themselves without the resources - or key information - needed to escape their home world, according to a study from Spain's Atlántico Medio university. Above, Jupiter's watery moon Europa, imaged by NASA's Galileo spacecraft

'The image of a planet whose gravity makes it difficult or impossible to leave suggested to me the metaphor of a fishbowl,' the new study's author Professor Elio Quiroga told DailyMail.com via email.

'I found it to be a powerful analogy,' Prof. Quiroga, who lectures at Atlántico Medio in Spain, said.

For one category of his Fishbowl Worlds, Prof. Quiroga calculated a value he termed the 'exoplanet escape factor' (Fex): a value comparing a given exo-planet's escape velocity to Earth's 7 miles-per-second (11.19 kilometers-per-second) escape velocity.

Escape velocity, the speed needed for a spacecraft to break free from the gravity of any celestial body — be it a moon, planet or asteroid — varies with that body's mass. 

Escaping the potentially habitable exo-planet Proxima Centauri b, four light-years from Earth, for example, is relatively easy: 5.9 miles-per-second, or an 'Fex' of just 0.85-times Earth's own.

But massive, yet hypothetically habitable planets like Kepler-131 b, 746 light-years from Earth, require a daunting speeds to break free: 21.8 miles-per-second to exit Kepler-131 b, for example, or an Fex of 3.13-times Earth.

Some so-called 'Super Earths' might be within their star's habitable zone, but so massive that their gravity makes interplanetary rocket launches all but impossible, like Kepler-131 b (left), 746 light-years from Earth. Others, like Proxima Centauri b (right), are easier to blast off from

'Many worlds, particularly super-Earths,' Prof. Quiroga told DailyMail.com, 'may be dismissed due to prohibitive escape velocities.'

But there were also interesting edge cases, like the habitable world GJ-1214b, 48 light-years from Earth, which has an escape velocity about 1.5 times that of Earth's. 

Planets like these, which also includes Kepler-103b, might prove harder for an advanced race to blast off from, but might not trap a species on their home world.

Prof. Quiroga's research, published in last October in the Journal of the British Interplanetary Society, theorizes that any escape velocity with an Fex more than 2.2 times Earth's own may lock a civilization onto their planet of origin for good.  

The more than 5,000 exoplanets confirmed in our galaxy so far include a variety of types - among them these mysterious 'super-Earths' that are larger than our world and possibly rocky

But Prof. Quiroga also pointed to unique cultural factors that might lock a species into their home planet, in one case discussing a more literal 'Fishbowl World.'

An advanced civilization on an ocean planet, he speculated, might have mastered long-distance communication via their own evolution, given that communication travels much further naturally in a fluid environment (think sonar) than in open air.

The dominant species on a watery planet, or a watery moon like Jupiter's moon Europa, may have enjoyed conversations that travel naturally for hundreds of miles.

In those worlds, Prof. Quiroga wrote, 'communication between individuals could be feasible without the need for communication devices,' stifling the urge to innovate advanced communication technologies.

There were also interesting edge cases, like the habitable world GJ-1214b (above), 48 light-years from Earth, which has an escape velocity 1.5 times Earth's: hard but possible to leave

'Telecommunications technology might never emerge on such a world, even though it could be home to a fully developed civilization,' Quiroga argued in his paper. 

'Such a civilization would not be "communicative" and would not be contemplated in the Drake equation,' the famous calculation formulated to predict the chance of finding intelligent life in the universe, Quiroga said.

An evolutionary feature like innate, biological, undersea sonar, in other words, could leave them both silent and unable to listen to humanity's SETI radio transmissions.

But, as Prof. Quiroga told DailyMail.com, his research is not a cause for astronomers and planetary scientists to drastically change policies or research plans just yet.

READ MORE: Could an advanced alien civilization wipe us out? Scientists identify two possible exoplanets where extraterrestrials may have had a five billion year head start on life on Earth

Cardiff University's Professor Jane Greaves has identified at least two potentially super-advanced worlds that might exist in our own Milky Way galaxy, orbiting stars 70 and 110 light-years away. 

'The initial step is to search for basic life, simple life-forms,' he said.

'We are making progress in this direction, but we require more advanced tools (such as the future Vera Rubin telescope) and improved methods for analyzing the faint signals emanating from these exo-planets.' 

'If we were to discover a world in another star system displaying clear and indisputable signs of intelligence,' Prof. Quiroga said, 'then we could contemplate whether those beings have achieved space travel, or if it's within their capability or not.'

What those first, successful signs, which researchers call techno-signatures, will turn out to be remains to be seen, he noted, in part due to the truly alien possibilities of life outside our world.

'We can only assert that we know of one civilization in the cosmos, and that is our own,' Prof. Quiroga said.

'Consequently, we tend to humanize or anthropomorphize everything; it's inevitable.' 

'However, there is something intriguing to consider,' he added. 

'We emerged as a species more or less in the middle of the lifespan of our star, the sun, which suggests something profound: intelligence takes a considerable amount of time to evolve.' 

Perhaps, Prof. Quiroga suggested, an answer to Enrico Fermi's famous paradox is that most alien species are taking just as much time to evolve as life did here on Earth.