While we have still not found life off-Earth, we have found many exo-planetary systems. The planets most likely to sustain life are eyeball exo-planets–and they’re drastically different than our home planet.

What is our planet made of? How does it work? Where does the Earth belong in the universe? Is life an unlikely happenstance?

Humans have pondered these delicate questions since the dawn of civilization.

According to the histories we’ve retained, Greek philosophers were among the first to try to answer these questions.

They perceived the Earth as a flat disk surrounded by an endless circular river. Above, there was a dome-like firmament of Heaven. Here, Zeus and Hera ruled the domain of the gods.

Read More: Everything you Need to Know About the Theory of Everything

Aristotle and Ptolemy defended the theory of Geocentrism. According to this theory, Earth is a motionless body at the center of the universe. Around this version of Earth, all celestial bodies, including the Sun and stars, orbit.

As you may know, the Geocentric model lasted until the end of the 16th century when Copernicus challenged it with his Heliocentrism theory, claiming that the Earth revolves around the Sun. This theory extrapolated that the Sun was now the center of the universe, not Earth.

Since then, we’ve come to know that neither the Earth nor the Sun is the center of the universe.

In fact, the Milky Way is only one of the hundreds of billions of galaxies in an expanding universe. In all, the human-centric theory of us being the focal point of the universe was rendered meaningless.

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This mysterious, infinitely enormous universe of ours gave rise to sentient life, at least here on Earth.

This raises the question: what are the chances of humans being the only sentient life in the universe?

Before we get to answer this question, we have to identify the “habitable” worlds that could support life.

The Goldilocks Zone, Habitability, and Earth Twins

An exoplanets’ theoretical ability to host life, or habitability, is largely based on one reference: Earth.

There are about 100 billion stars, give or take a few billion, in the Milky Way alone. As far as galaxies go, that’s on the low end. Estimates for this figure can go up to 400 billion stars.

Most stars have solar systems with at least one planet, so we can safely assume there are more planets than stars in the universe.

In 2016, researchers simulated the number of terrestrial (rocky) planets using a model of the universe that integrates all the known physical variables.

According to their conclusions, there are hundreds of billions of billions (≈2×1019 to ≈7×1020) of terrestrial planets orbiting around stars. Some of these stars are like our Sun, while many others are less-luminous types like red dwarfs.

In searching for alien life, scientists generally focus on worlds that resemble Earth. As such, in addition to being telluric, candidate exoplanets need to meet two major criteria: their size, and the distance from their home star.

To be eligible for consideration, a rocky planet must be neither too big nor too small.

They also have to be located in the habitable zone of its star for the temperature to allow for liquid water to exist on the surface.

As of writing this article, there are 3,605 exoplanets that have already been confirmed, with another 4,496 candidates needing further evaluation. Among these, 890 are rocky planets.

Taking into account the above criteria, the Habitable Exoplanets Catalog estimates the number of potentially habitable planets at 53, including 22 Earth-size ones.

The HEC even assigns an ESI score (Earth Similarity Index) to each of these planets, with Proxima Centaury b and Trappist-1e topping the list with 0.85s.

Red Dwarfs, Best Hope to Find ET Life

The hunt for alien planets likely to host life is in full swing.

In the last couple of years, more and more planets are being discovered outside our solar system, with many believed to be potentially habitable.

To most of these researchers, habitability boils down to liquid water flowing on the surface of a planet.

One of the most significant scientific landmarks of last year was the discovery of the alien solar system known as Trappist-1.

Recent studies revealed that these alien worlds are mostly rocky planets and are up to about 5% water. That’s 250 times more than Earth’s 0.02%!

These seven Earth-like planets orbit around the Trappist-1, which is a red dwarf star.

While the habitability zone or even the existence of liquid water can’t guarantee there will be life, this discovery should encourage astronomers to zero in on red dwarfs and planetary systems they may host.

The reason why we should make red dwarfs the main target is simple: red dwarfs are believed to be the most common star type (up to 70% of all stars) in the universe.

Then there’s also the fact that they tend to exist longer, sometimes for trillions of years, giving life a better chance of evolving.

Eyeball Exoplanets and the Twilight Ring

The habitability zone of a system depends on the nature of the star. The bigger the star, the farther out its habitable zone will be.

In the case of red dwarf stars, their habitable zone is much closer. Red dwarfs are about 20% of the mass of the sun and up to 50 times less radiant.

The relatively small size of red dwarfs means that any planet orbiting them would be much closer.

That also means that they orbit their host star more frequently, which makes their detection easier using the transit method or other observation techniques.

However, any planet lying near a red dwarf star is likely to be subjected to a strange phenomenon.

As it orbits the red dwarf very closely, these planets are usually forced by the star’s gravitational pull into synchronous rotation, which will make it permanently show the same side to the star.

This phenomenon, also known as tidal locking, isn’t actually so uncommon as a concept.

Earth exerts the same effect on the Moon, which takes four weeks to rotate around its axis and the same to turn around the Earth. That’s why we always see only one side of the Moon.

Tidally-locked exoplanets are called “Eyeball” planets because of the look they might give, with one side permanently facing heat and the other frozen in a night without end.

Your imagination can really go wild on these planets. For example, in some cases, there could be an ocean covering the bright side with ice all around it.

However, in between these two extremes, there is a twilight temperate zone that runs through the Eyeball planet’s middle and could gather the conditions for life to thrive.

Here’s how astronomer Sean Raymond describes the situation in his article: “Forget Earth-Like, We’ll First Find Aliens on Eyeball Planets”:

The day side is roasting and dry. The night side is frigid and icy. In between, it’s just right! The sweet spot—let’s call it the “ring of life”—is at the terminator, the boundary between night and day. The ring of life is bounded by deserts on one side and ice on the other. There is a constant flow of water from the night side to the day side—a series of rivers, all flowing in the same direction. The Sun is fixed in the sky right at the horizon, and the area is in permanent light.”

If the exoplanet is an “eyeball”, the hemisphere that is always facing the star would be the pupil. Per Raymond, there are many types of Eyeball planets, such the “hot Eyeball planets” and the “cold Eyeball planets”.

The concept of Eyeball exo-planets was first suggested with the discovery of Gliese 581g, the first exoplanet that was thought to have potentially habitable signs. It was then revealed that Gliese 581g might not exist at all.

Based on the hypothetical prevalence of tidally-locked Eyeball planets (based on the prevalence of red dwarfs), statistically-speaking, it’s most likely that we’d find alien life on one of these planets than other types of planets.

In all, the question of whether we are alone in the Universe or simply not looking with the right methods is a debate which is likely to continue until we either find life, or we run out of places to look. For now, these exo-eyeball planets give us hope for the possibility that we might not be the only intelligent life in existence.

Can you imagine what forms of life would exist in the twilight ring of Eyeball planets, between the scorching heat and the deathly cold? 

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