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Where to start looking? -

There are estimated to be up to one septillion stars in the known universe (that’s a one followed by 24 zeros!). And each of these stars could potentially have a dozen planets orbiting. So, how can we find alien life? The first step is to look closer to home.

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Earth and its conditions -

The Earth is pretty much the perfect place to live, astronomically speaking. The existence of water and a stable atmosphere has created an ideal environment for life to thrive.

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The Habitable Zone -

The distance of a planet from its star is crucial in understanding a planet’s viability in supporting life. Earth is located perfectly in what is called the Habitable Zone, where conditions are neither too hot, nor too cold to support life.

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Goldilocks Zone -

The Habitable Zone is also called the Goldilocks Zone, named after the fairy tale. In the story, Goldilocks chooses between three sets of items and ignores the ones that are too extreme (large or small, hot or cold), instead settling for the one that is “just right.”

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Venus -

Venus, which is similar to the Earth in mass, is too close to the Sun to be viable for life. Planetary scientists believe that Venus may have had surface water once, but the planet is now choked by carbon dioxide and a temperature of 864°F (462°C).

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Mercury -

Mercury is even closer to the Sun than Venus, not to mention that it is too small and doesn’t even have an atmosphere to support life.

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Narrowed down -

By understanding the criteria for the Goldilocks Zone, astronomers were able to figure out that there could be as many as 40 billion planets orbiting in habitable zones of the Milky Way.

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Written in the stars -

So far, astronomers have only been able to confirm 1,780 planets beyond our solar system. And of those planets, only as many as 16 are located in their respective habitable zones.

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Too big or too small - But it isn’t enough for a planet to be the right distance from a star. Size definitely matters. A small planet cannot maintain an atmosphere, while a planet that’s too big will have a crushing atmosphere.

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Earth as a scale -

In order for atmospheric pressure to be just right, a planet will need to be between 0.1 and 10 times the Earth’s mass. This is a very narrow habitable zone, especially when you consider how many planets in the universe are hundreds of times bigger than ours.

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Gliese 581c -

In 2007, the scientific community was abuzz with news of Gliese 581c, the first Super-Earth in the Goldilocks Zone. However, it was soon discovered that the planet’s surface conditions were brutally similar to Venus.

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Kepler-186f -

An Earth-sized exoplanet by the name of Kepler-186f was discovered in 2014, orbiting in a red dwarf star’s habitable zone. Astronomers are still trying to determine whether the planet can support life.

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Proxima Centauri b -

The closest exoplanet to Earth is Proxima Centauri b, which is in orbit around the closest star to the Sun, about four light years away. But although it is a Goldilocks planet, scientists are unsure whether it has an atmosphere.

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Other important criteria -

Although a planet’s Goldilocks zone and atmosphere are important in understanding its habitability, this isn’t where it ends. Planets must meet a plethora of other criteria, including the ability to sustain liquid water.

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Mars -

Mars is the only other planet in our solar system that is within the habitable zone. But the right atmospheric pressure can only be found at the planet’s low elevations, and there has been no confirmation that the planet has any liquid water.

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Desert planets -

Hypothetically, dry planets could retain water via oases, which would mean that such planets can orbit closer to a star and outside the Goldilocks Zone.

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Ocean planets -

It has been theorized that Earth’s oceans came into existence after icy bodies impacted our planet and subsequently melted. Other such planets could exist.

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Moons -

Planetary natural satellites, or moons, could also potentially be habitable. But these moons must be located within the Goldilocks zones of their host planets, and they must orbit far enough that they do not become volcanic worlds like Jupiter’s moon Io.

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Europa -

Jupiter’s fourth-largest moon, Europa, is thought to be home to a water ocean trapped beneath the planet’s surface, which could potentially harbor alien life.

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Magnetic fields -

Earth’s magnetic field is an example of another important criterion that determines the viability of life, as it shields the planet’s surface from radiation and cosmic rays. Other planets would need a similar field to allow complex life to flourish.

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Solar twin -

To date, astronomers have been unable to find any star that matches the Sun, a phenomenon called a solar twin. This has created much difficulty in the scientific community in finding extraterrestrial life that is similar to ours.

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Extraterrestrial life -

But what types of alien life could we potentially find? It has been largely accepted that carbon-based lifeforms are the only viable ways in which life can exist.

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Carbon chauvinism -

Many scientists have criticized the idea that alien life must be constructed from carbon. Hypothetically, extraterrestrials could be made up of vastly different elements that allow them to survive in places that are inhospitable to humans.

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Titan -

Saturn’s largest moon, Titan, is home to hydrocarbon lakes that could potentially support life. Titan is the only other object in space that has been discovered to have surface liquid.

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Are we unique, or just the first? -

Physicist Enrico Fermi (1901–1954) theorized that we haven’t encountered alien life because intelligent life is either rare or only just emerging in the universe.

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Close to the end -

Other physicists have claimed that we haven’t encountered alien life because we’re late to the party. Essentially, they say that extraterrestrials have mostly died and the universe is expanding too far for us to find their remains.

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The Great Filter -

Many anthropologists and astronomers believe that humankind is alone in the universe because of the Great Filter. Basically, there are so many improbable steps for life to evolve intelligence that it is unlikely to occur again.

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Zoo hypothesis -

Other scientists have hypothesized that extraterrestrial life does exist, but they are avoiding humanity in order to allow our primitive existence to unfold without intervention. Either way, space is a large frontier that may very well hold alien life.

Sources: (National Geographic) (NASA) (Astronomy.com) (Earth How) (Harvard John A. Paulson School of Engineering and Applied Sciences )

See also: The greatest discoveries and advancements in the history of astronomy

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