For years we've debated a fundamental question; Are we alone in the universe?

And our search for extra-terrestrial life usually has scientists studying distant, Earth-like

planets orbiting stable stars that are lightyears away.

But, what if we needn't look so far?

This is Unveiled, and today we're answering the extraordinary question; What if there

were two Earths in the solar system?

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There's actually a long-standing theory of a second Earth, or “Counter-Earth,”

dating back to the 4th Century BC where it was first hypothesised by the Greek philosopher

Philolaus.

The theory - which was initially proposed to explain an eclipse - says that the Counter-Earth

is always on the other side of the sun to us, meaning that it's never visible.

While we eventually learnt this isn't the way planets work, the Counter-Earth has become

a long-standing trope of science-fiction and a belief of many conspiracy theorists, who

still insist it exists just beyond our reach.

Scientifically speaking, if there really was another Earth we'd have found it by now,

by studying the orbits of the other inner planets or via satellites.

But even though the Counter-Earth doesn't exist, there are some other ways it'd be

theoretically possible.

One idea is that it'd follow an orbit that's the exact opposite of ours, which is why it

would remain on the other side of the sun.

There are already some solar system bodies that do something similar to this, called

trojans.

These are a type of object which stay fixed in the orbit of a larger body (usually a planet),

but don't orbit around that body as standard moons do.

Jupiter and Saturn, for example, have many.

Another type of shared orbit is a horseshoe orbit, where an object travels part of the

way around a planet and then turns around and goes back in the other direction.

Bizarre as this may sound, there are already instances of it happening in our solar system

- such as the Saturnian moons, Janus and Epimetheus.

In fact, even Earth boasts an object orbiting in this way, a very small asteroid called

2010 SO16.

While this still isn't a shared orbit in the way Counter-Earth theorists suggest, because

we know it's happening, it does provide an opportunity for another planet to exist

along the same path as we do.

However, if there was another Earth with the exact size, shape and (crucially) the same

orbit as ours, the results would almost certainly be catastrophic.

No matter how stable the two planets were, perhaps remaining equidistant for millions

of years, their orbits would eventually speed up until one crashed into the other.

Since the Earth travels at almost 19 miles per second, or 67,000 miles per hour, two

Earths smashing into each other would wipe out all life on each of them instantaneously.

A second Earth wouldn't necessarily need to share our exact orbit, though.

If “Earth 2” were simply another inner planet within our solar system's habitable

zone, then the planetary pile-up could well be avoided.

It's seasons would be different, perhaps its rotation speed would differ too, but it'd

still have the potential for life, either if we colonised it or if it developed life

of its own.

So, let's imagine that the Counter-Earth does exist in this way, and is capable of

maintaining its orbit without crashing into us; How different, or similar, to our own

planet would it be?

Its ability to sustain even basic life generally depends on the presence of amino acids - regarded

by scientists as the “building blocks” for life on Earth.

Given that there's some evidence that amino acids didn't naturally occur on Earth, but

instead crashed here on meteorites - couldn't the same thing happen elsewhere?

We can't say for sure that Earthly life only happened because of amino acid-carrying

meteorites, but that particular part of the puzzle really could be repeated all over.

So, say the second Earth does harbour the ingredients for life; What now?

Since humankind has yet to meet an actual extra-terrestrial, it's impossible to say

how that life would develop - even if we shared a star system.

In some ways it feels logical that any intelligent lifeform would be a mirror image of us, since

they'd have had to adapt to survive in almost identical conditions.

But there's every chance that evolution would take a totally different route, with

another planet to work on.

Instead of intelligent primates like us, the dominant species really could more closely

resemble reptiles, fish, birds, or something else entirely.

An optimistic outcome sees 'us' and 'them' communicate effectively, sharing knowledge

and widening perspectives without resorting to violence.

But, the threat of war is clear.

For centuries, there have been precious few moments in Earth's own history when there

hasn't been a war happening somewhere…

So, the assumption that we'd immediately start fighting any aliens we meet isn't

that unlikely.

In fact, even if the second Earth housed no life of its own but was instead colonized

by some future space-travellers from our planet… over time we'd likely see conflict arise

between the old and new worlds.

And, in the periods between actual, physical battles, our Earth and the other Earth would

have each other under constant surveillance.

Perhaps the problem would be resources, seeing as the second Earth could be turned into a

mining paradise - with visitors from our planet stripping it of all its earth-like materials,

in a bid to top up our own depleting reserves.

Clearly, if “Earth 2” was already populated by an intelligent species, this would be a

major point of tension - with either us taking advantage of them, or them of us.

But, of course, if this Counter-Earth actually was on the exact other side of the sun, then

getting there would be almost impossible in the first place.

Our Earth travels 584 million miles around the sun every year, meaning that the second

Earth would always be 292 million miles away.

Given that Mars was a measly 34 million miles away from us at its closest known approach

in 2003, and we've yet to figure out how to send people there, we'd need massive

technological advances to make the commute.

Perhaps the lack of terraforming required once we did arrive would be an equal trade-off

for the exceptional distance… but the journey would still demand whole generations of human

beings to be born, live and die on huge colonizing ships suspended in space.

The final destination could be a glorious, green utopia capable of hosting a population

of billions, full of new creatures to discover, places to explore and resources to enjoy.

But the paradise could also come at a cost, with the prospects of war, invasion and orbital

collision all clouding our connection with the brave new world.

And that's what would happen if there were two Earths in the solar system.

What do you think?

Is there anything we missed?

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