Two Earths, one solar system
This is 'What If,'
and here's what would happen if there
was another Earth in our Solar System
In 2015, NASA's super powerful
Kepler Space Telescope
discovered the closest
copy of Earth to date.
It's called Kepler-452.
Often nicknamed "Earth 2.0,"
Kepler-452 orbits a star about the
same size as our Sun every 385 days.
Earth 2.0 is positioned within a
habitable zone relative to its star,
meaning it's not too hot
and not too cold,
and it's likely to have
a rocky surface too.
While this planet could
plausibly be a cousin to Earth,
But suppose we had a second
Earth in our Solar System.
Two identical Earths, with
perhaps no differences
other than their
respective inhabitants.
How long would this scenario last?
Is it gravity or a rival species
that we need to fear most?
If we're going to fit another Earth
anywhere in our Solar System,
our best bet is somewhere
between our planet and Mars.
Our Earth is on the inner edge of
the Solar System's habitable zone,
and with Mars closer
to the outer edge,
anything past it might be a little
too cold for a species like ours.
Let's imagine another Earth
that's close enough to our own
so that its inhabitants are comfortable.
The prospect of two planets
sharing the same orbit is possible,
but it wouldn't last forever.
Eventually, the gravitational interaction
between both planets would either
cause the two of them to collide; 
or one would end up pushing
the other one towards the Sun,
and we'd be back to just one Earth.
I hope it's ours.
But don't let that scare you,
it's possible that two planets
could co-orbit in harmony
for billions of years before
anything bad happened.
Another possibility is
a binary planet system,
in which two Earths of comparable
size could have separate orbits,
with one orbiting outside of the other.
Think of the Moon,
the Earth, and the Sun.
See how the Moon orbits the Earth, but
how they both end up going around the Sun?
But since we're talking about two planets
of similar size and gravitational attraction,
it's more likely that these two
Earths would swap positions.
We've already got a real life example
of this with two of Saturn's moons,
Epimetheus and Janus.
About every four years,
whichever moon is closer to Saturn
has a shorter orbital period
and catches up to its cousin.
As they get closer, the two
moons pull on each other,
causing one to slow down
and the other to speed up.
The one on the inside
gains speed and moves out,
the one on the outside loses speed
and falls closer to Saturn. 
But then, of course, the closer moon
to Saturn adopts a shorter orbital period,
speeds up, and overtakes its cousin
in about another four years.
But while we've seen moons do this, we've
yet to discover a binary planet system.
Let's just suppose that's how it works:
two Earths either sharing an orbit
or swapping between two.
Now, assuming that this second Earth
is inhabited by intelligent lifeforms, 
and that they've evolved at relatively the
same speed as our own human species,
It's impossible to say whether
we'd look the same, or act the same,
much less speak the same language!
Twin planets don't necessarily
mean twin species.
But before we start dreaming up
intergalactic trade or warfare,
maybe we could start with
something a little simpler.
Maybe we start trying
to learn from each other,
establishing communication through
radio waves or satellites. 
If they speak the same language, perhaps
a friendly visit wouldn't be too far off;
and if they don't, it'll be
quite the learning curve.
But we'll get there.
Think about it, we put a
man on the moon in the '60s,
and we've accomplished
a lot more since!
Nope.
But if you're wondering what else could
be out there, keep watching 'What If.'
