Since the dawn of the space age, in the middle
of the 20th century, we’ve sent unmanned probes all over the solar system, which have
given us some incredible images of the planets and moons that are part of our system. There are plans to send humans to Mars, and
the Galilean moons of Jupiter shouldn’t be too long after that. But as exciting as it will be to colonize
our system, beyond Mars there are only a handful of moons that are even remotely fit for such
an endeavor. Our search for habitable worlds will inevitably
lead us outside of our own system. This poses a tremendous challenge, because
as far away as the other planets and moons in this system are, this distance is utterly
insignifcant compared to the vast distances to other stars. We will certainly need to innovate new methods
of propulsion in order to get there in a reasonable amount of time. But we will achieve this, and when we do,
where will we go first? The most logical destination once we achieve
the capacity for interstellar travel will be the closest star to our own sun. So which star is this? If we look to Centaurus, a constellation in
the southern sky, we will find this closest star. Or technically, not the single closest, but
rather, three stars that are extremely close together, which as a whole, represent the
system that is closest to our sun. This triple star system is called the Alpha
Centauri System. The closest of these stars is called Proxima
Centauri, named as such because of its proximity, and this star is almost certainly gravitationally
bound to the other two, which are called Alpha Centauri A and Alpha Centauri B, which together
form a binary system. So how far away are these nearest neighbors? They are farther than you think. The fastest probe we have ever built would
take over 50,000 years to get there. Even light, traveling at the universal speed
limit, would take more than four years to get there. Proxima Centauri is 4.23 light years away,
while the Alpha Centauri stars are 4.32 and 4.37 light years away. But if we can invent new methods of propulsion,
like fusion, or solar sails, or some other incredible new technology, it’s possible
that we could get there within a human lifetime. So what will we find when we get there? Well we would reach Proxima Centauri first. This is a dim red dwarf star, so it’s very
tiny and cool, as far as stars go. Its mass is only 12% of our sun’s, and its
radius is around 15% of our sun’s. It has one planet that we call Proxima Centauri
b, which is therefore the closest exoplanet to us, or the closest planet outside of our
solar system. It is roughly earth-sized, and has an orbital
period of around 11 Earth days. Its orbital radius is around 0.05 astronomical
units, or 5% of Earth’s orbital radius. Even though it is so close, because the star
is so much smaller and cooler than ours, Proxima Centauri b actually sits in the habitable
zone of its star, which mean it could harbor liquid water. Based on density calculations, it seems likely
that it has a rocky surface, potentially with features that would be familiar to us, making
it even more reasonable that this will be the first planet we set foot on outside of
our solar system. The surface temperature depends on the rate
of the planet’s rotation. There are two models that seem most probable. Proxima Centauri b may rotate in 3:2 resonance
with its star, which would allow for a relatively smooth distribution of heat from the star
as the planet rotates. But the planet may also be tidally locked,
exhibiting synchronous rotation, meaning that the same side always faces its star, like
the way the same side of the moon always faces the Earth. This would make one side very bright and hot
and the other side very dark and cold. The habitability of the planet will depend
greatly on which of these turns out to be correct. The atmosphere will also be a factor, and
this will depend on the strength of the planet’s magnetic field. The weaker the magnetic field is, the more
likely it is that activity from its star will have stripped away the atmosphere. The stronger the magnetic field is, the more
likely it is to have retained an atmosphere, despite stellar activity. We will continue to learn more about this
planet in the coming decades. There is also some evidence of another planet,
named Proxima Centauri c, that may orbit at 1.5 astronomical units, which is extremely
far from its star relative to the orbit of Proxima Centauri b. However, more data will have to be gathered
to verify its existence. Beyond this, there are two dust belts around
the star, one at around 1 to 4 astronomical units, and another at around 30. Together, these planets, one confirmed and
one unconfirmed, and some accompanying features, make this quite an interesting system. If we were to continue from here, it wouldn’t
be too long before we reach Alpha Centauri A and B. As we said, these make up a binary
star system, which means two stars that are quite close together, orbiting around their
center of mass, in this particular case once every 80 Earth years. Their orbit is elliptical so the distance
between them changes throughout the orbit, but on average they are about as far apart
as our sun and its outermost planets. They are both very sun-like, almost the same
as our sun in mass and size. It is possible that either of these stars
has one or more planets, and it is even possible that there could be a planet orbiting the
binary system as a whole. We do think it is highly likely that such
a planet exists, but there is not yet sufficient evidence to conclude this. We will continue to study this system from
here on Earth, as there is more data to gather, and hopefully later in this century, we can
send an unmanned probe in that direction, if we can figure out how to get it going fast
enough to make it worth our while. Perhaps if we can make it all the way there
and get some more information, we can shed some light on the habitability of the planet
or planets in this system. Could we ever colonize Proxima Centauri b? It is difficult to say. Because it is very close to Proxima Centauri,
which is a flare star, so it is subjected to intense stellar wind, which means plumes
of high-energy particles emitted by Proxima Centauri barrage the surface, with 2,000 times
greater intensity than our sun sends to us here on Earth. This would make it difficult to live there,
at least without protective habitats, and it also makes it pretty unlikely that we would
find any existing life when we get there, as the radiation would kill any microscopic
life we can conceive of. But we certainly don’t know for sure, and
we won’t know for sure until we set foot there ourselves. Hopefully we will unravel the secrets of the
Alpha Centauri system one day, and perhaps there are even people that are already alive
that will personally set foot on these worlds.
