Somewhere in a nondescript region of the Orion Arm of the Milky Way galaxy, which is a typical
spiral galaxy, there sits a typical G-type main-sequence star, roughly halfway through
its lifetime.
Moving in a bit closer, we can find a number of planets orbiting this star, some of which
are typical gas giants, and some of which are rocky planets.
The third closest of these from the sun glimmers unassumingly, but it quickly becomes apparent
that this planet is anything but typical.
Approaching more closely, we can see a thick atmosphere rich with molecular nitrogen and
oxygen, with clouds swirling in various weather patterns.
We see enormous bodies of liquid water covering the surface of the planet.
The land is covered with vegetation, the first sign of complex patterns of matter which we
call life.
But that’s not all.
There are animals too, some of which have become intelligent enough to alter the topography
of the planet.
These organisms have mastered not just the soil and plant life, but all the materials
at their disposal, creating tall buildings and all kinds of impressive technologies.
This is human civilization.
We are all a part of it.
And for as long as it has existed, it has been localized exclusively on this planet,
which we call Earth.
But will that always be the case?
As we move forward through the space age, new opportunities may arise to colonize other
worlds.
What might drive us to leave this perfect little planet?
Well for one thing, our safety is not entirely assured.
There is no telling when a rogue asteroid might head our way.
This is what killed the dinosaurs, and one might kill us too.
Then again, if we had the technology to leave Earth in large numbers, it’s pretty likely
that we’d be able to solve this problem somehow, by diverting or destroying the asteroid.
Even still, our planet has a finite lifespan.
When the sun nears the end of its life, it will become a red giant, swallowing up the
first three planets in the process, and vaporizing every last thing on them.
But of course this won’t happen for around five billion years, so it’s not a particularly
pressing matter.
In the end, if humanity ends up entertaining the idea of colonizing other worlds, it probably
won’t be to avoid extinction, but rather to explore and grow as a species.
After all, Earth may be our cradle, but every child must eventually learn to walk, and the
time to take our first big steps may be fast approaching.
Where will we go?
Everyone has heard of plans to send humans to Mars, and hopefully that will be underway
within a decade or two, but what comes next?
Well there are a number of moons in the Jovian and Saturnian systems that would be worth
a peek.
But should we find the solar system to be too small, there is an entire galaxy out there
begging to be explored.
\As we have known since the 1990s, other stars have planets around them too, which we call
exoplanets.
The first planet discovered around a sun-like star, 51 Pegasi B, was found in 1995, and
since then, thousands of others have been confirmed.
Earlier in this series we discussed a few nearby systems that harbor exoplanets, such
as TRAPPIST-1, Alpha Centauri, and Gliese 667.
But there are many more, such that exoplanets appear to be quite common, with many of them
being potentially Earth-like.
From around 2009 to 2018, the frontrunner in finding these worlds was the Kepler space
telescope, launched by NASA into an Earth-trailing heliocentric orbit for the specific purpose
of locating Earth-sized exoplanets orbiting in or near the habitable zone of their stars,
and it had a viewing range of around three thousand light years.
The searching was performed by monitoring the brightness of stars, looking for dimming
patterns indicative of exoplanets passing in front of them.
It found over 2,600 such planets during its tenure, the bulk of all exoplanets known,
so let’s take a look at a few of them.
Here’s the Kepler-11 system.
This features a sun-like star, found in the constellation Cygnus, a little over 2,000
light years from Earth.
The system is especially compact, with five planets all orbiting within a distance that
fits inside Mercury’s orbit around the sun, and the sixth orbiting just a bit outside.
All of these planets are larger than Earth, with some almost as large as Neptune, though
their compositions are not known.
Kepler-22, another sun-like star, is also found in Cygnus, but is only around 640 light
years away.
It has just one planet, Kepler-22b, which at 2.4 Earth radii, is somewhat larger than
Earth, though its composition is not known.
At the time of its discovery in 2011, it was the only relatively Earth-sized exoplanet
to have been discovered orbiting within a star’s habitable zone.
Then looking at Kepler-62, this star is a bit smaller and cooler than the sun, part
of the constellation Lyra, 1200 light years from Earth.
This one has five planets, two of which are in the habitable zone, and the outermost of
which, Kepler-62f, is likely to be a rocky planet.
Moving on to Kepler-186, this is an M1-type main sequence dwarf star, again in the constellation
Cygnus, only 582 light years away.
It has five planets, four of which orbit extremely close to the star, but the outermost, Kepler-186f,
is actually the first nearly identically Earth-sized planet in the habitable zone of its star ever
to be discovered.
Finally there is Kepler-452, a G-type main sequence star, again in Cygnus, about 1,400
light years away from Earth.
This one is just a bit larger and brighter than the sun, and its system is shown here
in comparison to both Kepler-186 and our own system.
As we can see there is one confirmed planet, Kepler-452b, the first potentially rocky super-Earth
ever found, given that it is approximately five times more massive than Earth, and is
likely to have a similar surface temperature, one of the most Earth-like ever discovered.
That barely scratches the surface of the worlds that the Kepler mission uncovered.
The mission showed us that there are more planets than stars in our galaxy, and that
small, rocky Earth-sized planets in habitable zones may be relatively common.
Some of these may even harbor life already, but either way, it’s interesting to imagine
the possibility of actually visiting these places in person.
So what are some of the things we are looking for when we examine exoplanets?
Apart from obvious things like size, mass, surface temperature, and orbital radius, we
can also gather information about the atmosphere of the planet.
We can do this by spectral analysis, which means analyzing the light we receive in our
telescopes.
This obviously works for bodies that are very close to us, like Venus, where the detection
of a molecule called phosphine, with the chemical formula PH3, has recently caused a bit of
a stir amongst astrobiologists who hypothesize that this may be a sign of rudimentary life
forms.
But we can also do this with distant bodies, not quite as directly, but when a planet passes
in front of its star, some of the light from the star passes through the planet’s atmosphere
on its way to us.
We can analzye that light in a similar way, and get an idea of what compounds are likely
to be present and in what amounts.
Although statistically speaking the probability is rather low, it’s not impossible that
we might find an Earth-like planet with an atmospheric composition that is almost precisely
the same as Earth, with mainly nitrogen, lots of oxygen, and trace amounts of other gases.
This means there could even be planets out there on which humans could breathe freely
without a spacesuit.
The possibility of exploring countless other worlds around the galaxy is as exciting a
notion as is conceivable.
Of course the enormous distances to these systems presents a staggering technological
barrier.
Whether this barrier proves to be completely insurmountable remains to be seen, but given
the potential for one or more paradigm shifts in propulsion, it would be naïve to say it
is impossible that we will one day begin to take our first few steps out into the Milky
Way, to see what we can find.
For the time being, we have our hands full taking our first steps out into our own system,
again with Mars being the current goal.
But humans setting foot on Mars seems to be very likely, and the planet could even serve
as a test piece for terraforming technology, should we figure out how to install a magnetic
field on the planet, thereby protecting any manufactured atmosphere from the solar wind.
If a few centuries into the future, Mars can be altered to fit our needs, then it should
follow that the same could be done with other worlds as well.
And if such practices can be mastered, the possibilities are truly endless.
There are billions of worlds in the Milky Way galaxy alone, and as unthinkable as intergalactic
travel may seem, should that ever become a possibility, there are billions of other galaxies
as well, which offer us practically infinite opportunities for exploration.
Just the way the ancients peered up at the moon, perplexed, while modern man has walked
upon its surface, perhaps future generations will look back on us with pity, as they hop
from system to system, the final odyssey for our species.
Only time will tell, but in the meantime, we can certainly dream of what comes next,
for the remainder of the 21st century and beyond.