Earlier today, NASA held a press conference to announce a
highly unusual star system known as Trappist - 1 that is currently setting the
fields of planetary science and exobiology afire. This system doesn't just
contain one earth-like world within its habitable zone, but at least an
astonishing three that could conceivably host life as we know it. And, they are
close at just under 40 light-years away.
The planets were discovered through a method known as transit
photometry initially with the Trappist telescope in Chile
and further studied with the Spitzer space telescope. Transits occur when
planets pass in front of their host star and block just a little bit of its
light. In fact, this is how we found out about Boyajian's star and many of the
known exoplanets. Looking at how much of the light was getting blocked told
scientists how big these worlds were. It turns out they're small, about the
size of earth. But how much like earth?
The key here is the availability of liquid water, like our
oceans. You have to be a certain distance from your star in order for water to
avoid freezing solid but also not evaporate into gas. Water is chemistry's
universal solvent, the medium where organic reactions can occur. This is key
for all life on earth, everything needs water to exist. There are a few other
ways life might manage to exist, but water-based organic chemistry is by far
the best chance.
That's where these new planets come in. They seem to not
only have the possibility of bearing liquid water like a planet like Proxima B
or Earth, but the conditions are such for all three that, as far as we know so
far, they probably do. And there are a further four planets in this system that
are also seemingly rocky and are about the same size of earth, but it's not
completely clear yet if they are also within the zone of habitability. In other
words, this is a star system that could have seven earths orbiting it.
Now two of these worlds are probably tidally locked to their
star. That puts them in the same league as Proxima B where they would have a
ring of habitability in the twighlight zone of the planets. But the sunlit
sides would likely be rather hot and the night side a frozen wasteland and this
could cause extremely high winds, we don't yet know. The third planet is on the
edge of the zone of habitability but is not tidally locked.
One thing going against life in Trappist-1 system is its
age. The system is only 500 million years old. This may not be enough time for
life to have developed, and isn't anywhere near long enough for intelligent
life as we understand it to develop. And, another hurdle is the type of star it
is. Trappist-1 is another red dwarf, similar to Proxima Centauri and its
planet.
Recent research suggests that red dwarfs are rather unstable
early in life, and this star is bizarrely metal rich and it's unclear why that
is or what that means or what implications that has, so the planets my still be
sterile even if there is liquid water present.
But that may not be true forever. This star will live an
unbelievably long life, at least 4 trillion years. When our sun expands into a
red giant and engulfs earth in a few billion years, this star will still be
young. When most of the stars of the universe have lived their lives, this star
will be among the few main sequence hydrogen burning stars left in the
universe. That leaves plenty of time for life to evolve there at some point in
the future. But this system is close enough that it seems more likely that if
we survive someday these planets will be colonies, more on that in a
minute.
The first thing scientists will do is determine of these
planets have atmospheres at all and what they are like. This discovery is so
important that the Hubble Space Telescope is doing just that already, which
will no doubt be the source of future news in the coming months. But a combined
spectrum we have for Trappist B and C rules out huge hydrogen envelope
atmospheres like our outer planets and is more consistent with a water vapor
atmosphere, or Venus-like atmosphere, or even Earth.
Where this will get interesting is if you can determine the
makeup of an earth-like planet's atmosphere and you see certain gases within
the atmosphere, such as oxygen, it is a dead ringer for the existence of
biological activity on that world. If it's there, we could have conclusive
proof of other life elsewhere in the universe in just a few years, or even
sooner.
Imagining what it's like in this system is mind-blowing.
Because the star is cool and dim, its zone of habitability is far closer to the
star's surface than it is with a larger star like the sun. The first planet
would have a year that is only a day and a half long. The second planet's year would be just 2.4 days and the third is
unclear and could be anywhere from four to 73 days. The star would appear ten
times larger or more in the sky. You would see planets occasionally pass by
that would appear as large as the moon, and everything would be very dim since
Trappist - 1 is a red dwarf. But it would be warm on these planets, since the
star radiates brightly in the infrared.
We have always wondered down here on earth what we would
eventually do when our sun expands into a red giant. But the truth of the
matter is, our changing sun will make life as we know it on this planet
impossible in just a few hundred million years. We arose at the tail end of a 3
billion year process of evolution on a stable planet that simply won't be
habitable for much longer.
As a result, we must either colonize other worlds, or
develop future technologies that can mitigate the effects of the changing sun.
All is not lost there, I'm confident that there will still be people on earth a
billion years from now. But we do need to branch out, and these seven worlds
offer us a place where we could conceivably survive until there's nothing left
in the universe to survive on.
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