Could Exoplanet K2-18b sustain life?
Hi.
We are living in the Age of Exoplanets!
A lot of Astrophysicists and Physicians, as well as Biologists, are currently involved
in projects which try to understand whether in the universe exists other forms of life
as we know it, or not.
Since we know that there are billions and billions of stars in the universe and since
we also know that our Solar System is actually a pretty common star system among others,
we suppose that potentially every star has its own planets orbiting around it.
That's why we search for exoplanets, hoping one day we'll find a habitable one, just like
ours, the Earth.
Not only earth-based telescope is being used to catch exoplanets.
In fact, we make large use of some space telescopes, such as the famous Kepler Space Telescope.
Among the huge number of exoplanets Kepler found (about 2700), there is one that caught
the attention of the scientific community.
Its name is K2-18 b, also known as...wait for it...
EPIC 201912552 b.
It is an exoplanet orbiting a red dwarf star, located about 111 light-years from Earth.
If you were able to reach half of the speed of light and travel at that constant speed,
you will take about 222 years to get to K2-18b.
And we don't even know how to reach that huge speed.
So I guess you and I will never go for a trip to K2-18b planet.
However, on cosmic scales, we could say that “it's right next to us”.
As we've said, the planet was discovered through the Kepler space telescope, using the transit
method.
What scientists found is that it has about eight times the mass of Earth and it completes
an orbit around its host star in about 33 days (it means that if we lived on that planet,
our year would last 33 solar days!).
It is also in the so-called Habitable zone of the planet, which is what makes it a potential
candidate for life as we know it.
But is it enough to be In the habitable zone in order to host life as we know it?
Well, things are harder than that.
First, we should search for some clues about water.
Is there water on K2 18 b?
With the help of the telescopes Kepler, Hubble and Spitzer (which works in the infrared range
of the electromagnetic spectrum), two independent research studies found that there are significant
amounts of water vapour in K2 18 b's atmosphere.
This was a groundbreaking discovery because it was the first time we had clues of water
within the habitable zone of a star.
But is the presence of water the only requirement in order to host life as we know it?
Of course, it is not.
Some other factors play a big role in this game.
For example, let's talk about K2 18 b size and let's see what we can say about the presence
(or not) of life.
K2-18b is about 2.7 times the size of Earth—so large that the planet must have a massive,
extended atmosphere, with a significant fraction of light hydrogen gas.
An atmosphere like this makes K2-18b much more akin to Neptune than it is to Earth.
In fact, spectroscopic analyzes suggested that the planet had characteristics similar
to a mini-Neptune, which is a smaller version of the gas giant.
The planet receives 94% of the radiation that the Earth receives from the Sun and its equilibrium
temperature has been estimated to be 274 K. With a mass of about 8.63 Earth-masses, depending
on its density it could be a mini-Neptune with no solid surface or a Super-Earth.
However, we know for sure that in its atmosphere there is a lot of hydrogen.
In a hydrogen-rich atmosphere, the temperature and pressure increase the deeper you go.
By the time the rocky core is reached, the pressure is expected to be thousands of times
higher than the surface of Earth, and the temperature can approach 5,000 degrees Fahrenheit.
These conditions are bad news for the formation of complex molecules, such as DNA, which aren't
stable at high temperatures and pressures.
Even if we're very open-minded about the conditions required for life to evolve, there's broad
consensus that complex molecules of some kind are necessary, to ensure that enough information
is provided for replication to occur.
Complex molecules cannot form on the deep surface of K2-18b.
That why it's not easy to believe that K2 18 b could support life as we know it.
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So what can we do with this information?
Is it a useless discovery?
Of course not.
Finding water on an exoplanet in a habitable zone helps to understand a lot of things about
exoplanets, for example, we can learn how they are formed.
This is usually done by simulation: we put all the things we know about the planet in
a simulation, and then we wait until it evolves, to see what we should expect, to see what
the simulation offers.
If the results of simulations match the reality, we are super happy.
If not, at least we know that there Is something wrong with our starter knowledge.
In all cases, we can keep do science.
A study conducted by astronomers from the University of Cambridge considered the internal
structure of the planet and found a number of possible solutions, from a rocky core wrapped
in a thick hydrogen envelope to a planet consisting mainly of water with a more subtle atmosphere,
such as an ocean planet could be.
What they found is that a subset of these solutions could allow the presence of liquid
water on the planet's surface, albeit at temperatures and pressures above standard conditions.
So what we learned today is that, unfortunately, the presence of water on another planet does
not imply the existence of life.
But one thing is sure: for life to exist on another planet, we surely need to find evidence
of the presence of water.
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