One of
the longstanding questions astronomers have had is, are we normal?
Is our Solar System a template for what we might expect to see as we look out into the
Milky Way at other star systems?
As the data continues to come in, the answer to that question really seems to be no, we’re
not normal.
Star systems seem to have a huge variety of planets orbiting them.
Familiar planets like our own terrestrial, gas and ice worlds.
But then there are also super earths, mini-neptunes, hot jupiters.
And it looks like there are planets, located in their star’s habitable zone, which are
completely covered with liquid water.
Like, oceans which are dozens and maybe even hundreds of kilometers deep.
What would it be like on one of these worlds, and of course, we always want to know, could
they be habitable?
About 71% of the Earth’s surface is covered by water.
But if you could scoop up all the water and collect it into a single sphere, you’d end
up with a ball of water that’s about 1,385 kilometers across, which is about half the
size of Pluto.
That’s about 1% the mass of asteroid Ceres.
The Universe, of course, has supplied us with an endless variety of planets out there that
we can compare ourselves too.
Of the 4,000 confirmed exoplanets found so far, many are right in the middle between
Earth-sized worlds and larger Neptune-sized worlds.
The problem is that most planets have only had their size measured, and not their mass,
which provides plenty of variety for what they could be made of.
There’s an intriguing idea that many of these worlds are gaseous at all but actually
made of mostly water surrounding a tiny rocky core.
Back in September, astronomers announced that they had discovered water vapor in the atmosphere
of an exoplanet, designated K2-18b, which is located in the habitable zone of its star.
This planet isn’t anything like we’ve got here in the Solar System, with a mass
of 8 times that of Earth.
This makes it a super-Earth or a mini-Neptune.
This was the first time that water vapor had been found in the habitable zone of a star,
which you know is a region where liquid water can be present.
Since there’s water vapor in the atmosphere, astronomers have inferred there could be water
on the surface of the planet - maybe a lot of it.
Does this mean the planet is habitable?
It’s definitely not a nice water world, that’s for sure.
It has that 8 times the mass of the Earth, and a radius that’s about 2.7 times the
size of Earth.
Exoplanet researchers have calculated that it’s pretty much impossible to have a planet
that’s bigger than twice the size of Earth without a very thick atmosphere.
Observations show that it has a thick atmosphere of hydrogen and helium - like, the surface
of Jupiter.
It probably has a rocky core, but that’s deep down inside an enormous amount of hydrogen,
helium and yes, some water.
Not to mention the fact that it orbits around a dangerous flaring red dwarf star which would
constantly be scouring the planet with deadly radiation.
What makes this discovery exciting is that astronomers were able to find a planet this
small, this close to its star, opening up the possibility for future discoveries.
Astronomers continued to study K2-18b, and were able to run the planet through a variety
of scientific models to try and figure out what’s actually going on there.
They recently came up with three possibilities:
It’s either a rocky world with a core that’s 94% iron with very little water and a very
thick atmosphere of hydrogen and helium.
It’s a mini-Neptune with an Earth-like core and more than 54% water.
Or a water world, with a small Earth-like core and almost 90% water with very little
hydrogen and helium.
Let’s talk about that third possibility since it opens up some interesting ideas about
the habitability of ocean worlds.
What if there was a world that had a small rocky core surrounded by almost entirely water?
Like, we’re talking about oceans that go down for hundreds, maybe even thousands of
kilometers.
What does that mean for habitability?
In a paper published in 2019, astronomers looked at a variety of worlds that have already
been discovered with 2-4 times the size of the Earth.
The prevailing theory is that these are mini-Neptunes, rocky cores surrounded by thick envelopes
of hydrogen gas.
But the researchers propose that these are actually water worlds, and they’re everywhere.
Go down deep enough, and the pressure would be overwhelming.
Liquid water would be compressed into exotic forms of ice that don’t occur naturally
here on Earth but have been made in the lab.
They would act like the Earth’s mantle but completely made out of ice.
A few hundred kilometers down, this hard layer of ice would completely cut off the surface
of the ocean from the rocky core underneath and prevent elements like carbon, iron, and
silicon from mixing into the oceans.
Clearly, we want water, but not too much water.
What if you’ve got deep oceans, a rocky seafloor, but no actual continents?
Can you have reasonable temperatures?
In order for a planet to be habitable, planetary scientists used to assume you needed something
like Earth, with both continents and oceans, to allow nutrients to cycle from land into
the oceans to support aquatic life.
Deep oceans smother volcanoes, stopping them from pushing carbon dioxide directly into
the atmosphere to help warm it up.
And without rocks on the surface, they don’t have a way to draw that carbon dioxide out
of the atmosphere again to cool back down.
But a recent study from the University of Chicago says that might not be the case.
They simulated thousands of randomly generated planets and tracked the evolution of their
climates over billions of years.
They found that about 10% of the waterworlds have the right mix of overall chemicals that
the ocean and the atmosphere can be in perfect balance, cycling carbon dioxide between the
air and water, keeping the temperatures nice and stable.
In fact, planets around red dwarf stars might be the ideal candidates since these stars
would give their planets billions and even trillions of years of constant light and warmth
(after a really turbulent early period).
There’s increasing evidence that the Earth itself used to be a waterworld for long periods
of time, without any solid ground.
We’ll get to that in a second, but first I’d like to thank:
Luke Wilhelm Michael Tyrrell
Brian P. Cox J MacNeill
Søren Riber Berg
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As I said earlier, the Earth is mostly covered by water today, but it still has plenty of
continents where life was able to thrive.
But there’s evidence that this wasn’t always the case.
Geologists were looking at exposed rock formations in Western Australia, which contains very
well preserved portions of the ocean crust from the Archaean eon, which formed about
3.2 billion years ago.
They analyzed the rock and were able to measure the isotopes of oxygen that gave them clues
about how the oceans and rocks interacted with each other at that time in history.
It seemed ton contain higher quantities of the heavier isotopes of oxygen that would
normally be drawn down by continents across the planet.
In other words, at that time in the Earth’s history, there were no continents, just oceans
of various depths across the entire planet.
It was only more recently that the continents themselves formed.
The big question, of course, is where did life on Earth come from.
The more established ideas is that it formed in some kind of nutrient-rich aquatic place
on land, but what if there were no continents on land at all at the time?
This means that the alternative explanation, that life formed around the volcanic vents
in the deep ocean, could be more likely.
And if life was able to form here on Earth in a purely aquatic environment, that gives
hope to the possibility that we’ll find life on other ocean worlds out there in the
Universe.
As new telescopes come online and astronomers continue to refine their techniques, it could
turn out that habitable zone ocean worlds are far more common than we ever expected.
Now the question is, which of these are truly habitable, and which are just enormous chunks
of sterile water.
Keep watching to see how it all unfolds.
What do you think?
Let me know your thoughts in the comments.
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This video was focused on ocean worlds located in the habitable zones of their stars, but
there’s growing evidence that the vast majority of habitable worlds are actually ice worlds,
like Europa and Enceladus.
There could be 1,000x as many of those places compared to planets like Earth.
Here’s a video that goes into it.