Imagine a planet more than twice the size of Earth orbiting a red dwarf star. Its atmosphere
is thick with hydrogen and helium. At first, it looks like a miniature Neptune. But underneath
its atmosphere lies an ocean that's unlike anything we have on Earth.
This ocean spans the entire globe, and it's hot - nearly twice the boiling temperature
of water on Earth. But on this planet, the water doesn't boil away because the atmosphere
bears down hard on the ocean, driving pressures to nearly 10 times what we feel on Earth.
It might seem like such a planet would be entirely inhospitable to life. But a team
of astronomers at Cambridge University found that planets like this one could be habitable
for heat-loving microorganisms called extremophiles. The astronomers call such planets Hycean worlds.
That is, ocean planets surrounded by thick hydrogen atmospheres. We have nothing like
them in our solar system, but they're among the most common types of planet in our galaxy.
You might think, 'well, that sounds interesting, but it's going to be decades before we have
a telescope capable of detecting life on other planets.'
Well, that would certainly be true if we constrained our search for life to small terrestrial planets
like Earth. But if these astronomers are right, the James Webb Space Telescope may detect
life on a Hycean planet in just the next few years.
Welcome to Launch Pad, I'm Christian. Ready, your friendly neighborhood astronomer. And
today, we're talking about how Hycean planets will expand our search for life beyond Earth.
Now, if you're curious about the search for life like I am, I'd like to take a moment
and recommend a mini series called Life in Outer Space, which is available on Magellan
TV, who I'd like to thank for sponsoring today's video. Part one of Life in Outer Space talks
about the search for life in our solar system, such as in the permafrost of Mars, the oceans
of Jupiter's moon, Europa and Saturn's moon, Enceladus.
It even explores the possibility of finding life in the methane lakes of Titan. Part two
explores the search for life beyond our solar system, the discovery of exoplanets, and the
methods by which astronomers could detect life on them, whether by finding biomarkers
in their spectra or even signals from alien civilizations.
This series, and many more like it, are available on Magellan TV, which features more than 3000
titles on nature, history, space, science and technology with new documentaries added
every week. I think you'll find Magellan just as inspiring and enjoyable as I do, and best
of all, Magellan is giving my viewers an extra month of their award winning content absolutely
free. You can claim this offer by using the link in the description of this video. In
2019, the Hubble Space Telescope detected water vapor in the atmosphere of the exoplanet
K2-18b. The planet was discovered four years earlier by the Kepler Space Telescope orbiting
a red dwarf star, 110 light-years away in the constellation Leo. So when Hubble detected
water vapor in K2-18b's atmosphere, it was kind of a big deal. In fact, it was the first
time water had been detected on any exoplanet!
Granted, the presence of water vapor alone doesn't prove that a planet is habitable,
let alone the presence of life. But as far as we understand, life requires liquid water.
So if there's water vapor in K2-18b's atmosphere, what's the rest of the planet like? Well,
K2-18b is about two and a half times larger than Earth. And that puts it firmly in the
size range between Earth and Neptune. And we don't have anything like that in our solar
system.
But most of the exoplanets out there fall into this range. And these planets are thought
to have compositions ranging from rocky "super-Earths" with relatively thin atmospheres to low-density
"mini-Neptunes", which are thought to have large atmospheres of mostly hydrogen surrounding
a liquid water ocean.
Kepler's observations indicate that K2-18b is about 8.6 times Earth's mass and is about
two and a half times larger. And that gives the planet a bulk density of about 2.7 grams
per cubic centimeter.
Now, that's a higher density than Neptune, but it's still quite a bit lower than Earth's.
So K2 18b is probably a mini-Neptune instead of a super-Earth or even an ocean planet.
Unfortunately, early studies of mini-Neptune's found that while they would have liquid water
oceans, their heavy atmospheres would drive the pressures and temperatures of the ocean
surface much too high to support life. But armed with data from both the Hubble and Spitzer
space telescopes, astronomers could narrow the range of possible conditions that K2-18b
could have. A team from Cambridge University, led by Nikku Madhusudhan, did just that.
They found that some of the allowed temperatures and pressures at the planet's surface could
be low enough for life to survive in the ocean. In other words, to K2-18b could be habitable!
Now by "habitable", I don't mean like anything we would find tolerable, such as the conditions
here on Earth's surface.
But we know that life does just fine around hydrothermal vents at the bottom of the ocean.
There, water temperatures can reach up to 464 Celsius! But instead of vaporizing, the
water remains liquid, thanks to the pressure of the rest of the ocean bearing down on it.
It doesn't matter if the water is highly acidic or highly alkaline. Life thrives in these
extreme environments. We call such life "extremophiles". But even these "habitable" conditions are
only a portion of the possibilities allowed by K2-18b's data.
At one extreme, the planet's hydrogen envelope could be as much as 6% of the planet's mass.
That's more than enough to drive the surface pressure to well above habitability. But most
of the solutions required much less atmosphere.
In fact, some scenarios required so little hydrogen, the pressure and temperature at
the water layer could actually be closer to those here on Earth. And that would make K2-18b
an ocean world with pressures around 1 bar and water temperatures around 27 Celsius.
Well, chances are K2 18 B has an atmosphere that's thicker than this minimum range. So
at best, we're probably looking at something much hotter and under higher pressure than
a super-Earth or even an ocean world. But it's quite likely not as high as that of a
typical mini-Neptune.
If so, K2-18b could be quite habitable for extremophile life. Well, that got the team
wondering if K2-18b was a potentially habitable mini-Neptune, then there should be a sub range
of mini-Neptunes that could also host life.
The team dubbed these worlds "Hycean planets". That is, a habitable ocean planet surrounded
by a hydrogen atmosphere. The team went to work calculating and modeling the different
combinations of planet mass, composition, and distances from the host stars that would
still result in a habitable Hycean planet.
They found a range of temperatures that these planets could have and still be habitable.
So a typical Hycean planet might have an ocean surface temperature around 200 degrees Celsius
under about three bars of atmospheric pressure. Again, lethal for us, but nothing that Joe
Extremophile would object to.
Next, the team looked into the range of distances the ocean planets could have from their host
stars. In other words, they wanted to figure out what was the Hycean habitable zone. Well,
this range depends on the star's temperature. The hotter the star, the farther away the
zone is and vice-versa. But it also depends on the planet's properties as well. For terrestrial
planets like Earth, the relatively low surface pressure means that liquid water is confined
to a fairly narrow range of distances from its host star.
But the team found that thanks to their thick atmospheres and internal heat, Hycean planets
can remain habitable to as far as 12 AU from a Sun-like star and as close to 0.01 AU for
a small red dwarf star.
And that alone is really interesting. However, we know that planets can get much, much closer
to their stars. When that happens, tidal forces from the star slow the planet's rotation down
until it matches its orbital period. The planet becomes tidally locked with one side always
facing the star, with the other in permanent darkness. With a never-ending onslaught of
radiation from the nearby star and internal heat pumping into the ocean, the dayside of
these planets would eventually become too hot to sustain life. But on the dark side,
things could be quite different. There, the ocean is warmed by the planet's internal heat,
as well as the energy flowing from the dayside.
Could this be enough to make the dark side habitable? Well, to find out, the team took
a conservative approach. They modeled these planets, assuming a very inefficient energy
distribution. Even then, the team still found an additional "Dark Hycean" habitable zone
that gets considerably closer to their stars.
And that's really good news because there's a number of possible mini-Neptunes that fit
within this new habitable zone, including some possible Hycean candidates! The team
also considered Hycean planets with little to no starlight at all. In other words, planets
that orbit very far from their stars and even free-floating planets without a star.
In this scenario, the only energy source for the atmosphere is the planet's internal heat.
They found that depending on the planet's mass radius and internal temperature, "Cold
Hycean" worlds can maintain habitable conditions down to a frigid -3 degrees Celsius and an
atmospheric pressure of 1000 bar.
And that's pretty much the extreme limit for cold loving extremophiles on Earth. But if
the planet has a slightly higher internal temperature, then life could swim in warmer
waters at lower pressures. It's amazing to think about a rogue planet drifting between
the stars, yet carrying with it life.
Now to be clear, these models don't make any prediction of life on Hycean planets. They
only predict a range of possible conditions that are otherwise sustainable to microbial
life. And therefore, we shouldn't expect to find evidence of large Hycean whales or other
intelligent creatures broadcasting messages to us, although that would make our jobs a
lot easier.
But if one of these planets had microbes swimming in the oceans underneath thick hydrogen atmospheres,
would we be able to detect them? Well, let's take a look at the atmosphere. If we took
a direct spectrum, we'd expect to see absorption due to hydrogen, helium, water, and probably
methane and ammonia, none of which are necessarily proof of life. So what else might we find?
Well, traditional biomarkers such as dimethyl sulfide, carbon disulfide, chloromethane,
carbonyl sulfide, and nitrous oxide have been proposed for both Rocky and terrestrial planets,
as well as for hydrogen rich atmospheres. However, it might be easier to detect these
biomarkers in Hycean planets rather than in the atmospheres of super-Earths for two reasons.
First, Hycean atmospheres are thicker, but their surface gravity's are lower, allowing
the atmospheres to grow taller. The larger and thicker the atmosphere, the more biomarker
absorption we can detect. Second, the biomarkers in question are thought to come from aquatic
microorganisms, and these planets are nothing but oceans. Without any annoying landmasses
in the way, these microbes would have the entire planet to themselves, belching their
biomarker gases into the air. So on the one hand, biomarkers should be easier to detect
on a Hycean planet than on a rocky super-Earth. But would it be easy enough for the James
Webb Space Telescope to detect?
Well, it would be great if Webb could just take a direct spectrum of the planet, but
that's not going to happen. Even though K2-18b orbits a small red dwarf, it's much too close
to its star for Webb to directly distinguish.
But in the last few years, astronomers have come up with a clever trick. Instead of trying
to take the spectra of an entire planet directly, astronomers take two spectra of the same star;
once when the planet is out of its way, and once again when the planet is transiting in
front. When the star's own spectrum is subtracted from the combined spectrum, what remains is
the spectrum of just the planet. This is a technique called transit spectroscopy. Given
what's known about K2 18b, its host star, and Webb's capabilities, the Cambridge team
calculated what the planet's spectrum would look like to Webb if it had biomarkers in
its atmosphere.
They simulated observations taken over three transits of the planet. Sure enough, absorption
lines from biomarkers were present in the resulting spectra. They even repeated the
calculations for two other Hycean planets and got very similar results! This is why
I'm so interested in these Hycean planets, because if these models are correct, and these
planets really do turn out to be Hycean worlds, and if any of them have life, Webb could detect
it. Granted, there's much about Hycean planets and habitability that we still don't know
very much about, particularly when the planet orbits close to a red dwarf star.
Are their atmospheres really strong enough to minimize UV radiation from the star? Are
the magnetic field strong enough to withstand flares that are up to 10000 times stronger
than those on the Sun? What Hycean planets recycle atmospheric gases to regulate their
temperatures, or would they eventually lose their habitats to a runaway greenhouse effect?
Well, there are a range of possible answers to these questions, and the majority of exoplanets
are within the size range between Earth and Neptune. And this study allows us to expand
our search for life into this mid-sized planet region.
Not only are there more of them out there, but they could support life over a wide range
of distances from their stars. But most importantly, we may actually be able to detect life on
a Hycean planet with the James Webb Space Telescope!
Say what you want about how long it took to build, but it's going to start studying exoplanets
within its first year of operation. Imagine if we detect life with it in the next five
years instead of the next 50.
I think that might make it into the papers. I want to thank my Patreon supporters for
helping to keep Launch Pad Astronomy going, and especially for helping to give me the
time to study and research videos like this. And speaking of videos like this, if you want
to stay up to date on the latest developments with new planets and the James Webb Space
Telescope, make sure that you subscribe and ring that notification bell so that you don't
miss out on any new videos. Until next time. Stay curious, my friend.