The term "habitable zone" comes up a lot when discussing exoplanets and extraterrestrial life these days. But what exactly does it mean?
Host | Matthew S Williams
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Episode Notes
The term "habitable zone" comes up a lot when discussing exoplanets and extraterrestrial life these days. But what exactly does it mean? Interestingly, the way we define "habitable" has changed in recent decades, thanks to the number of confirmed exoplanets and our growing knowledge of how life evolved here on Earth.
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Resources
NASA Science - The Habitable Zone: https://science.nasa.gov/exoplanets/habitable-zone/
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For more podcast Stories from Space with Matthew S Williams, visit: https://itspmagazine.com/stories-from-space-podcast
What is a Habitable Zone? | Stories From Space Podcast With Matthew S Williams
[00:00:00] The authors acknowledge that this podcast was recorded on the
traditional unceded lands of the Lekwungen peoples. Hello, and welcome to
another episode of Stories from Space. I'm your host Matt Williams, and today I
wanted to get into a very fascinating topic, and one which is very near and dear
to my heart, and it concerns a subject which is vitally important to the search for
habitable planets beyond Earth, and of course the search for life in our universe,
aka astrobiology, The concept of a circumsolar habitable zone, or habitable
zone for short, which is to say, the orbit that a planet would need to have around
a star in order for life to be able to emerge and thrive there.
Now, over the years, with the huge explosion in the number of confirmed
exoplanets, this concept has evolved. And part of this has to do with the fact
that our very [00:01:00] notions of habitability have also evolved thanks to the
large sample of exoplanets that we have been able to study thus far. Prior to the
Kepler Space Telescope and the thousands of exoplanets it was responsible for
detecting, scientists were largely confined to using our solar system as an
analog for other star systems, and It was sort of naturally assumed that what we
have here is essentially the standard by which habitability and habitable star
systems can be measured.
But that has changed in recent decades, as I said, because of the sheer number
of exoplanets we've discovered. The more exoplanets we can confirm, and the
more we can learn about them in turn, it influences our very perception of what
type of planets are most common in the universe, and what conditions are likely
to exist on their surfaces.
And, of course, there's also the fact that scientists [00:02:00] have been looking
at how life emerged on Earth, and there are a number of important
considerations that need to be factored in when we talk about what does it take
for life to exist. In the past, the concept of a habitable zone really came down to
one thing, the distance that a planet would need to orbit from its star in order to
receive enough sunlight and enough warmth that water could exist on the
surface in liquid form.
However, even by this simple definition, there are a number of caveats that have
to be taken into account. For starters, the distance that a planet would need to
orbit its star depends entirely upon the star itself. Whereas your larger classes of
stars, which are typically much hotter and brighter than our Sun, for example,
their circumsolar habitable zones tend to be much farther out.In addition, they'll have a wider circumsolar habitable zone, which means that
more than one planet [00:03:00] could in fact fit into this region. Whereas your
G type and K type stars, our sun being an example of a G type main sequence
star, or yellow dwarf, and K types being orange dwarfs, these tend to have
tighter habitable zones, so closer to the star and less wide, less broad.
Whereas your smaller red dwarf stars, also known as M type stars, which are
typically much smaller, dimmer, and cooler than other classes, they have rather
tight and constrictive habitable zones, so really quite close in, and rather narrow
by comparison. So you might think that's pretty straightforward. The bigger,
more massive, and hotter the star, the more distant its habitable zone is going to
be, and the more extensive it's going to be.
But even there, there's another all important consideration, which is the age of
the star. For example, when dwarf stars [00:04:00] first form in their planetary
systems of fully accreted and are now orbiting it, they have not yet entered what
is known as their main sequence, which is the longest stretch of their lifetimes,
in which their color, brightness, and the intensity of the radiation they emit will
evolve with time.
For example, hundreds of millions of years ago, our sun was actually dimmer
than what it is today. Earth received less in the way of solar radiation. So it was
necessary for our atmosphere to have higher concentrations of carbon dioxide in
order to maintain the kinds of temperatures that were stable over time.
And as our sun has gotten brighter, atmospheric carbon dioxide levels have
dropped off accordingly. What's more, stars like our sun, they remain within
their main sequence phase for about 10 to 11 billion years, whereas much
larger, more massive stars have much shorter lived main sequence phases. And
[00:05:00] given that our solar system formed roughly 4.
6 billion years ago, that means in about 5 billion years, Our sun will have run
out of hydrogen fuel in its interior, and at this point, it will expand considerably
to become a red giant. And scientists currently predict that when our sun does
become a red giant, it's going to expand to the point where just about all the
inner planets, including Mercury, Venus, Earth, and maybe even Mars, will be
consumed, at which point, The habitable zone of our solar system will have
moved or relocated much farther out.
At this point, what is known as the frost line, the boundary in our solar system
beyond which volatile elements freeze solid, which is roughly located in the
asteroid belt, beyond that will now be the new habitable zone of our solarsystem. And so you see, even by the simplest definition there, the [00:06:00]
distance required to maintain liquid water on the surface of a planet, that the
concept of a habitable zone, it changes with time, it evolves.
And as the example of Earth and its varying levels of carbon dioxide in the
atmosphere of its greenhouse gases, the chemistry of a planet's atmosphere also
plays a very, very important role in determining, or defining, What a habitable
zone is and what its extent is. And this brings us into another key area of
astrobiology, the search for life in our universe, and how that applies to
circumsolar habitable zones in the very definition of habitable planets.
And it comes down to what is required for life to emerge and to thrive and for
the climate of a planet to remain stable over time. As its own geological history
indicates, [00:07:00] there will be shifts and changes due to changes in the
planet's axial tilt or just the dynamics between the star and the planet itself.
Nevertheless, over the long haul, the presence of certain elements changes. are
what ensure that life can continue to survive and continue to emerge and evolve.
And, as we addressed in the previous episode, the search for biosignatures,
water is considered foremost among them, because, as we know, all life forms
on Earth rely upon water in order to carry out basic functions and to survive.
And not only that, but based on the most recent, up to date fossilized evidence,
It is now known that the earliest lifeforms, they emerged roughly 4. 1 billion
years ago, and they did so within Earth's oceans around hydrothermal vents.
And this indicates that life emerged almost immediately [00:08:00] after Earth
had oceans to begin with.
And the next most obvious is carbon dioxide, because, as we know, Earth's
primordial atmosphere, it consisted predominantly of gases released from
Earth's interior through volcanic eruptions, so a process known as volcanic
outgassing. And the largest component of that was carbon dioxide. And so, over
time, this led to the first cyanobacteria and other photosynthetic organisms,
which were still single celled and relatively simple by this time.
Nevertheless, these organisms are what allowed for oxygen to be introduced to
the atmosphere. By combining water and carbon dioxide with an energy source,
in this case, sunlight, they were able to produce glucose as a food source and
oxygen gas as a waste product. And over time, this had the effect of drastically
transforming our atmosphere, leading to the Great [00:09:00] Oxygenation
Event.And at first, this triggered mass extinctions there, where much of the organisms
that depended upon a carbon dioxide atmosphere began to die off. But of
course, that led to eventually an equilibrium being achieved, where oxygen
consuming organisms emerged. And this led to the first complex lifeforms that
were not only multi celled and had mitochondrial DNA, But we're much larger
and have complex systems of organs in order to metabolize oxygen and
nutrients.
So naturally, oxygen is also considered a biosignature. And since nitrogen plays
such an important role through the nitrogen cycle in the lives of plants and trees
and other foliage that are essential to maintaining the balance of carbon dioxide
and oxygen in our atmosphere, Nitrogen is also considered a biosignature, not
only in the form of nitrogen [00:10:00] gas, which acts as an important buffer in
our atmosphere between oxygen, CO2, and other trace gases, but also in the
form of ammonia.
As ammonia is a natural solvent, much like water, predominantly composed of
nitrogen, and the nitrogen byproducts like nitrites and nitrates are essential for
plants as a food source, and it enriches the soil. So ammonia is there too. And
last, but hardly least, there is methane. Methane is yet another solvent that is
found naturally occurring in nature.
It's made up of hydrocarbons, which is why it is combustible and flammable.
But those hydrocarbons are key to life. Carbohydrates themselves are a main
source of food energy for complex animals like humans. And those essentially
consist of long chains of hydrogen bonded with carbon and oxygen. [00:11:00]
And when organic matter dies and begins to decompose, methane is what is
released.
And it is, in some organisms, also a byproduct of their digestion process, such
as cows. And, as I'm sure our listeners know, it plays a rather important role in
the dynamics of our atmosphere, because methane is a super greenhouse gas, as
is ammonia. Thank you for listening. So these two are not only essential to
organic life as we know it, they also help maintain the atmospheric balance and
stable temperatures over time on a planet.
And so, when looking at how planet Earth itself has evolved over time, how its
atmosphere has changed, how its dynamics have ultimately led to the
emergence of life and How the presence of life itself changed the way the planet
works. These are all rather complex considerations that scientists need to take
into account when attempting to determine if a [00:12:00] planet is potentially
habitable or not.As recent studies have shown, planets orbit red dwarf suns. They are
particularly good at forming systems of rocky planets. In fact, within 50 light
years of our solar system, we have identified a total of 30 star systems where
potentially habitable rocky planets reside. And of those, 29 are red dwarf suns.
And these planets, many of which were super Earths, but several were
comparable in size to Earth, too, they tended to orbit within the circumsolar
habitable zone, so just the right distance from their sun that they would get
enough heat to maintain liquid water. But! Because, as I said earlier, red dwarfs
have a rather up close and tight and constricted habitable zone.
That means that these planets orbit very close to their stars, and as [00:13:00] a
result of that, the gravitational pull of the star, it has led to a situation known as
tidal locking. And what this means is, is that the planet's rotation has become
synchronized with its orbit around the star, so that one side is constantly facing
the star, and the other is dealing with perpetual darkness.
And along the terminator, or the boundary between the sun facing side and the
dark side, you have perpetual twilight. And this is very much the situation we
have here on Earth with the Moon. The Moon is tidally locked to us, which is
why we always look up and see the same face, and why we talk about the dark
side of the Moon, or rather the far side, because it still gets plenty of
illumination from the Sun.
It's only dark in the sense that we don't see what's there at any given time.
What's more, scientists have noted from [00:14:00] spectral observations of
these systems that they appear to have a lot of water, and this is evidenced by
the loss of hydrogen to space, what's known as atmospheric escape. When you
have water on the surface of a planet that's bombarded by solar radiation, the
hydrogen and oxygen atoms of water, they chemically disassociate.
And whereas the oxygen gas created from this process will be retained by the
planet, the hydrogen gas is lost to space. And so, from this, scientists have
theorized that there may be many red dwarf planets out there that have plenty of
oxygen in their atmosphere. But this does not necessarily make them habitable.
Because not only was this oxygen created through inorganic processes, so it's
known as abiotic oxygen, But, using Earth as a template again, life would have
an [00:15:00] incredibly hard time emerging on this planet because the presence
of an oxygen atmosphere would be toxic for simple organisms. It was only
through the transition of carbon dioxide to an oxygen rich atmosphere thatsimple organisms gave rise to more complex organisms that use oxygen as a
fuel source.
And so this has led to a term known as transiently habitable, where a planet
could be habitable to life as we know it now, as it has come to evolve, so we
could seed planets with our own life and they would be able to thrive there. Life
in the indigenous sense would not emerge on its own, at least as we know it.
And this is seen as particularly likely around red dwarf suns because they are
very long lived. Whereas our sun and similar G type or K type dwarfs tend to
live for about 10 billion [00:16:00] years before they leave their main sequence,
it is estimated that red dwarfs can remain in their main sequence for hundreds of
billions of years, even trillions.
And a consequence of that is that they take a while to get into their main
sequence phase. So, in fact, what is their habitable zone once they are in their
main sequence is not what their habitable zone was before that. And so planets
that are located in what will become a red dwarf's core Goldilocks zone, another
term used for it, would be subject to a lot of radiation from this newly formed
red dwarf star for billions of years.
And this could have the effect of turning water that is on their surface into a
viotic oxygen, and thus creating a world that would be hospitable to advanced
life. But, very hostile to emergent life, at least as we are familiar with it.
[00:17:00] And, as also mentioned earlier, Earth's primordial atmosphere was
the result of volcanic outgassing.
And a look at the geological record shows that plate tectonics have played a
pretty vital role in the evolution of life. In fact, scientists believe that The
Cambrian Explosion, as it's referred to, which took place roughly 530 million
years ago, this was characterized by a wide variety of animal species, the
majority of the taxonomies that we are familiar with today, they suddenly Quote
unquote appeared in the fossil record.
Now, of course, this happened over the course of many millions of years, but it
is referred to as an explosion, and the term sudden is used in the sense that this
seemed like a very brief period of time for such an explosion to happen. And of
course, subsequent research since the late 19th century, when this Cambrian
explosion was first [00:18:00] identified in the fossil record, it has shown that
there was actually a lead up to this, it wasn't an explosion, so much as a faster
than normal rate of evolution for terrestrial species.Nevertheless, The latest explanation as for why this explosion took place, when
it did, which is certainly the most comprehensive explanation offered to date,
because it manages to synthesize several other theories, has to do with a slight
increase in oxygen levels that occurred during the same period, as well as the
existence of shallower marine environments, which led to more oxygenated
waters.
And this coincided with the breakup of the Panacea supercontinent, so in
essence, the movement of tectonic plates, the breakup of the supercontinent,
form all these shallower oceans and new niche environments between the New
continents that existed by the Cambrian period. This is what allowed [00:19:00]
for rapid evolution and diversification of life.
On a geological time scale at least. What's more, tectonic activity plays a very
key role in the maintenance of Earth's carbon cycle. Carbon is sequestered into
the Earth repeatedly by the convection of the mantle plate. And this is what
results in the formation of carbonous rocks in the interior, which store carbon
dioxide in them.
And carbon dioxide is then released again into the atmosphere through volcanic
activity. So, again, the presence of plate tectonics and the planet being
geologically active would seem to play a very key role in the evolution of life
and its eventual diversification, complexity, and as well, the maintenance of a
stable environment that can host it.
And, interestingly, this is something that is unique within our solar system to
Earth. Earth alone appears to have tectonic [00:20:00] plates with sharp
boundaries that are constantly moving and shuffling, whereas the Moon,
Mercury, Venus, and Mars all appear to have just one plate, which covers the
entire surface.
There are no boundaries, no breakups, no movement. So, this is what's known
as a stagnant lid planet. Thanks for watching! But, interestingly, Venus and
Mars are not also geologically dead in that respect. There is evidence that Venus
experiences volcanic activity on its surface semi regularly, and that Mars itself
still has magma in its interior, and that there's still heat in the movement of lava
beneath its surface.
And so far, scientists are unsure whether or not this would apply to exoplanets
as well. There is research that considers super Earths and indicates that they are
likely to not have tectonic activity, that they are most likely stagnant lid worlds,
but this remains to be [00:21:00] seen and tested. All that we can say at thispoint is that the presence of tectonic plates and their movement as part of a
geologically active world is vital to life as we know it, and certainly has been
here on Earth.
And what is especially interesting is that Tectonic activity does seem to take
place in icy bodies in the outer solar system. And we're talking Europa,
Ganymede, Triton. And evidence of plate movements were observed by the
New Horizons probe on Pluto. However, rather than it consisting of a sheet
silicate minerals, this tectonic movement was ice sheets.
Either composed predominantly of water ice, with other volatiles mixed in, or
nitrogen ice, in the case of Triton and Pluto. And that is quite similar to
cryovolcanism. It is a type of geological activity involving [00:22:00] the
Movement of material in the interior that is liquid due to tidal flexing, much like
lava, but instead we see water, and it is spewing out of mountainous features
that are made of ice and other volatiles instead of rock.
And this brings us to yet another consideration when defining things like
habitable zones. If, in fact, the outer solar system is full of ocean worlds, and it
certainly is, and if, in fact, these can host life in their interior, does that not
mean that the habitable zone, as we know it, is far too constructive?
That beyond the frost line of any solar system, um, Icy moons at orbit, gas
giants, and have all the necessary components there. Tidal flexing in their
interiors, the exchange of material between the surface and the interior, liquid
water, hydrothermal vents, and the necessary chemical elements for life, as well
as an energy [00:23:00] source, which can take the form of heat provided by the
hydrothermal vents, as well as the decay of radioactive elements in the
atmosphere.
rocky and metallic cores of these worlds, then is it not a safe assumption that
life can exist well beyond the boundary where water can exist in liquid form on
its surface? And should astrobiologists therefore consider gas giants that orbit
beyond the so called habitable zone to be a potential abode for life, provided
they have exomoons?
So, as you can see, Our very concept of what constitutes a habitable zone, or a
habitable planet, the very notion of habitability for that matter, it has evolved
with time thanks to a lot of new discoveries, new research, and these have come
to challenge the traditional notion that the solar system is the archetype, or the
predominant model for judging [00:24:00] what it takes for a planet to host life.We are reconsidering the possibility that water and oxygen themselves are
guarantees that life would exist. We're also reconsidering whether or not
terrestrial planets are the only place where life could exist. And of course, we're
taking into account the evolution of stars and planets themselves, and how the
presence of life itself is responsible for altering the environment in such a way
that Habitability is extended.
More life can exist. So, in that respect, having the necessary chemical
ingredients that we associate with life today is not nearly as important as having
the necessary ingredients for life to emerge in the first place, do its thing, and
ultimately transform a planet into something that we would recognize today as
being habitable.
And so, in this respect, astrobiology is just like the search for extraterrestrial
intelligence, or SETI, [00:25:00] in that it all comes down to the framework
within which we're operating, it's limited by our current knowledge, our current
examples, and, of course, like SETI, that's bound to change in the coming years.
Not only has the explosion in exoplanet discoveries and the presence of many
space telescopes that are highly adept at finding and characterizing exoplanets,
not only are they presenting us with many, many, many more examples of what
is out there and what life would have to deal with, at the same time, our
exploration of the solar system, which is going to involve a few very interesting
missions sent out to the outer solar system in the coming years.
And that includes the Europa Clipper mission, the European Space Agency's
Jupiter Icy Moons Explorer, or JUICE, which is already launched, and the
Dragonfly mission to Titan, as well as a possible Enceladus [00:26:00] orbiter
and a Europa lander. All of these missions are going to be tasked with picking
up where previous missions left off, and looking for evidence of life and
biosignatures.
On the surface of these bodies and as well as of their interior oceans, sampling
them to see if there are the necessary ingredients for life, or indeed indications
that life could be down there. And what we learn from these future missions and
from our next generation telescopes, such as the James Webb Space Telescope,
that's already in operation and revealing some wonderful things, which will be
followed as well by the Nancy Grace Roman Space Telescope, Hubble's direct
successor, the European Space Agency's Planetary Transits and Oscillation of
Stars mission, otherwise known as PLATO, Hubble Space Telescope.As well as the ESA's Atmospheric Remote Sensing Infrared Exoplanet Large
Survey mission, a. k. a. [00:27:00] ARIEL. All of these missions are going to
teach us about the possibilities and potential for life, not only within exoplanet
systems, but also within our own solar system. So, in the coming years, we'll be
able to search for life, both at home and abroad.
And what we find, it'll be complementary of course, if we find that life can in
fact exist within icy moons orbiting gas giants, this knowledge will go a long
way towards informing exoplanet and exomoon studies, and will also mean that
rogue planets, rogue gas giants that have carried along their systems of
satellites, that they too could be transporting life as they interstellar medium.
So this will have tremendous implications for astrobiology, but also for what we
consider to be habitability and where life can be found. And depending on what
we find, we may learn that Earth is, [00:28:00] in fact, a very type of rare
environment, very rare jewel, and that life can only exist under very, very tight
and constrained conditions, the kind that we only find so far here on planet
Earth, or that life takes many forms, that it's ubiquitous in the universe.
And this, in turn, could have implications for SETI. We may learn that
intelligent life is a lot more common than we thought, just not in forms that we
would immediately recognize as such. Or that while intelligent life may be rare,
the product of long, long phases of evolution, stretching out billions of years,
that basic life could be absolutely plentiful.
So, as I often say at the end of these episodes, exciting times lie ahead, and be
sure to tune in again as upcoming episodes will be dealing with Aztec
astronomy as part of our ongoing segment on indigenous astronomy, [00:29:00]
as well as the final proposed resolution of the Fermi Paradox in our series
known as the SETI Paradox.
We'll also look at the emerging Chinese space program and what it has in store
for us in the near future, as well as India's. And we'll be taking a look at other
commercial space companies that are giving SpaceX a run for its money, are
pushing the envelope on innovation, and working towards making space
accessible for all people.
In the meantime, thank you for listening. I'm Matt Williams, and this has been
Stories from Space.