Hunting for Alien Worlds (Part 3): The Habitability Debate
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Artist’s illustration of a Super-Earth planet Gliese 667C c (4.54 Earth masses), compared to Earth and Mars.
CREDIT: PHL@UPR Aricebo
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At the 2012 Astrobiology Science Conference, Astrobiology Magazine
hosted a plenary session titled: “Expanding the Habitable Zone: The Hunt
for Exoplanets Now and Into the Future.”
Originally formulated as part of our "Great Debate" series, this panel
of exoplanet hunters and thinkers held a lively discussion about some of
the most important issues facing the search for and understanding of
alien worlds orbiting far-distant stars.
Part One of this series examined the ways
exoplanet science is changing today, and Part Two explored the
science of habitability. Here is Part Three:
David Grinspoon: Dirk mentioned the possibility of a 5
Earth-mass Super-Earth at the orbit of Mars that might be a habitable
planet. Which is interesting that perhaps our notion of the outer edge
of the classical habitable zone is too constrained by the fact that we
have this wimpy little planet called Mars there, and if we had a real
proper big terrestrial planet occupying that position, that we might
have a wider view of the habitable zone. The problem with that is of
course nobody's been able to solve the problem really well of how you
make even early Mars have a warm/wet environment. It's a bit of a
puzzle of course, you can't just pump up the CO2 if you have a
Super-Earth.
But some recent work that Ray Pierrehumbert presented at ExoClimes
showed that in fact if you have a Super-Earth at a Mars-like orbit, it
could be habitable in the classical sense, because if you have a very
large hydrogen envelope on a cold big Super-Earth, that
collision-induced opacity gives you enough warming. So that to me is an
example of how we may, with new theoretical developments, be able to
expand our notion of the habitable zone. [
Planets Large and Small Populate our Galaxy (Infographic)]
However, Ray also pointed out that if life developed on that planet,
and if there were methanogens, they would quickly destroy the greenhouse
and render the planet uninhabitable in a sort of anti-Gaia effect. So,
this also is an example of how there are a lot of subtleties that will
perhaps become apparent once we actually start discovering these worlds
and trying to understand what we're seeing.
The early Earth was a very different world from the planet we experience today.
CREDIT: Planetary Science Institute/William K. Hartmann
Dirk Schulze-Makuch: David, was it the same thing that the cyanobacteria did to the poor bacteria on early Earth? Right?
David Grinspoon: Cyanobacteria are very irresponsible planetary stewards.
[Laughter]
Vikki made a really interesting point in her intro about their
Earth-over-time project. How do we deal with the fact that we can't
help but
look for Earths?
But of course through most of Earth's history Earth was not an Earth in
the sense that we often often think of that. Do we know enough about
early Earth? Does that help us because in a certain sense we have more
than one example of a known habitable planet because we know enough
about early Earth? Or does it hurt us because it limits our ability to
even think about the problem because we are looking for something that
in fact may be fleeting?
Vikki Meadows: I think looking at the early Earth is a
very nice transition from looking at modern Earth where we know a great
deal and can learn a great deal, out to
exoplanets
where we will know very little. And in fact we should not have any
preconceptions about what we see with the extrasolar planet population.
But looking at the early Earth, you can get some geological and
biological constraints, it is difficult, but it allows us to explore an
alien habitable environment with the chance of getting more data on it
then we can currently get on exoplanets. So, I think that is
particularly important.
I also want to point out that the stars in the local solar neighborhood
go from just born to ten billion years old or so. There is a very wide
range of ages. So the planets around them will also be a very wide
range of ages. And so when we finally do
find extrasolar planets
in the local solar neighborhood that we can study, they will not
necessarily be 4.6 billion years old, but they will be a very different
stage of evolution. And I think that is one of the most interesting
things about looking for local extra-solar planets, is that we get to do
not only the comparative planetology in the real sense that this is a
different type of world with a different environment, but you also get
to see terrestrial planets at different stages of evolution. And so
again, the Earth through time feeds into that concept as well.
Eric Ford: One of the challenges I think will be when
we ask, “How would you know if you saw a habitable planet?” One of the
most common things we appeal to is out-of-equilibrium chemistry, when we
pull up something like oxygen or ozone and some other reducing gas.
Well, for most of Earth's history that signature wasn't here. And so
maybe even if we have an inhabited planet, we see it and we characterize
it with spectroscopy, it would be hard to know that it is inhabited.
Sara Seager: That's so true. I would call it [one of]
the dirty little secrets that we don't tell you. We are looking forward
to the day that we have data, so we will be happy either way. In some
cases we'll assign you high probability, like when we see oxygen and it
looks just terra-centric, so we could be 99 percent sure that planet has
life that is generating that gas. In many other cases we'll just say
50/50, we don't really know. We're working towards having a quantitative
understanding as much as possible about which ones could and which ones
couldn't.
I know the topic today is not current exoplanet atmosphere observation
and theory, but I will tell you one of the most important lessons we're
learning is to live with uncertainty. We get to a point where we have
limited data and we make interpretations and the best we can do is give
you a range of possibilities. And the people in what I call “the real
world,” they’re not used to that way of looking at the world. But that
may be how it evolves for our generation for habitability. [
The Strangest Alien Planets (Gallery)]
Vikki Meadows: Can I just respond to Sara's particular
point and Eric's as well, about equilibrium signatures. Those are
incredibly valuable because you don't really have to have any a priori
sense of what the metabolism is or anything like that. So they're very
good general signatures. But you can also do a different type of study,
which is to figure out what a new bio-signature might be, by going
through the three steps that it takes to generate a bio-signature.
Which is to understand a metabolism and the by-products of that
metabolism, how that’s pumped into the atmosphere. To understand how
those by-products would survive in that atmosphere against the flux from
the parent star and against local chemical reactions. And then to look
at what remains, what builds up in the atmosphere and whether or not
that's detectable by a spectrometer onboard a telescope.
So we can actually do those studies, the VPL does those kinds of
studies, and the recent work done by Shawn Domagal-Goldman looking at an
anoxic atmosphere, the classic early Earth atmosphere with a sulphur
bio-signature at the bottom of it, and finding that in fact,
counter-intuitively, the sign of life was ethane. This was a sulphur
bio-sphere, because methyl radicals come off a lot of these biogenic
sulphur molecules and ended up producing more ethane in the atmosphere
than you get from photolysis of methane alone. So it may be possible to
start to learn about some of these signatures that are not just
equilibrium bio-signatures, but at least to run through a series of
possibilities for different metabolisms.
Sara Seager: The question is, are there any other pathways to form that excess ethane?
Vikki Meadows: Yes, by photolysis of methane.
Sara Seager: I was just trying to push you to say
whether you would be 100 percent sure if you saw this excess ethane. Or
whether it would be kind of sure, but not 100 percent.
NASA has lately been “following the water” in the
search for alien life. Will aliens need water in order to exist, or is
the presence of water just one more planetary condition that life must
learn to adapt to?
CREDIT: WHOI
Vikki Meadows: But that brings me to another sort of
paradigm of bio-signatures: you must understand the context of the
planetary environment in which you see it. And that will allow you to
try and determine whether there is enough ethane in the system, say from
geothermal activity that could produce the signature versus life. So
you do have to learn a little bit more about the environment.
Dirk Schulze-Makuch: Yeah, that's a very good point.
Because in some way you have this issue with Titan. Because you have a
lot of methane, and the question is, “Where does the methane come
from?” So the traditional view is from a formation theory of Titan.
But there is another view, it could actually be an end product of some
kind of a metabolic process.
On the other hand, you can envision a scenario where you have a lot of
oxygen in the atmosphere but it is not a bio-signature. Just imagine
Europa and you have a huge meteorite slamming into
Europa,
basically melting all the ice, then you have a water atmosphere, you
have this huge radiation from Jupiter, the hydrogen escapes to space
and, voila, you have an oxygen atmosphere, but you may have no life
whatsoever. So we have to really put it into context.
And one more point. Yes liquid water is important: we, our organisms,
our life on Earth learned to live with it. But that is not necessarily
because water is this greatly wonderful solvent. It has its problems.
If you come from Washington State where I am right now, there are no
leaves on the trees. Why is that? Because the volume of water expands
when it freezes and it tears up the cellular membranes. And so that
doesn't work. And life on Earth adapted to live with it. In winter
there are no leaves on the trees, no grass is growing, and in summer
everything starts blooming again. The point being is that water is not
that great, but Earth had a lot of water in it, and life on Earth had no
other choice than to adapt to water on our planet. If you are on a
very different planet with very different conditions, the result could
have been very different from that. And the real challenge is to see
what is universal with life, and what is really adaptation of life to
Earth's conditions.
Eric Ford: We heard yesterday about how important it was when we arrive on
Mars
to understand the context of what you are measuring. And it is similar
to some of the points you have made. But the challenges as astronomers
is that there are only some things that we can do for you. And the
quality of data that you are going to get is going to get very difficult
to understand anywhere near the level of contexts that some planetary
scientists are used to. So, mass, density, radius, maybe a little bit
about the fraction that’s in rock, ice, gas. Maybe a few atmospheric
signatures, but those will be very difficult to get. And that is an
optimistic view of what you'll have to work with. And so one of the
challenges will be taking what we are able to measure and then relating
it to what we'd like to know, even though there are so many other
missing variables.
You can watch the full "Great Exoplanet Debate" here: https://connect.arc.nasa.gov/p68qflmgnhk/?launcher=false&fcsContent=true&pbMode=normal
