Welcome to "Hunting for habitable worlds."
This lecture takes us away from our own planet
to look at what we currently know about planets orbiting around other stars.
Before the early 1990s, the only planets we knew for sure that existed
were the worlds that orbited around our own Sun,
but as our instruments became sensitive enough to spot
the dim whisper of a planet around other stars in our galaxy,
we discovered our planetary system was one of multitudes.
We now know of thousands of extrasolar planets or exoplanets -
planets that orbit stars other than our Sun.
This results in an obvious question:
could any of these newly discovered worlds be habitable?
The problem with that question is, while we have discovered many worlds,
we actually know very little about each planet.
The majority of planets we have discovered so far
have been found by one of two techniques:
the radial velocity technique used by ground-based telescopes,
such as the ESO Observatory in Chile,
or the transit technique used by instruments such as
the Kepler space telescope and its successor - TESS.
The radial velocity technique,
sometimes known as the "Doppler wobble,"
detects a planet via the tiny wobble it excites in the star.
While we normally think of the star as stationary and the planet in orbit,
in truth, both the star and planet orbit their common center of mass.
As a star is so much bigger than the planet,
this center of mass lies very close to the star's own center,
causing its orbit to be just a tiny wobble
in comparison to the planet's wide circuit.
This wobble causes the star to move periodically
slightly further away and then closer to the Earth.
As the star moves slightly from the Earth,
its light waves stretch out and redden slightly.
Conversely, as a star moves back towards us,
the light waves compress and become bluer.
This regular shift from red to blue
is what astronomers can measure to detect a planet.
The second main method for planet detection
is the transit technique.
Here, a slight dip in the star's brightness
is detected as the planet passes in front of the star
as seen from Earth.
These two methods give you just two properties about the planet.
The transit technique gives you an estimate of the planet's radius
while the radial velocity technique
tells you about the planet's minimum mass.
This may be significantly less than the true mass of the planets
as the radial velocity technique only measures the wobble of the star
directly towards the Earth.
If the planet's orbit is tilted with respect to us,
then part of the star's motion will be directed away from us.
We won't measure this and so underestimate the planet mass.
Both techniques also tell you about the amount of radiation
the planet receives from the star.
But, this can be very different from the surface temperature
as it does not allow for the heat trapping effects
of the different atmospheric gases.
The challenge we're trying to determine -
if a plant is habitable -
is therefore that we can only measure two or three properties
and none of these actually tell us what it's like on the planet surface.
This will change as the next generation of telescopes
will be able to detect light that passes through the planet's atmosphere.
Different molecules in the atmosphere absorb different wavelengths of light,
providing a fingerprint of missing wavelengths
that indicate atmospheric composition -
our first hint at what is happening on the planet's surface.
But, this brings us to a new problem:
such atmospheric spectroscopy for rocky, temperate planets
is time-consuming and difficult.
We therefore need a way of selecting planets
most likely to reveal interesting results.
But how do we select planets best suited for habitability
without knowing any surface properties?
Let's think about what we want to find.
It's going to be easiest to recognize Earth-like life,
that is, water and carbon-based chemistry.
Also, this needs to be detectable,
which means the water needs to be on the surface of the planet,
not a subsurface system like Europa.
Based on this, we can ask the question:
how much insulation does an Earth-like planet need?
The answer to this is a "Classical Habitable Zone."
The Classical Habitable Zone is where an Earth-like planet,
that is, a planet with our surface pressure,
atmospheric gases and geological processes
can support water on the surface.
Often, in exoplanet literature,
this is simply referred to as the "habitable zone"
as we don't yet know about planets other than the Earth
that can support life.
At the inner edge of the habitable zone,
it is too warm for surface water on the Earth and it evaporates.
At the outer edge, carbon dioxide condenses into clouds
and is no longer able to provide the thermal insulation
of a greenhouse gas - so the planet freezes.
Climate models predict that the habitable zone should stretch
between 0.99 au and 1.67 au
where 1 au is the average distance of the Earth from the Sun.
Our planet, therefore, sits right on the inner edge.
A slight extension to this is known as the "optimistic habitable zone,"
which can broaden these limits based on the idea
that Venus and Mars probably have supported surface water in their past.
So, an earth-like planet could have a period of habitability
just outside the habitable zone edges.
The edges of the classical habitable zone
are only calculated for the Earth.
This is easily demonstrated as,
while Venus sits outside the habitable zone,
both the Moon and Mars orbit within it
but neither are Earth-like enough to support liquid water in this region.
Different planets might have different habitable zones at different locations,
or they may not have a habitable zone at all.
Of the planets we found so far orbiting in the classical habitable zone,
almost 15 times as many are large enough
to likely have thick, Neptune-like atmospheres
compared to planets that might be rocky.
We have discovered planets that are the right size to be rocky
and orbit entirely within the habitable zone.
Are these Earth-like enough to support liquid water in this region?
We don't know.
They may have very different atmospheric gases
or geology that makes surface water impossible.
The only thing we can say is that
if another habitable, Earth-like planet is out there,
it would be in the habitable zone, but being in the habitable zone
does not mean you're Earth-like enough for life.
So, in conclusion, we've discovered thousands of exoplanets,
many of which are similar in size to the Earth.
But, at the moment,
we have no way of knowing what their surfaces are like.
Note, in particular,
that the Earth and Venus are both very similar in size -
so, they are both Earth-sized planets.
Our next generation of telescopes
will be able to detect the atmosphere of these worlds
and tell us something about their surfaces for the first time.
The habitable zone is a useful concept
for selecting planets for these new telescopes
but it offers no guarantee that a planet is actually habitable.
If you'd like to try playing with
a simple climate model of an Earth-like planet,
you can head over to "earthlike.world" or the associated Twitter feed.
This website lets you see
how different a planet might be from our own world today,
even if it did have the same geological cycles as our own.
The NASA NExSS "Many Worlds" blog
covers the latest news for exoplanets and many origin of life stories.
There's also a more technical overview of the search for biosignatures
in a paper led by Yuka Fujii, published in "Astrobiology" last year.
These are the references that were mentioned during the lecture.
Thank you very much for listening.