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.