ORIGINS OF LIFE - ASTROBIOLOGY
AND GENERAL THEORIES OF LIFE

EXOPLANETS - EXOPLANET
ATMOSPHERIC CHARACTERIZATION

Welcome to the lecture.

Astronomers have discovered

thousands of extrasolar planets
or exoplanets.

Is any of them inhabited like the Earth?

How can we search for it?

In this lecture, I will talk about

the techniques to study exoplanet
atmospheres and possibly surfaces,

which is an essential step towards
finding life on exoplanets.

Detection of exoplanets typically
comes with two properties:

the size and the orbit,

including the distance from the host star.

For evaluating their potential for life,

we definitely need to get
more information.

But, how?

It has to be reminded that
exoplanets are light-years away.

This means that it's not realistic
to actually visit there

and it's not possible either
to get the spatially resolved images,

unlike the case of solar system planets.

They are just point sources

and we have to rely on remote
observations of these faint dots.

But, observations of these faint dots,
if possible at all,

can in principle give us hints
about the nature of the planets.

For example,
if you observe an Earth Twin,

this is the overall spectrum
you would get.

At shorter wavelength,
the planet illuminates

by scattering the light
from the host star.

And, it's blue features depend on,
for example,

surface composition,
atmospheric pressures and clouds.

On the other hand,
in the invisible infrared range,

the planet emits light
because of its thermal energy,

and its baseline depends on
the temperature structure

of the surface layers.

Imprinted in these baselines
are the lower features

due to absorption
by atmospheric species.

In the case of an Earth Twin,

they include astrobiologically important

water vapor, oxygen
and ozone features.

We could also potentially use

the time variation of these features
due to planet rotation,

which essentially allows us
to scan the planet

and highlight the regional features.

However, it's not straightforward
to detect the light from exoplanets.

Seen from afar, an exoplanet is
very close to its own host star,

and the star is several to ten
orders of magnitude brighter.

It's equivalent to seeing a firefly
right next to a lighthouse.

Suppose you try to take a picture
of an exoplanet.

Then, the host star is always there,

and, on the imaging plane,
the star is blurred

and the planet is in the skirt of it.

Compared to the peak intensity,

the skirt is orders of magnitude darker,

but planets are often even fainter,
so the signal is varied.

In order to identify the planetary signal,

we need to suppress the light
from the host star

using special instruments.

The idea of such direct imaging
observations of Earth-like planets

dates back to the 1990s.

But, the starlight suppression
is technically challenging.

In the past decade,
direct imaging has been successful

for young, luminous, giant,
gaseous planets at wide orbits.

Earth-like planets are about
ten times smaller in diameter

and much fainter than
these successful targets.

The efforts are ongoing to achieve
the hyper-suppression level

to be able to detect an Earth Twin.

Meanwhile, the discovery
of transiting planets

opened up new possibilities
to study exoplanet atmospheres

without using special instruments.

A transiting planet is a planet which
passes right in front of the star

because its orbital plane
is close to the line of sight.

During the transit, a small portion
of the stellar light

is filtered through
the planetary atmosphere.

By analyzing the spectrum
of this tiny portion

and finding the scattering
or absorption features in there,

we can learn about
the atmospheric composition

and the presence of condensates
such as clouds.

This technique is called
"transmission spectroscopy."

In many cases,

transiting planets also pass
behind the host star

and that's called "planetary"
or "secondary" eclipse.

At the time of the eclipse,
the planetary flux is blocked by the star.

So, the difference between
the out-of-eclipse total flux

and the in-eclipse flux

corresponds to the brightness
of the planetary dayside.

This way, using planetary eclipse,

we can identify the spectrum
of the planet

without directly separating the star
and the planet in the imaging plate.

In addition, while the planet
orbits the star,

the varying portion of the planetary
dayside faces us

and the planetary flux
changes in time accordingly.

In turn, although we cannot resolve
the star and the planet,

the time variation in the total flux

can be attributed to
the planetary component

assuming that the star is stable.

These three techniques
have been successful

with hot Jupiter-like planets
and have revealed

some of the atmospheric species
and thermal structures.

In order to apply them to smaller,
potentially habitable planets, however,

we need to push the current
technology to its limit.

The techniques introduced so far
have pros and cons

and the relevant targets vary.

So, we will use all of them
to study various aspects of exoplanets.

In the next decade,
new powerful observatories

with spectroscopic capability
will come into play.

The main targets will probably be
large gaseous planets

but some basic investigations
of terrestrial sized planets

are also being attempted.

Future missions aimed at

the most detailed study
of potentially habitable planets

are currently under discussion.

In this lecture,
we discussed the key observations

to study the nature of exoplanets
beyond their size in the orbit.

I encourage you to think further

about how you would find life
on an Earth Twin light-years away

using these techniques.

It will provide and unique view
on Earth's biosphere.

Here are the suggestions
for further reading

and this is the information of
the studies referred to in this lecture.

Thank you for listening!