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!