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With thousands of exoplanets in the bag, the game has moved on to characterizing those
exoplanets. From the Doppler method we get the mass and the distance of the planet from
the star. From the transit method we get the size of the exoplanet. Going beyond this requires
exquisitely detailed observations that have only so far been possible in a handful of
cases. Spectroscopy is a critical tool in this work................................
then another spectrum that just has the star and subtracting them to get a spectrum of
the exoplanet. That's the principle of how we might characterize an exoplanet using spectroscopy.
In practice, this method has only been used for a handful of targets. It requires the
extraordinary stability of the space environment and so has been done by the Hubble Space Telescope.
but actually only for jovian planets not for terrestrial exoplanets. It works like this:
as the planet passes in front of the star, light from the star is blocked by the planet
but some of that light filters through the atmosphere of the giant planet and comes to
us. The spectrum of a star has absorption lines imprinted from the cooler outer layers
of the star and those absorption lines give the spectral elements and the chemical composition
of the star, typically hydrogen and helium but also potassium, sodium and other heavy
elements in trace quantities. When we're just observing the star we get the stellar absorption
spectrum. When the exoplanet is in front of the star some of the star's light is filtered
through the atmosphere of the exoplanet and extra absorption is imprinted by that atmosphere.
In principle, differencing the two would give the spectrum of the exoplanet atmosphere.
In a handful of cases where its been used, several chemical elements have been detected
in the atmospheres of giant exoplanets: sodium, carbon dioxide, and water vapor, or steam. These
are hot jupiters, and so water is in the form of steam. These observations are proof of concept
for the eventual detection of biomarkers. Where we try to look for oxygen or ozone being imprinted
in the spectrum from the atmosphere of the exoplanet; that would be a sign of life. As
data accumulates, more and more exoplanets have both detections by the doppler method,
and transits. If you have a mass from the
doppler method, and a size from the transit method. You can use the two to get a density.
So, the fraction of exoplanets where both pieces of information are available are extremely
valuable. All you get, however, is one number—one mass and one size, and so that's a mean
density. By comparing that mean density to the mean density of rock or metal or water or gas,
it's possible to say what the average composition of the exoplanet is, and in principle distinguish
terrestrial planets from gas giants COROT-7b was an example of an early planet where this
technique was used. It turns out to have a similar density to the Earth, and is a super-earth
(several Earth masses sized object), however it orbits its star in only 20 hours and is
so close to that star that its surface its probably molten. Its nothing like an Earth-like
world. Another earth like planet found in the last few years has a size just under three-times
the size of the Earth and a mass about 6x the mass of the Earth. It's mean
density of 1.8 grams/per cubic centimeter implies the planet may be composed primarily
of water which has a density of one gram per cubic centimeter. This is exciting: the possible
detection of a water world. This planet is relatively nearby, only 40 light years away.
In principle, if there are living creatures on there, our light waves and radio communications
have swept over them since the dawn of the electronic age. Unfortunately, a single mean
density does not uniquely define an exoplanet.
There's wants called a redundancy or degeneracy in the models. In other words, there are many
different chemical compositions and layerings of a planet that can produce the same mean
density. It could happen if you had a mostly rocky planet or a planet with a gaseous envelope
and a small metallic core. Those would give the same mean density. So we need more information
before we can confidently talk about finding water worlds or earth's. In fact the zoo of
exoplanets contains extraordinary diversity. Based on these mean density measurements and
other chemical indicators, there appear to be exoplanets primarily made of metal, primarily made of
silicates like rock, primarily made of carbon and perhaps primarily made of water. An extraordinary
diversity of exoplanets, including some completely alien from the planets in our solar system.
Kepler has found earthlike worlds and giant worlds in many different orbits. Most of these
orbits, like the early exoplanet discoveries, are very close to their parent stars, so the
equilibrium temperatures on the rocky planets found by Kepler so far, are extremely hot.
These are not typically habitable worlds. Characterizing an exoplanet goes beyond the
simple discovery and the measurement of the mass or size. It either involves combining
information, such as from the doppler and transit method to give the mean density and
some sense of what the planet might be made of, or with much more difficulty in obtaining
a transmission spectrum of the atmosphere of the exoplanet. This is a proof of concept
for the detection of biomarkers, the way we might actually find life on another world.