[Music]
This is Exoplanet Radio. I'm Tony Darnell.
Over the course of this show, we've talked many, many
times about the transit method for detecting exoplanets.
It is simply the measure of a star's decrease in
brightness as a star passes through our line of sight.
This once difficult measurement has now become commonplace and can even be done
with advanced amateur astronomy equipment under a reasonably dark sky.
From this measurement, we can infer a few things about the planets passing by.
We can get an indication of its size and, if we measure several transits, we can
get the period of the orbit around the host star.
We can also get an estimate of the period and
eccentricity of the orbit by the width of the dip in the light curve.
But there is another, even more difficult measurement we can make using transit
telescopes that are an important tool for
learning about planets around other stars.
Exoplanet Secondary Eclipses
To visualize a secondary eclipse is tricky. When the planet passes in front of the
star and blocks some of its light, we have a transit, or primary eclipse.
That part isn't so hard to visualize, and this measurement is much easier to make.
But now, imagine that as the planet travels in its orbit around the star, the
reflected light from the planet starts to add brightness to our observations
that gets ever so slightly brighter as it
travels around towards the back of the star.
The planet's reflected light is at its brightest just as it begins to pass behind
the star, and the moment it begins to go behind it,
the reflected light diminishes and shows up as a
secondary dimming in the measurement from the telescope.
This is a very difficult measurement to make, and it requires extreme precision
from the telescope, extraordinary detector sensitivity, and very careful
calibration and processing of the images.
From the secondary eclipse, astronomers get information
about the thermal structure of the atmosphere of the exoplanet.
As an example, in 2016, astronomers used the extensive Kepler dataset from the
exoplanet HAT-P-11b, which is a Neptune-like
planet that orbits its star once every five days.
The data spanned over five years of looking at the same spot in the sky.
Carefully analyzing these observations revealed a secondary eclipse.
From this tiny signal, astronomers inferred that the temperature of the planet
varied a lot as it went around the sun, and had a highly eccentric orbit.
They deduced this because there were large temperature variations from the highly
varied star-to-planet distances in the orbit.
The variations changed according to where in the orbit the planet was.
The analysis revealed that HAT-P-11b was hot, with a boiling sky.
The atmospheric temperature ranged from 300 Celsius to over 600 degrees Celsius.
Exoplanet secondary eclipses are brightest in the infrared, and while astronomers
have been able to tease out secondary eclipses from
data archives of the Kepler and TESS space telescopes,
with the deployment of the James Webb Space Telescope,
which is specifically designed to look in the infrared,
astronomers should begin to get more
characteristics from exoplanets from their secondary eclipses.
Learning more about exoplanetary atmospheres from planets around the galaxy will
fill in many blanks from these exotic worlds.
Exoplanet Radio is produced by Deep Astronomy. The music
is by Geodesium and available at Loch Ness Productions.com.
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