Episode for October 4th, 2023 Download

Exoplanet HAT-P-11 b

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 the 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.

As you can imagine, 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 data from the exoplanet HAT-P-11b - 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; it 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 explanets from their secondary eclipses. Learning more about exoplanetary atmospheres from planets around the galaxy will fill in many blanks from these exotic worlds.