What is the geocorona, and how can modelling it help us find habitable exoplanets?

 

As a planet passes in front of a star, the size of the shadow it casts is different at different wavelengths. Typically the solid surface of the planet blocks all light. Above that, the wavelength variation of the absorption in the thin sliver of atmosphere seen against the star is used to help us understand the environment of that planet. But if you go to lyman-alpha (121.6 nm) wavelengths, the greatest absorber is hydrogen escaping from the top of the atmosphere. This sparse hydrogen region, called the geocorona, can be many times larger than the radius of the solid surface, blocking a much larger fraction of the star's light. The image above shows Earth's geocorona as seen from the moon, taken by Apollo 16 astronauts in 1972. https://www.esa.int/Science_Exploration/Space_Science/Earth_s_atmosphere_stretches_out_to_the_Moon_and_beyond

by Justin Kerr

For today’s PVL blog post, I am going to be giving you a brief introduction to my main research project with the team and eventual Master’s thesis. My research is focused on developing a better understanding of the hydrogen coronae that we expect to surround exoplanets in order to direct future searches for life with UV telescopes. 

But just what is a hydrogen corona? 

It may surprise you to hear that a small portion of the Earth's atmosphere extends past the orbit of the moon – this very outermost portion of Earth’s atmosphere consists of atomic hydrogen (so not the gas H2 that we are familiar with on the surface) which has yet to fully escape the gravity of the planet and is known as the geocorona. The geocorona is the hydrogen portion of the exosphere, which itself is defined as the region of the atmosphere where densities of atmospheric particles are so low that they can be described as collision-less. At the orbit of the moon, recent studies have shown that the density is so low that you would only find about one single atom of hydrogen per cubic meter. The geocorona extends past the moon to a radius of at least 100 times that of the Earth before eventually merging with empty space as the influence of the solar wind makes it impossible for the Earth to keep its grip on the atoms.
    
Now that we know what the geocorona is, how can we actually detect it? A single atom per cubic meter of empty space isn’t exactly something you could count visually or easily collect for sampling, after all. Luckily for us, hydrogen happens to be an exceptionally well studied element and we can borrow one of the most famous ways astronomers detect it throughout our galaxy. From quantum mechanics we know that the electrons in atoms can only occupy specific energy levels, and when they move between these levels they will either release or absorb light of a specific wavelength related to the energy difference between the two levels. 

For hydrogen, we know that when an electron moves between the first and second energy levels it will absorb or release light with a wavelength of 121.6nm. This specific line of light in the spectrum is known as the Lyman-alpha line. Since this wavelength falls within the ultraviolet (UV) portion of the electromagnetic spectrum of light, we can then detect it with UV telescopes. In the case of the geocorona, we can see this Lyman-alpha light around the Earth when 121.6nm light from the sun is absorbed by the geocoronal hydrogen and re-emitted in different directions. It is this light that the picture taken by the Apollo 16 astronauts shown above is seeing. This works slightly differently for exoplanets since the Lyman-alpha light produced by the planet’s hydrogen corona won’t make it all the way to Earth. Instead of looking for emissions from them, we look for drops in the Lyman-alpha light that should normally be produced by the star as some of it will have been absorbed by the corona and re-emitted in random directions.

So, we can see a hydrogen corona floating around an exoplanet. How does this relate to finding habitable worlds? It comes down to how a hydrogen corona is actually produced around a planet. From studying the geocorona, we know that it’s size is directly linked to the amount of water vapour located at the base of the exosphere. This is because the atomic hydrogen making up the geocorona is produced by the photo-dissociation (breaking apart of the molecule by light) of water. The size of the hydrogen corona around an exoplanet with an Earth-like atmosphere should also be linked to the amount of water vapour in the same location. While we can currently detect the hydrogen corona of gas giants with hydrogen-dominated atmospheres using the Hubble space telescope, we will need to wait for more powerful instruments such as the proposed LUVOIR telescope to see them around potentially habitable exoplanets.

The goal of my research is to use computer models of exoplanet atmospheres to understand how the size of the hydrogen corona changes with different atmospheric characteristics below the exobase. While telescopes such as LUVOIR which could be used to confirm our models are still a long way from launching, this work will help develop analysis plans and proposals for research so that we are ready when the necessary data becomes available for study. Even though it is just one potential piece of the massive effort put into the attempt to find alien life, I hope that my efforts here with PVL will one day play a role in this grand undertaking to better understand our place in the universe.