by Jim Shih
Jim Shih has a MSc degree in Astronomy from the University of Amsterdam. After completing his degree, he worked as a guest researcher at the Anton Pannekoek Institute for Astronomy, also at the University of Amsterdam. He is interested in studying exoplanets and learning about the unthinkable properties and diverse compositions they hold with models and observations – they are just so mind-blowing.
In his free time, he likes to sing and photograph things he finds beautiful (my photography account IG@jimmyshootttt). He is also a huge fan of monster movies, especially Godzilla.
What will happen to a gaseous planet if it gets too close to a hot star? Would it be on fire? Melt? Or even evaporate? At first glance, this question may seem very silly and trivial, but figuring out the answer to this problem can actually help us explain one of the most intriguing features observed among exoplanet populations – the identification of radius valley!
Stars at short distances – The perfect “trainers” for weight-loss
When a planet with an atmosphere orbits in close proximity to its host star (typically less than 0.08 au), a process called “photoevaporation” could take place. Under the exposure to starlight at such short distances, the intense high-energy radiation (i.e., X-ray and Extreme-Ultra-Violet photons) will heat up the gas in the atmosphere of the planet. Just like putting raw dough inside a hot oven, this constant supply of heat energy from the star will cause the atmosphere to expand. As a result, the planet would appear to be larger in size to an outside observer. Eventually, the atmosphere will be so extended or “puffed up” that it becomes difficult for the planet to hold on to it because the now “heated” gas would have enough energy to overcome the downward gravitational force and escape outward into space. A similar analogy to how water molecules, initially bound together as a liquid, turn into unbounded water vapour and evaporate off the surface of the liquid water when heated.
All in all, the escape of gas from a planet’s surface into space can be triggered with extensive heating, leading to the planet gradually losing its material over time. Or, in other words, the planet is undergoing weight-loss!
What does atmospheric loss look like?
So, if we could teleport ourselves in front of a planet with an escaping atmosphere, what would it look like? It might be tempting to picture in our head – a gaseous sphere with gas shooting outward in all directions. But one must not forget that a planet is also a body that is in constant motion as it orbits around a star. Therefore, just as your hair would be blown back when running directly into a strong wind, a planet losing its atmosphere will leave a trail of gas behind it as it orbits around the star (Fig. 1). Moreover, what is interesting about this “planet-tail” is that astronomers can use it to deduce valuable data, such as information about the chemical makeup of the atmosphere, its density, and even the rate at which the atmosphere is being lost.
How does this affect the evolution of the planet?
One of the most famous and intriguing results from the study of exoplanets after decades of population surveys is the identification of the so-called “Radius Valley” or “Radius gap”. It turns out, peculiarly, that if we gather all the exoplanets we’ve observed so far and sort them out by their corresponding size in radius, there is a drop in the number of detected planets between 1.5 and 2.0 Earth radii (see Fig. 2). While several models have been put forward to explain this observational feature, a leading and perhaps the most prominent theory for explaining this missing gap is the atmospheric loss of planets during their evolution – whereby the planets above the gap hold on to their atmosphere while those below the gap experience either partial or complete loss of their atmospheres over time.
As more sophisticated models and observational evidence become readily available, astronomers have been able to gain insights into the details of atmospheric loss processes such as photoevaporation (discussed above) or core-powered mass-loss. That being said, atmospheric loss remains a very active area of research and a better understanding of it is expected to help us uncover the conditions that contribute to the exoplanet’s eventual demographic and even the habitable potential they may harbour.
Astrobite edited by Jessie Thwaites
Featured image credit: Made by Stickers and edited by Jim Shih, via Flaticon.
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