Hot super-Earths stripped by host stars

We live 150 million km away from our nearest star. It rises each morning, sets each night, and is our primary source of energy. Conditions in the hottest deserts may be almost unbearable for us but it could be much worse. 

Planets that lie much closer to their stars than Earth does to the sun (much closer even than the planet Mercury), may find themselves in a hot super-Earth desert. This is a harsh environment, but very different from the extreme deserts found on our planet. Here we are talking about a desert or a void characterised by a lack of observed ‘close-in’ planets around other stars. But why are these close-in planets absent from our observations?

According to a study published in the journal Nature Communications, planets that lie very close to their host stars are bombarded by a torrent of high-energy radiation. Because they are so close to the star, the heat that the planets suffer means that their atmospheric ‘envelopes’ have been blown away. This violent ‘stripping’ occurs in planets that are made up of a rocky core with a tenuous, gaseous outer layer.

We needed to obtain detailed information on the host stars to reach these conclusions. The planets were discovered by the ‘transit method’, in which they block light from the star as they orbit their hosts. Therefore, by knowing only the properties of the planets as well as that of the stars, we detect planets indirectly.

In this study, we used asteroseismology to characterise the stars, and the planets in levels of accuracy not achieved before for these systems. Asteroseismology uses the natural resonances of stars to reveal their properties and inner structures.  These natural resonances occur because  stars like the sun trap sound inside. The pitch of this instrument is dominated by the size of the star. This means we can work out the properties of a star by listening to their sounds.

But it’s not that simple; the sounds of stars do not travel through space as a sound that we could hear. Instead of listening directly to the stars, we watch them. Sound is a pressure wave that compresses and rarefies the medium it travels through and stars are just big balls of hot gas. As the sound compresses a star, it gets a little bit hotter and brighter. And similarly, when the sound rarefies the gas, it gets a little bit cooler and dimmer. 

We used NASA’s Kepler space telescope to observe a number of stars that have planets around them for nearly 4 years, continually measuring how bright these stars were. By analysing this data, we can work out the pitch of the star and then how big the star is. It is also possible to assess how far away the star is from the planet it is revolving around and also the planet’s size.

We were able to precisely characterise many stars and planets using these methods. The results showed a lack of close-in planets that were a little bigger than Earth. For these close-in planets, it is like standing next to a hairdryer turned up to its hottest setting. There has been much theoretical speculation that such planets might be stripped off their atmospheres. We now have observational evidence to confirm this, which removes any lingering doubts over the theory.

These results have important implications for understanding how stellar systems, like our own solar system, and their planets evolve over time and the crucial role played by the host star. Our results show that planets of a certain size that lie close to their stars are likely to have been much larger at the beginning of their lives. Those planets would have looked very different. 

We expect to discover and characterise many more of these ‘stripped systems’ using a new generation of satellites, including the NASA TESS Mission, which will be launched next year and ESA’s PLATO 2.0 due for launch in 2024. By examining the fate of faraway planets exposed to the full force of their host star, we can understand the implications of these discoveries closer to home.

(The author is with the University of Birmingham, UK and was part of the study.)