Element 56 Discovered as the Heaviest Element within an Exoplanet Atmosphere
The research of exoplanet atmospheres primarily focuses on bodies known as ultra-hot Jupiters. As the name suggests, these are very large and very hot exoplanets. These tend to be such popular targets due to their large size resulting in high atmospheric scale heights (the height at which the atmospheric pressure of a body falls by a factor of 1/e), while their high temperature results in very large and easily observable atmospheres.
In the study led by Tiago Azevedo Silva, it was discovered that the atmospheres of two exoplanets, WASP-76 b and WASP-121 b, contained significant traces of ionised Barium within their upper regions. This is the heaviest element ever discovered within an exoplanet atmosphere. However, this discovery is quite puzzling, as even within the extreme environments of these ultra-hot Jupiters it was not expected to see such a heavy element, so high up.
In general, there are two competing effects that will influence the composition of a planetary atmosphere. These are the mass of the body, and the temperature of the surface. The former pulls particles towards the centre of gravity. This is what keeps atmospheres in place, as gaseous particles become gravitationally bound to the planet. The latter effect is directly correlated to the average kinetic energy of particles, and causes these to exist at higher altitudes.
Bodies will attempt orbit where their gravitational potential is twice their kinetic energy, and can escape bound orbit when these two are equal. In general, heavier particles escape less easily, as they will have lower velocities for a certain kinetic energy, than lighter particles. They will also be more strongly attracted by gravity and will require a greater escape velocity. This balance dictates what elements will remain in a bound orbit and make up an atmosphere. As such, extremely heavy elements, such as Barium, tend to remain significantly closer to the gravitational centre, and thus not be detectable within a planet’s atmosphere.
The data used concerned two distinct transits of the exoplanets across their home stars, Wasp 76 and Wasp 121 respectively. During these periods, light emitted by the star will pass through the planetary atmospheres. From Earth, we only observe certain wavelengths of light that have been transmitted, since certain wavelengths will be scattered and absorbed differently by particles, causing troughs within perceived spectrum. Using a spectrograph, these wavelengths can be captured and a spectrum is plotted. These plots are then matched with known patterns to identify the composition of the atmosphere and the abundance of certain elements.
Azevedo Silva et al cite recent developments in the instrumental accuracy of spectroscopic observations as a major contributor to the discovery, namely the Echelle SPectrograph for Rocky Exoplanets (ESPRESSO) at the Very Large Telescope Observatory, which produced the spectrum used, between λ = 3800 Å and λ = 7880 Å. The discovery itself is cited to be an accidental one, as the team were not looking to add to the known composition of these planet’s atmospheres. Instead, they were hoping to use this higher resolution imagery to describe the dynamics of these systems.
The study mentions that this discovery remains exactly that, merely the identification of an unusual phenomena. Despite these two being popular targets for exoplanetary research, no mechanisms by which ionised Barium could appear that high have been discovered. Additionally, this raises the possibility that many other ultra-hot Jupiters will exhibit similar atmospheric compositions, but have yet to be fully probed.
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Journal Source: T. Azevedo Silva et al, Detection of barium in the atmospheres of the ultra-hot gas giants WASP-76b and WASP-121b, Astronomy & Astrophysics, Vol. 666, 2022
Cover Image: NASA/JPL-Caltech