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Barium in Atmospheres of WASP-76b & WASP-121b


Exoplanets and the Transit Method

Since the discovery of the first exoplanet in 1992, over 5,200 planetary bodies have been confirmed (Exoplanet Catalog | NASA, 2022). With each discovery, humanity deepens its understanding of Earth’s place among countless planetary possibilities. Many attributes of a planet can be calculated using data from the radial velocity and transit detection methods and spectroscopy. The transit method was utilized to extract information about WASP-76b and WASP-121b, two gas giants ~634 LY and ~880 LY away, respectively (Azevedo Silva et al., 2022). The transit method measures the dip in a star's luminosity as a planet passes by the star during its orbit. Through careful and repeated observation, this slight decrease in brightness is detected at regular intervals and later confirmed as a planet’s orbit. Larger planets cause a larger dip in starlight during transit, visible as an increased transit depth on a graph. This means that larger planets, often gas giants, are most frequently detected with the transit method. The size of an exoplanet can be calculated using the stellar radii of an exoplanet’s host star and the transit depth created as the exoplanet orbits across the face of the star. (What’s a Transit?, n.d.)


Transit Spectroscopy

The atmosphere of exoplanets can be studied through a variation of the transit method, referred to as the transit spectroscopy method. During a transit, the light from a planet’s host star passes through the atmosphere of the exoplanet. When the photons interact with the exo-atmosphere, the elements in the atmosphere change the spectra of the light. The light detected on Earth contains a unique chemical footprint corresponding to the elements the light filtered through in the exo-atmosphere. These spectra can be decoded to determine atmospheric composition. However, the light also passes through Earth’s atmosphere, further influencing the spectra. To isolate the spectra of the exo-atmosphere and correct for Earth’s absorption lines, scientists use software tools, such as the European Southern Observatory’s (ESO) Molecfit. (Kausch et al., 2015; Smette et al., 2015). Since the first exo-atmosphere was observed in 2001, high-resolution spectroscopy has significantly progressed (Charbonneau et al., 2002). Developments in observing planetary spectra offer pioneering insights into the atmospheric conditions of the universe’s largest planetary bodies: ultra-hot gas giants.


Detection of Barium

Researchers detected barium in the atmospheres of two separate gas giants, WASP-76b and WASP-121b, using this spectroscopy technique. Barium is the heaviest element ever detected on an exoplanet. Both exoplanets are gas giants, categorized as Hot Jupiters. This type of planet, as its name suggests, is a class of exoplanets with conditions akin to those on Jupiter. They often undergo planetary migration and hold orbits extremely close to their host stars. The research was presented in “Detection of Barium in the Atmospheres of Ultra-Hot Gas Giants WASP-76b & WASP-121b” in Astronomy and Astrophysics on October 13, 2022 (Azevedo Silva et al., 2022).


The discovery of barium was made using the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) instrument. The instrument is located under the platform for the Very Large Telescope (VLT) on Cerro Paranal, a mountain in the Atacama Desert of northern Chile. The VLT is composed of four unit telescopes (UTs) and four auxiliary telescopes (ATs). When working together, the ATs and UTs create an interferometer -- a tool that measures interference patterns -- that is capable of observing astronomical bodies at a higher precision than if either type of telescope operated individually (European Southern Observatory, n.d.). The VLT observes transits and feeds ESPRESSO data. ESPRESSO then extracts spectroscopic information to be interpreted by astronomers. To account for Earth’s absorption lines in the received spectra and the Barycentric Earth Radial Velocity (BERV), Azevedo Silva et al. used the Molecfit5 pipeline within the EsoReflex environment (ESO - EsoReflex, n.d.). Doppler shifts were present due to the gravitational pull of the exoplanets and their host stars. To correct this, Keplerian models were applied to the spectra. Among other elements, Azevedo Silva et al. detected barium when analyzing data from VLT’s observations of two transits of each planet.


Barium

Barium (Ba), atomic number 56, is highly reactive and cannot be found as a free element on Earth. This means that it is always found combined with other elements. It is an alkaline earth metal and falls into group two on the periodic table with five other elements (beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and radium (Ra)). These elements all contain a full outer s-orbital. Alkaline earth metals frequently lose the two electrons of their outer shell to form +2-charged cations. With a Mohs hardness of 1.25, barium is considered a very soft metal. Barium is the fourteenth most abundant element in Earth’s crust, with an abundance of 0.0425% (compared to oxygen, silicon, aluminum, and iron, which hold 46.1%, 28.2, 8.23%, and 5.63% abundances, respectively). In Earth’s crust barium exists primarily as the minerals barite and witherite. With a weight of 137.327 atomic units (AU), barium is nearly two and a half times as heavy as iron (“Abundance of Elements in Earth’s Crust,” 2022; “Barium,” 2022; Barium | Uses, Compounds, & Facts | Britannica, n.d.).


WASP-76b

Located ~634 LY away, WASP-76b was announced by scientists in 2016 and orbits an F-type star -- one that is slightly larger and hotter than the Sun -- in the Pisces constellation (WASP-76 | NASA Exoplanet Archive, n.d.). The exoplanet has a mass of 0.92 Jupiters, an orbital distance of 0.033 AU, and an orbital period of 1.8 days. The planet is also tidally locked, meaning the same side of the planet constantly faces its host star. At extreme dayside temperatures of ~4,400 F (2,426 C), iron and other metals exist in a gaseous state (Exoplanet-Catalog – Exoplanet Exploration: Planets Beyond Our Solar System WASP-76 b, n.d.). Azevedo Silva et al. confirmed previous spectrographic detections that reported H, Li, Na, Ca+, V, Cr, Mn, and Fe. (Kesseli et al., 2022; Tabernero et al., 2021). Azevedo Silva et al. did not recover K, due to regions excluded because of high telluric contamination, or Co, Ni, or Sr+. It is thought that the latter three species were not detected due to differing retrieval processes from Kesseli et al., who did report their presence in the atmosphere. Azevedo Silva et al. notably reported their finding of Ba+, with a strong absorption signal during both transits.


WASP-121b

WASP-121b was announced in 2016 and is ~880 light-years away from Earth, orbiting an F-type star (WASP-121 | NASA Exoplanet Archive, n.d.). It is tidally locked, reaching dayside temperatures of ~5,840 F (3,227 C) at the top layer of the atmosphere (Strickland, 2022). With a mass of 1.157 Jupiters and an orbital distance of 0.02596 AU, it takes WASP-121b only 1.3 days to complete its orbit. Powerful gravitational forces pull the planet towards its host star, deforming the shape of WASP-121b into an oval (Exoplanet-Catalog, n.d.). As previously reported by Borsa et al., Azevedo Silva et al. detected H, Li, Na, Mg, Ca+, V, Cr, Mn, and Fe, as well as Ca and Ni reported by Merritt et al. (Borsa et al., 2021; Merritt et al., 2021). New species detected by Azevedo Silva et al. include Ba+, Co, Sr+, and the potential presence of Ti+, which would require further research to confirm.


Implications for Astrobiology

A heavy element like barium would be predicted to exist in lower layers of the atmosphere on such high-gravity planets. Thus, the detection of barium in the upper layers of both exo-atmospheres directly contradicts current understandings, suggesting it is more than an anomaly. The presence of this heavy species may be evidence of a new planetary process. It implies that barium may be found in the upper layers of other Hot Jupiters. What keeps the element from falling into lower layers is unknown. Azevedo Silva et al. also commented on the ionization trend of the detected alkaline earth metals (Ca+, Sr+, and Ba+). The pattern of heavy ionized species at high altitudes in gas giant atmospheres may be further evidence of unknown planetary dynamics.


Short orbital periods of gas giants provide frequent transits to observe and more data to collect. Scientists plan to further examine both WASP-76b and WASP-121b in an attempt to understand the role of planetary atmospheric escape and the mechanisms behind their extreme conditions. The discovery of barium is indicative of how little is known about the possibilities of planetary composition. Soon, high-resolution spectroscopy will not be limited to examining the atmospheres of gas giants. Perhaps most exciting, investigating smaller exoplanets that have previously been unavailable for atmospheric analysis will increase the possibility of detecting evidence of life. Such evidence would be collected through the detection of a biosignature -- likely an atmospheric gas that could only be produced by a lifeform.


Although the discovery of over 5,000 exoplanets is a milestone in modern science, that number is staggeringly small when considered in perspective. As transit spectroscopic techniques improve, more data will be available to extract from each planet. Instruments such as the James Webb Space Telescope (JWST), ESPRESSO, or the high-resolution Armazones High Dispersion Echelle Spectrograph (ANDES) on the Extremely Large Telescope (ELT) currently being built in Chile will allow deeper investigation of exoplanets and their properties, offering a chance to learn about an untold number of new planets.


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