The Titanium Chemistry of WASP-121 b: A High-Precision Look at an Ultrahot Jupiter

Ultrahot Jupiters, like WASP-121 b, are some of the most extreme exoplanets known. Their close proximity to their host stars results in intense heating, atmospheric ionization, and powerful winds. These planets are key laboratories for studying atmospheric chemistry under conditions that do not exist in our Solar System. WASP-121 b, with its inflated atmosphere and high temperatures (about 2,358 K), is an excellent candidate for detailed atmospheric studies. Previous observations suggested that titanium compounds, such as TiO, might be present, but their detection has remained elusive due to limited signal quality.

Observations and Methods

This study used the ESPRESSO spectrograph in its 4-UT mode at the Very Large Telescope (VLT) to observe two partial transits of WASP-121 b. By using high-resolution spectroscopy, the researchers performed a cross-correlation analysis, a method that enhances the detection of atmospheric elements by comparing observed spectra with model templates. They also applied narrow-band spectroscopy to detect individual spectral lines. These methods allowed them to identify multiple elements, including hydrogen, sodium, magnesium, calcium, iron, and notably, neutral titanium (Ti I).

Key Findings

One of the most significant results of this study was the high-confidence detection of Ti I in the atmosphere of WASP-121 b, reaching a statistical significance of about 19σ. This is surprising, as previous studies suggested that titanium might be depleted due to atmospheric processes that cause elements to condense and rain out. The study also found that Ti I appears to be concentrated at lower latitudes, within the planet’s equatorial jet stream, as evidenced by a blueshift in its velocity signature. This suggests that Ti I is affected by strong atmospheric winds that transport material across the planet.

Another major finding is the absence of TiO, which was expected to play a role in creating thermal inversions in the atmosphere. The researchers suggest that inaccuracies in the TiO line list, combined with possible depletion through condensation, might explain why TiO remains undetected.

Implications for Exoplanetary Atmospheres

These findings challenge existing atmospheric models, which predict that elements like Ti and V should behave similarly. However, the study shows that titanium is significantly depleted compared to vanadium. This suggests that the atmospheric circulation and chemistry of ultrahot Jupiters are more complex than previously thought. The study also highlights the need for improved theoretical models and more precise observations, particularly with space telescopes like the JWST, which could provide further clarity on the presence of TiO.

Conclusion

The ESPRESSO observations provide some of the most detailed insights into WASP-121 b’s atmospheric composition to date. The high signal-to-noise ratio achieved demonstrates the power of combining multiple telescopes in high-resolution spectroscopy. The study also sets the stage for future investigations using the Extremely Large Telescope (ELT), which will push the boundaries of exoplanet atmospheric science even further.

Source: Prinoth