Recent Progress: Researchers Characterize Exoplanet Atmospheres Using a 4-meter Ground-based Telescope and Achieve Significant Advancements
In recent times, researchers from the National Astronomical Observatory utilized a 4-meter aperture ground-based telescope located in Chile to obtain optical wavelength transmission spectra of two hot Jupiters, namely WASP-69b and WASP-121b. Through model analysis, they were able to provide important constraints on the atmospheric properties of these planets. The research findings were published in the Monthly Notices of the Royal Astronomical Society and Astronomy and Astrophysics Research, respectively.
Hot Jupiters are a type of extreme exoplanets that orbit very close to their host stars, with orbital periods generally less than 10 days. Due to long-term and intense stellar radiation, the dayside temperature of these planets typically exceeds 1000K. As a result, their atmospheres are relatively inflated, exhibiting strong spectral signals, making them one of the most important targets for atmospheric studies. Transmission spectra are crucial data used to investigate exoplanet atmospheres. They represent the difference between the spectra during planetary transits and the out-of-transit spectra, carrying information about the temperature structure and chemical composition of the planetary atmospheres. Therefore, transmission spectra can be utilized to characterize the atmospheric properties of these planets.
WASP-69b has a radius of 1.057±0.047 RJup, a mass of 0.260±0.017 MJup, and an orbital period of approximately 3.868 days. In the transmission spectrum, the researchers, including authors Ouyang Qinglin and Wang Wei, observed a slope caused by Rayleigh scattering, inferring that the atmosphere of this planet is primarily composed of hydrogen. Additionally, the transmission spectrum of WASP-69b exhibits significant oscillation signals in the range of 700-900 nm. Combining atmospheric models, the authors suggest that this may be attributed to titanium oxide absorption. Titanium oxide is commonly considered as the main component responsible for generating temperature inversions in hot Jupiter atmospheres. However, its presence has previously only been detected in the atmospheres of ultra-hot Jupiters (Teq > 2000K), as maintaining a gaseous state for titanium oxide requires sufficiently high temperatures (> 1500K). Thus, this is the first evidence of titanium oxide’s existence in a classical hot Jupiter.
Figure 1: Transmission spectrum of WASP-69b and the corresponding best-fit inversion models. The blue and red lines represent the best-fit inversion models for optical transmission spectrum and combined optical + near-infrared transmission spectrum, respectively. The lower panels show the posterior distributions of the parameters for the two inversion models. Both inversion models exhibit relatively high metallicity.
WASP-121b has a radius of 1.865±0.065 RJup, a mass of 1.183±0.064 MJup, and an orbital period of approximately 1.275 days. The authors found that the transmission spectrum they obtained differs from previous spectra, and there are also significant differences among the spectra obtained by different researchers. Through literature research and reanalysis of previous data, they believe that these differences are likely to be real and unlikely to be attributed to data processing procedures or stellar activity. In other words, the atmospheric composition of this planet may vary over time. Inversion analysis reveals the presence of absorption signals from titanium oxide and vanadium oxide in the atmosphere of WASP-121b. However, the abundances of these components are inconsistent with previous results, further supporting the possibility of temporal variations in the atmosphere of this planet.
Figure 2: Similar to Figure 1, the transmission spectrum of WASP-121b and the corresponding best-fit inversion models are shown. The lower panels display clear posterior distributions of the inversion parameters, depicting the posterior distributions of titanium oxide and vanadium oxide abundances, corresponding to the absorption of these components in the atmosphere.
Both studies represent the first observational research on exoplanet atmospheres using the SOAR telescope. The SOAR telescope, whose full name is Southern Astrophysical Research Telescope, is operated jointly by astronomical institutions from Brazil, Chile, and the United States. The relevant observational data were obtained through the application of the lead author, Wang Wei, during their tenure at the South American Astronomical Observatory of the Chinese Academy of Sciences. The authors of the studies point out that, compared to the current mainstream large-aperture ground-based telescopes and space telescopes, 4-meter telescopes such as SOAR have slightly lower precision. However, they can still provide important constraints on the atmospheric properties of exoplanets. Therefore, 4-meter ground-based telescopes like SOAR can be a cost-effective yet reliable option for studying exoplanet atmospheres, facilitating future research on a larger sample of planetary atmospheres.
Figure 3: The SOAR telescope located in Chile, with an aperture of 4.1 meters.
Paper 1 link: https://academic.oup.com/mnras/article/521/4/5860/7091929
Paper 2 link: https://doi.org/10.1088/1674-4527/accbb2