Exploring Exoplanet Atmospheres: Low-Resolution Spectroscopy of Three Hot Jupiters with the Himalayan Chandra Telescope

This study, led by Athira Unni, explores the atmospheres of three "hot Jupiters"—giant planets with high temperatures due to their proximity to their host stars. The planets observed, HAT-P-1b, WASP-127b, and KELT-18b, were studied through low-resolution transmission spectroscopy using the 2m Himalayan Chandra Telescope (HCT) in Hanle, India. This method allows astronomers to examine the spectrum of light passing through the planet's atmosphere during transit, revealing its atmospheric composition. The research marks the first successful use of the HCT for transmission spectroscopy and introduces the potential for smaller telescopes in exoplanet studies.

Observational Setup and Methodology

Data collection took place at the Indian Astronomical Observatory, with the HCT equipped with the HFOSC spectrograph, capturing wavelengths from approximately 3800 Å to 8350 Å. A significant part of the study was choosing appropriate reference stars for each exoplanet, which helped track slit losses and environmental effects, critical for maintaining data quality over long observations. Data reduction involved calibrating for wavelength shifts, applying bias correction, and constructing a “white light curve” for each star—essentially a measure of the star's light intensity across all observed wavelengths during transit. The research team used the Common Mode Correction (CMC) technique to improve data precision by reducing systematic errors common across wavelengths.

Transmission Spectra of Three Planets

The study presented transmission spectra for each planet. HAT-P-1b’s data, collected using the CMC technique, aligned closely with previous observations, revealing a spectrum hinting at Rayleigh scattering—a scattering process similar to how Earth's atmosphere scatters blue light. WASP-127b, an unusually puffy, low-density planet, also showed strong Rayleigh scattering, supporting findings from other studies that suggest a hazy atmosphere. Meanwhile, the spectra for KELT-18b, presented here for the first time, lacked notable features, which the authors attribute to lower precision and an inherently small atmospheric scale height.

Modeling the Observed Atmospheres

Using a set of atmospheric models known as ATMO, the authors analyzed the transmission spectra to estimate atmospheric characteristics. They modeled factors like temperature, metallicity (elements heavier than hydrogen and helium), and haze/cloud factors. For HAT-P-1b and WASP-127b, the team detected substantial Rayleigh scattering but no distinct absorption signatures of individual atoms or molecules due to the low resolution. For KELT-18b, the lack of signal limited the model’s ability to constrain its atmospheric properties accurately.

Combining Ground and Space-Based Observations

To refine their models, the authors combined HCT data with infrared observations from space-based telescopes like the Hubble Space Telescope (HST) and Spitzer. This approach provided better constraints on haze levels, temperature, and other properties for HAT-P-1b and WASP-127b, highlighting the complementary nature of ground and space-based observations. The authors suggest that HCT could serve as a valuable partner to larger telescopes for multi-wavelength studies of exoplanet atmospheres.

Implications and Future Directions

This proof-of-concept study demonstrates the capability of smaller telescopes like the HCT for atmospheric studies of exoplanets, particularly with the right observational techniques. The authors recommend a careful selection of targets with high signal-to-noise ratios and suitable reference stars to maximize precision. By demonstrating the effectiveness of a relatively modest telescope, this work opens the door for broader participation in exoplanetary science using smaller, accessible facilities.

Source: Unni