Spectroscopic time series performance of the Mid-Infrared Instrument on the JWST

TL;DR

This paper demonstrates the first mid-infrared spectroscopic time series observation of an exoplanet using JWST's MIRI instrument, achieving high precision and validating its potential for atmospheric studies.

Contribution

It provides the first performance assessment of MIRI's Low Resolution Spectroscopy mode for exoplanet transit observations, with independent analyses confirming its high precision capabilities.

Findings

Achieved ~50 ppm spectro-photometric precision in 7-8 micron range.

Recovered known transit depth within 1 sigma.

Measured noise slightly higher than simulations, with some excess noise at shorter wavelengths.

Abstract

We present here the first ever mid-infrared spectroscopic time series observation of the transiting exoplanet \object{L 168-9 b} with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope. The data were obtained as part of the MIRI commissioning activities, to characterize the performance of the Low Resolution Spectroscopy (LRS) mode for these challenging observations. To assess the MIRI LRS performance, we performed two independent analyses of the data. We find that with a single transit observation we reached a spectro-photometric precision of $\sim$50 ppm in the 7-8 \micron range at R=50, consistent with $\sim$25 ppm systematic noise. The derived band averaged transit depth is 524 $\pm$ 15 ppm and 547 $\pm$ 13 ppm for the two applied analysis methods, respectively, recovering the known transit depth to within 1 $\sigma$. The measured noise in the planet's transmission spectrum is approximately 15-20 \% higher than random noise simulations over wavelengths $6.8 \lesssim \lambda \lesssim 11$ $\mu$m. \added{We observed an larger excess noise at the shortest wavelengths of up to a factor of two, for which possible causes are discussed.} This performance was achieved with limited in-flight calibration data, demonstrating the future potential of MIRI for the characterization of exoplanet atmospheres.