Scaled up from Galilean Moon
HR 8799 planetary system has 4 planets whose period ratios are almost 1:2:4:8, very similar to those of Galilean moons of Jupiter. I have been studying the system since 2017 and published 4 papers on the system. Almost every paper comes with a surprise.
The first paper in 2018 surprised me in that we only detect water but not methane in the atmosphere of HR 8799 c. Methane was once thought to be abundant for a planet with temperature around ~1000 K, so we set up to detect its 3.3 micron feature using the Keck telescope. We came away empty-handed for methane but detected water in the atmosphere. This was one of first few papers that directly detected molecules in a directly-imaged exoplanet.
The first paper in 2018 surprised me in that we only detect water but not methane in the atmosphere of HR 8799 c. Methane was once thought to be abundant for a planet with temperature around ~1000 K, so we set up to detect its 3.3 micron feature using the Keck telescope. We came away empty-handed for methane but detected water in the atmosphere. This was one of first few papers that directly detected molecules in a directly-imaged exoplanet.
The second paper in 2020 was written during the pandemic. We had new data for HR 8799 c from CHARIS at the Subaru telescope and LMIRCam at the Large Binocular Telescope, together with archival data, we attempted to understand the chemical composition of the planet's atmosphere. We were surprised again by that different data resulted in different answers even though we knew that the answers should be the same. Either the data were not good enough or different methods were not convergent. Nonetheless, this work raised the issue for the first time and set the stage for more careful spectral analyses in the future.
The third paper in 2023 was a breakthrough of studying HR 8799 c in both the data and the analysis. We harvested high-resolution data from the Keck Planet Imager and Characterizer (KPIC), for which I contributed to conceptualizing and building. I had also improved the spectral analysis method by incorporating clouds, which are common for exoplanets. The model fit well data, both the KPIC data and other archival data. We were able to conclude that the planet HR 8799 c formed beyond the carbon-monoxide snowline and accreted solids to enrich its atmosphere.
But how much solids had been accreted? The fourth paper in 2025 answered the question, but with a surprising answer: HR 8799 c accreted ~40 Earth's masses solids during the formation process, like many other directly-imaged exoplanets. Where can one planet find that much solids, it is almost as much as adding all solids available in the solar system! The answer is that these planets formed very early, as early as when the system was younger than 2 Myrs, that is when this much solids might be available. An OSU news about this paper can be found here.
