AN EXOPLANET'S MIGRATION HISTORY WRITTEN IN THE CHEMICAL FINGERPRINTS OF ITS ATMOSPHERE

Never before had the chemical composition of an extrasolar planet's atmosphere been studied in such great detail. Now, an international team of scientists led by the Italian National Institute for Astrophysics (INAF) has for the first time revealed the simultaneous presence of six chemical compounds - water, carbon monoxide, hydrogen cyanide, methane, ammonia and acetylene - in the atmosphere of the hot-Jupiter HD209458b. The discovery was possible thanks to observations performed with the GIANO-B combined with an innovative data analysis technique. The presence of these molecules indicates that the planet's atmosphere is richer in carbon rather than oxygen, suggesting that HD209458b formed at a greater distance from its parent star and has later migrated to its current, closer position.

HD209458b has a density lower than Jupiter's and orbits its star at a distance of just over 7 million kilometers - twenty times smaller than Earth's distance from the Sun. This results in a high planet temperature, around 1200ºC, and a very short orbital period of 3.5 days. Moreover, we see it passing in front of the parent star.

The research team led by Paolo Giacobbe has gathered data over four planetary transits of HD209458b, observing in the near-infrared portion of the electromagnetic spectrum with GIANO-B. The observations were performed as part of the INAF Global Architecture of Planetary Systems (GAPS) large programme. While the planet transits in front of the star, starlight is filtered by the planet's atmosphere, thus marking the characteristic "fingerprints" of the molecules it contains. Such a transmission spectroscopy allowed astronomers to study the planet's atmosphere at the terminator - the region that separates the planet's dayside, illuminated by the star, from its nightside. The spectra obtained with GIANO-B enabled the researchers to identify, for the first time simultaneously, the six molecular species in the atmosphere of HD209458b thanks to their thousands of resolved spectral lines. This discovery opens new horizons to be explored.

According to current theoretical models of exoplanetary atmospheres, discovering so many molecules in the atmosphere of HD209458b, many of them carbon-bearing, would indicate an atmospheric chemistry that is richer in carbon rather than in oxygen. This feature suggests that the planet formed beyond the water snowline, several astronomical units away from its parent star, where the gas in the protoplanetary disk was expected to be richer in carbon. In Solar System terms, HD209458b would have formed beyond the orbit of Mars, most likely between the orbits of Jupiter and Saturn, and would have later migrated towards its star at the distance we observe it today, one tenth of the Mercury-Sun distance. This validates the theories that hot Jupiters formed much farther away from their current position.

Link to the paper Five carbon- and nitrogen-bearing species in a hot giant planet's atmosphere by Paolo Giacobbe et al., published in Nature https://dx.doi.org/10.1038/s41586-021-03381-x.

Credits to Marco Galliani, MediaInaf