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Studying the birth of exoplanets with chemistry

A new study led by Elenia Pacetti, PhD student at La Sapienza University and INAF, jointly uses ultra-volatile, volatile, and refractory elements in the atmospheres of giant planets to develop a unified method to shed light on how and where giant planets form. The new work, published in The Astrophysical Journal, paves the road to the exoplanetary studies of the ESA mission Ariel
Studying the birth of exoplanets with chemistry

Diagram of formation of a giant planet in a protoplanetary disk around a star. Credits: Elenia Pacetti (Sapienza/INAF)

Studying the birth of exoplanets with chemistry

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Using chemistry to study history. This is not an outlandish idea but our best shot at finding answers, when by history we mean that of giant exoplanets and by chemistry that of their atmospheres. Chemical elements are distributed between the gas and dust of protoplanetary disks from which giant planets are born in different proportions according to their volatility and the distance from the star. During their formation, therefore, giant planets interact with multiple and chemically diverse environments in protoplanetary disks. The composition of the material they capture in the process is what shapes their planetary atmospheres. By studying the relative abundances of the chemical elements in planetary atmospheres, we can learn how the forming planets interacted with their native disks and where their life began.

Studies have been exploring the information encoded in exoplanetary atmospheres for the past decade, focusing on the cosmically abundant carbon and oxygen. The list of elements observable in circumstellar disks and planetary atmospheres, however, is longer and each additional element can provide a different piece of the story. Investigating what the various elements can tell us, when they supply reliable indications, and when they can mislead us is the topic of the study led by Elenia Pacetti, PhD student at La Sapienza University of Rome and the INAF-IAPS, published yesterday in The Astrophysical Journal.

The study led by Pacetti combines the information provided by ultra-volatile, volatile, and refractory elements into a unified picture to shed light on how and where giant planets form. "We provide universal guidelines to link the elemental abundances in the atmospheres of giant planets to their formation history, breaking some of the intrinsic degeneracies of previous methods" explains Elenia Pacetti. "Our work shows how the abundance ratio between carbon and oxygen in giant planet atmospheres is highly influenced by the chemical structure of protoplanetary disks. Not accounting for this fact introduces serious biases in pinpointing the nature and the formation history of giant planets. However, our methods allow us to turn to our advantage this dependency of the C/O ratio from the disk chemistry, using it as a window into the natal environments of these planets".

In the study, the team led by Pacetti explores the effects of different disc chemical structures and different formation and migration histories on the final composition of giant planets’ atmospheres. The study focuses on the atmospheric abundances of ultra-volatile nitrogen and refractory sulfur, together with carbon and oxygen. The study team finds that the abundance ratios of these four elements (C/O, C/N, N/O, and S/N) can be jointly used as robust tracers in planet formation studies. The study shows how the planet formation process produces giant planets with atmospheric elemental ratios significantly deviating from those of the host star and the native disk. The different chemical behaviors of the four elements can be used to constrain where giant planets start forming, in which environment, how far they migrate, and what is the dominant contributor to their metallicity, whether gas or solids.

The study is born from the joint efforts of the ERC Synergy project "ECOGAL" and the Planet Formation Working Group of the ESA mission Ariel. The ECOGAL project is devoted to studying star formation across all spatial scales combining observational, theoretical, and numerical techniques and its contributions to this study are led by one of its principal investigators, Sergio Molinari, from INAF. The Ariel Planet Formation Working Group is led by Diego Turrini from INAF, also co-author of this study, and its goals are to develop new methods to link the atmospheric characteristics of the hundreds of exoplanets that Ariel will observe to their formation history and that of their host star.

Giovanna Tinetti, Ariel’s principal investigator and professor at the University College London, not directly involved in the study, comments: "I am delighted to see this important study being published! This paper represents an important step forward about how to best constrain the provenance of exoplanets through the chemistry of their atmospheres. These questions are key to the success of the Ariel mission". Giusi Micela, Ariel’s Italian Co-PI and INAF researcher not involved in the study, adds "The Ariel mission will observe one thousand exoplanetary atmospheres that will be characterized by extremely diverse physical conditions. The work by Elenia and her collaborators provides new insight on the paths to deliver the different chemical elements from the disks to the forming giant planets and allows for more reliable interpretations of future observations of planetary atmospheres. Their method will therefore be an extremely valuable tool for the analysis and the interpretation of Ariel’s data".

"We have already been able to successfully apply our methods to interpretating exoplanetary data collected at the Telescopio Nazionale Galileo by the Italian program GAPS in recent works directed by other INAF researchers and associates" concludes Pacetti. "These first applications confirm the importance of comparing the planetary elemental ratios with those of the host stars, instead of the characteristic values of the Sun, to avoid introducing biases in the interpretation of the atmospheric elemental ratios. We look forward to applying our methods to the incoming data of the James Webb Space Telescope".

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