The exoplanet Osiris’ atmospheric chemistry hints at a past journey

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New facts about Osiris' atmosphere have been discovered, telling us more about where this "hot Jupiter" formed. (NASA/European Space Agency)

Scientists have observed an array of molecules in the atmosphere of a well-known "hot Jupiter" that indicate the exoplanet formed much farther from its sun than previously thought.

The team identified water vapor and five other carbon- or nitrogen-containing compounds, and say their methods could be applied to understanding potentially habitable worlds, as well. The study of the exoplanet "Osiris" was published April 7 in Nature.

"The key findings of these discoveries are that the chemistry of exoplanets is indeed as complex as theory had predicted, and that we now have the tools to investigate it in detail," said Paolo Giacobbe, a postdoctoral fellow at the Italian National Institute for Astrophysics Astrophysical Observatory of Turin and first author of the study. 

Hot Jupiters are gas giants that are extremely toasty because they orbit so close to their host star and were the first kind of planet to be discovered outside our own solar system.

"They are very interesting objects to study because they exhibit extreme physical conditions not found in our solar system," Giacobbe said. "They are essentially the only 'laboratory' available to us for studying highly irradiated planets."

The exoplanet his team focused on, HD 209458b, is located about 150 light-years away from the solar system in the constellation Pegasus. Being a hot Jupiter makes this planet particularly easy for scientists to observe.

This is partly because researchers track distant planets as they cross in front of their stars. The closer a planet is to its star, the more frequently it blocks the star's light, and the more likely it is to be found. HD 209458b, which also goes by the nickname Osiris, was in fact the first planet spotted using this technique. 

Additionally, Giacobbe says, hot Jupiters have very thick atmospheres that produce an especially distinctive signal.

HD 209458b's bright star also helps make the planet easier to detect. "We observe the footprint that the exoplanetary atmosphere leaves in the light spectrum emitted by the star," Giacobbe said. "In order for these chemical fingerprints to be observable with our instruments, it is necessary to collect as many stellar photons as possible."  

Previously, researchers had detected water vapor, carbon monoxide and hydrogen cyanide in the atmosphere of Osiris. Giacobbe and his colleagues observed the planet as it passed in front of its star four times in 2018 and 2019. They used an instrument called GIANO-B mounted at the Telescopio Nazionale Galileo in La Palma, Spain, which allowed them to detect a wide range of chemical signatures simultaneously. 

The researchers identified water, carbon monoxide, hydrogen cyanide, methane, ammonia and acetylene in the planet's hot atmosphere. The team then used atmospheric models to determine that the planet's atmosphere is unexpectedly rich in carbon relative to oxygen. This ratio can give researchers a clue about where Osiris formed in the protoplanetary disk of dust and gas that surrounded its young sun. 

The surprising abundance of carbon-based molecules — hydrogen cyanide, methane, acetylene and carbon monoxide — suggests that the planet accreted gases rich in the element during its formation. This could only happen if the planet was born beyond its solar system's snowline, where oxygen can condense in water ice while carbon remains in gaseous form. 

Osiris likely formed at a similar distance to its star as Jupiter or Saturn are in our own solar system and later migrated inward, Giacobbe says. The planet might have interacted with gases in its protoplanetary disk, resulting in a relatively smooth voyage. Or perhaps it undertook a more violent journey caused by gravitational interactions with other planets after the disk dissipated.

"The presence of a hot Jupiter has a great impact on the planetary system under consideration and on the presence of any other planets, including rocky ones," Giacobbe said. If the hot Jupiter's migration was violent, it might have pushed neighboring planets inward toward the star, or, more likely, ejected them out of the system to wander "free" in space. 

"It also creates strong instabilities in the disc, reducing the possibility of the formation of other planets on stable orbits, a fundamental parameter for the habitability of a planet," Giacobbe added.

The kind of measurement he and his colleagues used is highly dependent on theoretical models of planetary atmospheres. 

"Certainly, as our knowledge increases and our atmospheric models are refined, we can expect changes in the interpretation of what we have observed," Giacobbe said. 

Still, applying these techniques to other planets could allow scientists to understand how hot Jupiters differ from each other. The ability to detect several molecules simultaneously could also make it easier for researchers to determine whether smaller and cooler exoplanets might be habitable.

"All this is very promising for the future, when there will be instruments capable of detecting the extremely weak signals from the atmospheres of Earth-like rocky planets," Giacobbe said.

The study, "Five carbon- and nitrogen-bearing species in a hot giant planet's atmosphere," published April 7 in Nature, was authored by Paolo Giacobbe, Aldo S. Bonomo, Alessandro Sozzetti, and Mario Damasso, INAF—Osservatorio Astrofisico di Torino; Matteo Brogi, INAF—Osservatorio Astrofisico di Torino and University of Warwick; Siddharth Gandhi, University of Warwick; Patricio E. Cubillos and Luca Fossati, Austrian Academy of Sciences; Gloria Guilluy, INAF—Osservatorio Astrofisico di Torino and Università di Torino; Ilaria Carleo, Wesleyan University; Monica Rainer, Alberto Galli, Nicoletta Sanna and Andrea Tozzi, INAF—Osservatorio Astrofisico di Arcetri; Avet Harutyunyan, Rosario Cosentino, Aldo F. M. Fiorenzano, Adriano Ghedina, Marco Pedani and Ennio Poretti, INAF—Fundación Galileo Galilei; Francesco Borsa, INAF—Osservatorio Astronomico di Brera; Lorenzo Pino, INAF—Osservatorio Astrofisico di Arcetri and University of Amsterdam; Valerio Nascimbeni, Luca Malavolta and Giampaolo Piotto, Università di Padova; Serena Benatti, Antonio Maggio, Jesus Maldonado, Giuseppina Micela and Marcello Lodi, INAF—Osservatorio Astronomico di Palermo; Katia Biazzo, INAF—Osservatorio Astronomico di Roma; Andrea Bignamini, INAF—Osservatorio Astronomico di Trieste; Katy L. Chubb, Netherlands Institute for Space Research; Riccardo Claudi, Silvano Desidera and Daniela Fantinel, INAF—Osservatorio Astronomico di Padova; Elvira Covino, INAF—Osservatorio Astronomico di Capodimonte; Antonino F. Lanza, Giuseppe Leto, Isabella Pagano and Gaetano Scandariato, INAF—Osservatorio Astrofisico di Catania; Emilio Molinari, INAF—Osservatorio Astronomico di Cagliari; and Sergei N. Yurchenko, University College of London. 

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