Giant Protoplanets Where You Don’t Expect Them (Planetary Science)

Traces of giant protoplanets have been detected by the Alma radio telescope at a great distance from the star in the system HD 100546. The new model allows us to interpret previous data that testified to the presence of water in the disk. Leading the study, published in Astronomy & Astrophysics, Davide Fedele of the National Institute of Astrophysics

Contrary to what standard models of planetary formation predict, giant planets the size of Jupiter can also form at very large distances from the star, in the coldest, outer region of the protoplanetary disk. New simulations, applied to the data obtained with Alma ‘s observations of the HD 100546 system  , have led to a new interpretative model that predicts the existence of two planets revolving around a young star (about five million years old): a giant outermost gas, 110 astronomical units away from the star and with a mass equal to over eight Jupiter masses, and another smaller one of mass equal to three Jupiter masses orbiting about fifteen astronomical units from the star.

The new Alma images and this new model also allow us to solve a puzzle concerning the presence of water in the disk of HD 100546. Previously, the signature of ice water and water molecules in the outer region of the protoplanetary disk was detected, at over forty astronomical units from the star. Thanks to a study  led by Davide Fedele of INAF of Arcetri, published last week in the journal Astronomy and Astrophysics,we know that the traces of water found in this system are due precisely to the presence of the outer protoplanet. This work was possible thanks to the great potential of Alma, which allows us to study in detail the birth and evolution of exoplanetary systems and the interplanetary medium.

Davide Fedele. Credits: Inaf Arcetri

“Starting from previous observations carried out with Alma, we studied this system because it presented clear evidence of a protoplanetary disk in the range between twenty and forty astronomical units and an indication of much more external faint rings”, explains Fedele, an expert in star and planetary formation. “Our analysis confirmed the presence of rings up to almost 250 astronomical units.”

«With this model», continues the researcher, «we have obtained two important results. In the first place, we have seen that there are giant gas planets at distances of more than one hundred astronomical units from the star, a fact not foreseen by the standard theories of planetary formation which predict the existence of Jupiter planets at a maximum of forty astronomical units. Secondly, the presence of such a distant massive planet made it possible to coherently interpret the observations of the spectral emission of ice grains and water molecules. In fact, the evidence of these molecules is due to the presence of the outer planet, which, sweeping away part of the gas and dust in the interplanetary ring between the two planets, it allows stellar radiation to penetrate into the innermost and colder regions of the disk. When the star’s ultraviolet photons interact with water ice, they release energy and the water molecules evaporate from the ice,photoevaporating ».

In summary, with the new interpretation of this protoplanetary system, on the one hand the problem of the formation of giant planets even very far from the star is questioned and on the other it is shown that the effect of the outermost planet on the interplanetary medium induces a phase transition of the water, finally explaining the observations.

Featured image: The protoplanetary disk around the star HD 100546: the characteristic ring structure is due to the presence of two giant protoplanets that perturb the disk. Credits: Alma.


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