Astronomer finds gas giant exoplanets formed earlier than previously thought
The Ohio State University study's results provide new information about the timing of accretion - the process of accumulating a large amount of gas as well as solid particles that are rich in carbon and oxygen to make large planets, like Jupiter.
Planets are formed from protoplanetary disks, spinning clouds of dust and gas that are the perfect ingredients for planet formation. This new study suggests the accretion takes place early, when disks are massive and much younger than researchers previously believed.
While the number of newly confirmed exoplanets has continued to grow, the origins of these worlds and the factors that impact their formation is a puzzle scientists are still aiming to solve. Jupiter-like exoplanets, for instance, were initially thought to take nearly 3 to 5 million years to fully form; recent observations now suggest that for a gas giant like Jupiter, this process is likely closer to about 1 to 2 million years.
This discovery challenges researchers' existing theories regarding at what "age" of the protoplanetary disks these planets were formed, said Ji Wang, author of the study and an assistant professor in astronomy at Ohio State. The results could lead scientists to re-evaluate and revamp their theories of planet formation for the solar system and elsewhere.
"Everything we know about exoplanets can be put in the context of the solar system and vice versa," said Wang. "Usually planet formation is a bottom-up scheme, meaning it starts with small objects that build up to form a bigger planet, but that way takes time."
Though exoplanets refer to planetary objects that orbit far beyond the confines of our solar system, understanding more about how they form could help researchers gain more insight into the evolution of the solar system and early Earth, whose formation was much later than Jupiter's, but was still greatly impacted by it.
The "'bottom-up" interpretation of planetary formation is called the "core accretion theory," but another possible formation mechanism is when planets are formed through gravitational instability - when the clumps in a disk around a star are too massive to support themselves and collapse to form planets. Because a planet's accretion history could be closely linked to these two compelling yet complementary formation mechanisms of evolution, Wang said, it's important to determine which process is more often the case.
The study was recently published in The Astrophysical Journal.
The study analyzed a sample of seven gas giant exoplanets whose stellar and planetary chemical properties had already been directly measured by previous studies and compared them to data on the gas giants in our solar system, Jupiter and Saturn.
Wang showed that the early formation of these exoplanets is consistent with recent evidence that Jupiter formed much earlier than previously thought. This finding is based on the surprisingly high amount of solids these exoplanets accreted.
All the materials accreted at the beginning of a planet's formation increase the metallicity of its atmosphere, and by observing the traces they leave behind, researchers are able to measure the amount of solids the planet once gathered.
The higher the metallicity, the more solids and metals - anything on the periodic table more massive than hydrogen and helium - scientists can assume were accreted during the formation process, said Wang.
"We can infer that on average, every one of the five planets sampled accreted the equivalent of 50 Earth masses worth of solids," he said. "Such a large amount of solids can only be found when a system is younger than 2 million years, but in our solar system, the total solids available is only on the order of 30 to 50 Earth masses worth."
This new data implies that the building blocks used to form the exoplanets were available at an earlier stage of the protoplanetary disk's evolution than once expected and their availability of these building blocks greatly decreased over a span of millions of years. Because scientists usually don't expect to find proof that planets formed that early, it's a finding that current theories will likely struggle to reconcile, Wang said.
"These exoplanets formed so early that there was still a large reservoir of metals available," said Wang. "This is something that the scientific community was not fully prepared for so now they'll have to scramble to come up with new theories to explain it."
Because gas giants pull in huge amounts of matter during accretion, their formation and migration through space also affects the development of rocky planets elsewhere in a protoplanetary disk. In the solar system, this phenomenon is believed to have caused Jupiter and Saturn to push Mercury out of its original orbit, and caused Mars to become much smaller than the Earth or Venus.
That said, to aid astronomers looking to do similar planetary formation analyses in the future, the work also provides a statistical framework for inferring the total mass of solid accretion for any other exoplanet, which the study notes can be an ideal tool for investigating other kinds of complex elemental data as well.
And while this research relied purely on archival data, Wang expects his work to be further complemented with new high-resolution data collected by better instruments, such as more powerful ground-based astronomical observatories or next-generation technologies like the James Webb Space Telescope.
"By expanding this work with a larger sample of exoplanets, we hope to see the trend of evidence found in this paper continue to hold," said Wang.
Research Report:Early Accretion of Large Amounts of Solids for Directly Imaged Exoplanets
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