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A new study provides key insights into how planetary evolution occurs, revealing details about infant planets and their formation process. These young planets aggressively consume the surrounding gas and dust within their star-circling protoplanetary disks, which dissipate rapidly, generally within a few million years. Thanks to this study, astronomers now possess a more refined understanding of this crucial phase.
New Insights into Protoplanetary Disk Evolution
An international team of astronomers conducted the research using the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALMA Survey of Gas Evolution of PROtoplanetary Disks (AGE-PRO). The AGE-PRO team investigated the gas surrounding 30 protoplanetary disks around sun-like stars, discovering that gas and dust components in these disks evolve at varying paces. The remaining amount of gas determines the type of planets that can form within these systems, according to the researchers.
These findings may help scientists gain a deeper understanding of planetary system formation and evolution, including our own solar system.
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The AGE-PRO data has led to an impressive 12 research papers from different teams, highlighting its significance.
“These studies have revealed how protoplanetary disks evolve over time,” stated Anibal E. Sierra Morales, an AGE-PRO researcher at the Mullard Space Science Laboratory at University College London (UCL), to Space.com. “The extraordinary results are an essential step toward understanding the initial conditions that lead to the formation of Earth-like planets.”
Understanding Planet Formation from Protoplanetary Disks
The development of protoplanetary disks starts when clumps of dense, cool gas collapse due to gravity within interstellar molecular clouds, giving rise to stars. These young stars, known as “protostars,” continue to accumulate material from their initial gas envelope.
Eventually, a main sequence star emerges, encircled by a flattened, swirling cloud of gas and dust—the protoplanetary disk. Within this disk, particles collide and merge, steadily gaining mass and forming planetesimals. These planetesimals continue to gather material, eventually growing into planets.
Protoplanetary disks typically exist around young stars for millions of years, providing the necessary timeframe for giant planets to form.
Key factors such as the initial size and mass of the protoplanetary disk, as well as its rotational speed (angular momentum), dictate the types of planets that can emerge. The lifespan of gas within the disk determines the time available for matter to accumulate and evolve into asteroids or planets.
Furthermore, these elements can also influence planetary migration, where planets shift positions within the disk, either closer to or farther from their star, ultimately shaping the architecture of the planetary system.
Prior to this investigation, researchers had examined dust content in protoplanetary disks. However, changes in gas content over time were not as thoroughly understood.
“AGE-PRO provides the first measurements of gas disk masses and sizes across the lifetime of planet-forming disks,”, stated Ke Zhang, principal investigator of the research at the University of Wisconsin-Madison.
Key Findings from the ALMA Observations
The team employed ALMA to intensively study 30 protoplanetary disks of varying ages, from 1 million to over 5 million years old. These disks resided in star-forming regions within the constellations Ophiuchus, Lupus, and Upper Scorpius.
ALMA’s sensitivity enabled the team to monitor specific chemical “tracers,” which revealed gas and dust masses during crucial phases of protoplanetary disk evolution, encompassing initial formation and eventual disintegration over millions of years.
While carbon monoxide is a commonly used chemical tracer in astronomy, the AGE-Pro team also utilized diazenylium. ALMA also detected chemical signatures of other molecules, including aldehyde, deuterated cyanogen, and cyanomethane, providing an unprecedentedly detailed view of the chemical evolution of protoplanetary disks.
“This is the first large-scale chemical survey of its kind, targeting the 30 disks with a broader range of ages to characterize the gas masses,” stated John Carpenter, co-leader of the research at the Joint ALMA Observatory.
Implications for Planet Formation Timeframes
The research offers hints regarding the timescales associated with the formation of gas giant planets like Jupiter and Saturn, compared to smaller, terrestrial planets such as Earth and Mars.
“AGE-PRO reveals that the median average of the gas disk mass goes from several Jupiter masses in the early ages of less than 1 million years to less than a Jupiter mass in the first 1 to 3 million years,” explained Paola Pinilla, an AGE-PRO researcher at the Mullard Space Science Laboratory at UCL. “This means that disks have the reservoir to form giant planets in the young disks, but as they mature, the fuel for forming giant planets significantly decreases.”
“However, it is surprising that the disks that survive for longer times of between 2 million to 3 million years, maintain a very similar gas disk mass as the 1 million to 3-million-years-old examples.”
Gas and Dust Consumption Rates
Another notable discovery was the different rates at which gas and dust are consumed within aging protoplanetary disks. The gas-to-dust ratio undergoes a shift as these clouds evolve.
“The most surprising finding is that, although most disks dissipate after a few million years, the ones that survive have more gas than expected,” noted Zhang. “This fundamentally changes our estimation of the atmospheric accretion of planets formed at a later time.”
By comparing AGE-PRO’s data on gas evolution in protoplanetary disks with other studies, researchers are developing a more extensive and detailed understanding of planetary system evolution.
The 12 associated research papers are scheduled for publication in an upcoming issue of The Astrophysical Journal.