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Researchers may have discovered a hidden ingredient in planetary growth that causes some worlds to grow rapidly in disks of gas and dust around infant stars. The model could have particular value in understanding the growth of the solar system’s giant planets.
Scientists have a pretty good understanding of how the Solar System’s rocky inner planets, Mercury, Venus, Earth, and Mars, formed and evolved. However, the current model of the collision and aggregation of asteroid-like bodies called “planetesimals” and the accumulation of gas over millions of years leaves much to be explained when it comes to the formation of the gas giants Jupiter and Saturn and the ice giants Neptune and Uranus.
All of these giants appear to have formed too far from the sun to be so large, and the growth of the outer ice giants is particularly puzzling. Jupiter is 15 times farther from the sun than our planet, and Neptune is 30 times farther from our star than Earth.
To help solve this planetary puzzle, a team of scientists from the ORIGINS Cluster of Excellence, the Max Planck Institute for Solar System Research (MPS) and the Ludwig Maximilian University of Munich (LMU) have developed a new model of planetary evolution that can explain these apparent discrepancies.
The scientists say their model is the first to bring together all the physical processes needed for planet formation.
“This is the first simulation to track the evolution of fine dust into giant planets,” team member Tommy Chi Ho Lau, a doctoral candidate at LMU, said in a statement.
The researchers’ simulation suggests that disruptions in the disks of gas and dust around newborn stars, called “protoplanetary disks,” could lead to the rapid formation of large numbers of gas giants.
Their findings align with recent observations of young extrasolar planets, or “exoplanets,” and may suggest that giant planets may form much faster and more efficiently than currently thought.
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Even giant planets start small
Young stars are born when extremely dense patches form in interstellar clouds of gas and dust. As this clump of gas accumulates material, it eventually collapses, giving birth to a “protostar.”
This protostellar body continues to accrete material until it reaches a mass that produces enough pressure and temperature to trigger nuclear fusion in its core, converting hydrogen into helium; this process defines the “main sequence” lifetime of a star.
The remaining material in the newborn star’s prenatal cocoon flattens out as it orbits the star, forming a protoplanetary disk. As the name suggests, planets emerge from this disk of gas and dust.
The team’s model suggests that millimeter-sized dust particles accumulate aerodynamically in the turbulent protoplanetary disk. This disruption traps the dust and prevents it from moving toward the central infant star. This encourages the accumulation of planetary “building blocks” in a relatively small region of the protoplanetary disk. These are the conditions needed for a planet to form.
“When a planet grows large enough to impact the gas disk, this leads to renewed dust enrichment further out into the disk,” says Til Birnstiel, Professor of Theoretical Astrophysics at LMU and member of the ORIGINS Cluster of Excellence. “In this process, the planet pushes the dust out into space beyond its orbit – like a sheepdog chasing its flock.”
This could lead to the formation of planets from stars as far away as 200 times the distance between Earth and the Sun, more than enough to explain the formation of giant planets in the Solar System about 4.5 billion years ago.
The team’s model also implies that the reason the Solar System stopped producing planets after Neptune was because it ran out of planetary building blocks.
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The results produced by the team’s simulations appear to agree with observations of young planetary systems made by the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 radio antennas located in northern Chile.
This research could not only help us understand the birth and evolution of giants in our own planetary system, but could also give astronomers clues about the diversity of exoplanets in the Milky Way Galaxy.
The team’s research was published in the journal Astronomy & Astrophysics.