- 3239 - PLANETS - how do they form around stars? Circumplanetary disks are vital to the formation of an exoplanet, since they control how much material the growing planet accumulates. Planetary disks also set the budget for satellite formation, determining how much material will be left over for moons to coalesce from.
------------------ 3239 - PLANETS - how do they form around stars?
- Astronomers have clearly detected a dusty disk around a young giant planet that may go on to form moons. Disks of gas and dust left over from stellar formation can create circumstellar disks, shrouding a newborn star in planet-making potential.
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- Planets that form use their gravity vacuum through the dust to trace out rings and other structures, perhaps gathering their own personal dusty disk, called a circumplanetary disk, in the process.
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- Astronomers had previously found a circumstellar disk around “PDS 70“, a young star nearly 400 light-years away in the constellation Centaurus, the Centaur. In 2020, they confirmed the presence of two planets, a Jupiter-Saturn pair, dubbed PDS 70b and c.
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- These baby planets dwell in a cavity between two rings of dust, one close to the star, the other farther out. High-resolution data from the Atacama Large Millimeter / submillimeter Array (ALMA) clearly shows PDS 70c has a disk of its own, separate from the larger, encompassing circumstellar disk.
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- This giant world is the first exoplanet to have a directly detected circumplanetary disk, making this system. Such a discovery confirms astronomers’ theories about how moons and planets form.
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- Circumplanetary disks are vital to the formation of an exoplanet, since they control how much material the growing planet accumulates. Planetary disks also set the budget for satellite formation, determining how much material will be left over for moons to coalesce from.
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- Both PDS 70b and c are still gathering mass, slowly, but only PDS hosts its own disk. This planet, a few Jupiter masses at most, resides 34 times farther from its star than Earth does from the Sun (1 astronomical unit, or a.u.), that’s a little farther than Neptune’s orbit.
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- At its largest, PDS 70c’s disk would be 1.2 a.u. in diameter, holding roughly 3 lunar masses (about 3% Earth’s mass) of material.
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- PDS 70c’s circumplanetary disk could make moons in different ways. Small dust particles can get trapped in the disk, creating conditions just right so that these particles stick together, forming pebbles, then rocks, then moons.
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- If larger particles get trapped, they can spontaneously congregate into clumps, causing them to collapse into the beginnings of a satellite.
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- The other planet, PDS 70b, shows signs of tenuous dust near its orbit, but nothing actually encircling the planet. Maybe the dust is trapped at one of the gravitationally stable points along 70b’s orbit, or maybe it’s part of a stream of material, linking each planet to the inner disk surrounding the star. Either way, PDS 70b doesn’t have the circumplanetary disk that its companion has.
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- Since PDS 70b is closer to the star than its sibling, perhaps its realm of influence is much smaller, meaning that any dust that would orbit PDS 70b is instead pulled towards the star.
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- Another explanation is that PDS 70c starves b of dust. Circumplanetary dust must come from the outer, cooler ring of the stellar disk, and since PDS 70c is closer to that region, it may catch what it can and allow only a trickle of tiny dust particles to drift towards 70b.
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- Astronomers have found the first signposts of a gravitationally unstable disk around the young star Elias 2-27. Protoplanetary disks of gas and dust leftover from stellar formation are known as the birthplace of planets. Astronomers understand that these disks give way to planets, but they’re still working to determine the exact evolution from dust to new worlds.
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- There’s more than one way to form a planet, and one path might be gravitational instability, when disks become so massive that they begin to fragment and cave in on themselves, directly collapsing into planets or forming spiral arms that trap material for future planet formation.
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- An already-formed giant planet or interactions with a nearby star can also create spirals, but spiral structure born out of gravitational instability carries special characteristics.
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- In 2016, scientists at the Atacama Large Millimeter/submillimeter Array (ALMA) first saw spiral arms in the disk of Elias 2-27. The finding makes these spiral arms the first convincing evidence for a gravitationally unstable disk.
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- ALMA was used to observe the dust and gas in Elias 2-27’s disk. The results show that the expansive spiral arms are symmetric, with similar shapes and sizes, as predicted if gravitational instability were at work.
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- The key piece of evidence for instability, however, is the wiggle. The team used ALMA to observe the motions of carbon monoxide, which traces the harder-to-observe, but more abundant, hydrogen gas, and discovered the sought-after signature. This disturbance coincides with the spiral arms in most observations. This confirmation of velocity perturbations that so closely resembles what was predicted.
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- These signals of gravitational instability also led to the first direct measurement of the mass in a planet-forming disk. Elias 2-27’s disk has 17% the mass of its star, creating conditions ripe for gravitational instabilities.
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- In thinner disks, the star with its powerful gravity governs the motions of the disk. But for a massive disk like the one around Elias 2-27, the disk’s own gravity starts to influence its dynamics.
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- While the spiral, wiggle, and mass all indicate the disk is experiencing gravitational instability, gaps in that same disk are throwing astronomers for a loop. There is a gap in the middle of the disk that is devoid of dust, a trait typically attributed to the commotion of a forming planet.
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- A planet forming a gap of this size wouldn’t be large enough to form the spiral structure. Even if there were a planet at this location, it would get sucked into the star. On the other hand, gravitational instability cannot explain the gap, even though it explains the spirals.
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- The gas in Elias 2-27’s disk is unexpectedly asymmetric, such that the gas is thicker on one side of the disk than the other. The varying layers of gas indicate that material could still be falling onto the disk from the cloud that formed the Elias 2-27 system. This inbound gas might have ignited the gravitational instability and even caused a disk warp that morphed into the currently observed dust gap.
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- Although more observations are needed to solve the mysteries of Elias 2-27’s disk, the evidence for a massive, gravitationally unstable disk is quite compelling.
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- Astronomers still need to work out how gravitational instability leads to planets, via direct collapse or indirectly, inciting spiral structures that help funnel material. Elias 2-27 and others like it will help astronomers piece together the planet formation puzzle.
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- August 3, 2021 PLANETS - how do they form around stars? 3239
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