ALMA Spies Baby Stars’ Planetary Workshops

Planetary formation remains one of the biggest puzzles in modern astronomy. Although we know that the vast majority of stars possess systems of planets — from tiny Mercury-sized rocky worlds to massive gas giants that would dwarf Jupiter — mysteries remain as to how material accretes to form small planetoids and how long it takes for these planetary embryos to plump-up into what we would consider to be planets.

Now, with the help of the awesome Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have zoomed-in on a selection of very young stars, revealing never-before-seen detail in the planet-forming regions surrounding them. And what they found were monster planets, several times more massive than Jupiter, hiding inside the dusty planetary workshops.

When a star is born, it will often be accompanied by a protoplanetary disk. As the star settles and disk matures, small dusty particles accrete (clump together), eventually creating gravitationally-dominant protoplanets that rapidly vacuum up more and more material, growing bigger and more massive. Of particular interest to astronomers are transitional disks that have a surprising lack of dust in their centers, in the region between the disk and star.

This may not seem surprising; astronomers have explained away these features as either a consequence of stellar radiation pressure (as the star matures, its radiation blasts any nearby dust away), or massive planets could be lurking in this zone, having cleared their orbits of dust through their gravitational dominance.

We’ve been stuck at this impasse for some time; how can we tell whether this dust gap is caused by radiation pressure or planetary formation?

This ALMA image combines a view of the dust around the young star HD 135344B (orange) with a view of the gaseous material (blue). The smaller hole in the inner gas is a telltale sign of the presence of a young planet clearing the disc. The bar at the bottom of the image indicates the diameter of the orbit of Neptune in the Solar System (60 AU).

This is where ALMA comes in. The array of radio antennae are sensitive to emissions from the gas these transitional disks contain and through studies of 4 young stars, astronomers have found that inside these dust gaps, there are also gas gaps, but they are 3 times thinner. Only with ALMA’s precision observations could these gas gaps be pinpointed and they can mean only one thing.

“Previous observations already hinted at the presence of gas inside the dust gaps,” said astronomer Nienke van der Marel, of Leiden Observatory in the Netherlands. “But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc.”

Although we are looking at very alien star systems, it’s studies such as these that will ultimately reveal how the planets in our own solar system formed, likely clearing up many mysteries surrounding our understanding of planetary evolution. And as observatories become more sophisticated answers are likely to come sooner rather than later.

“Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look,” added Ewine van Dishoeck, also of Leiden Observatory and the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

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