Dusty disc reveals young planets

A ring of dust surrounding a star is 240 billion km across and lopsided, showing it has interacted with its surroundings, and that exoplanets do not grow up in isolation.

This is a Hubble Space Telescope photo of a vast, complex dust structure, about 150 billion miles across, enveloping the young star HR 4796A. A bright, narrow inner ring of dust is already known to encircle the star and may have been corralled by the gravitational pull of an unseen giant planet. This newly discovered huge dust structure around the system may have implications for what this yet-unseen planetary system looks like around the 8-million-year-old star, which is in its formative years of planet construction. The debris field of very fine dust was likely created from collisions among developing infant planets near the star, evidenced by a bright ring of dusty debris seen 7 billion miles from the star. The pressure of starlight from the star, which is 23 times more luminous than the Sun, then expelled the dust far into space.

The dust ring around HR 4796A is created by infant planets colliding together. Credit: NASA/ESA/G. Schneider (University of Arizona)

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A huge ring of dust measuring around 240 billion km (1,600 times the Earth-Sun distance) across has been uncovered around a young star whose planetary system is still in its formative years.

The observations of the ring with the Hubble Space Telescope show it has interacted with its surroundings, squashing it on one side, emphasising how planetary systems do not grow up in isolation.

The star at the centre of the ring, HR 4796A, is only eight million years old: an important time in the creation of a planetary system.

Infant planets in orbit around the star are still colliding, creating huge amounts of fine dust that is then being formed into a ring by pressure from the starlight.

But the star is 23 times brighter than the Sun, which is why the structure is so large and complex.

These protoplanets also appear to have the effect of pulling the dust into a bright ring around 11 billion km from the star.

Meanwhile, the puffy outer structure looks like a donut that has been squashed so that it extends more in one direction than in the other.

This strange shape could be due to the dust being swept back by the interstellar medium as it moves through space, creating a structure like the bow wave of a ship.

Alternately, it could be that the star’s binary companion – HR 4796B, which is 87 billion km from the primary star – is tidally tugging on the dust.

“The dust distribution is a telltale sign of how dynamically interactive the inner system containing the ring is,” says Glenn Schneider of the University of Arizona, Tucson who mapped the dust around the star.

“We cannot treat exoplanetary debris systems simply as being in isolation.

The gross asymmetries of the outer dust field are telling us there are a lot of forces in play (beyond just the host-star radiation pressure) that are moving the material around.

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“We’ve seen effects like this in a few other systems, but here’s a case where we see a bunch of things going on at once,” Schneider concludes.