Young star caught cooking up planets

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This image compares the size of the Solar System with HL Tauri and its surrounding protoplanetary disc. Although the star is much smaller than the Sun, the disc around HL Tauri stretches out to almost three times as far from the star as Neptune is from the Sun. – ESO / NAOJ / NRAO

For the first time, astronomers have spied a distinct clump of dust sitting in a ring right beside a young star, which they claim is the first stage of planet formation.

In a paper to be published in The Astrophysical Journal Letters, an international team of astronomers led by Carlos Carrasco-Gonzalez from the National Autonomous University of Mexico spotted the promising region near the million-year-old star HL Tau, only 450 light-years from Earth.

"We believe this clump of dust represents the earliest stage in the formation of protoplanets, and this is the first time we've seen that stage," says study co-author Thomas Henning from the Max Planck Institute for Astronomy.

"This is quite different from the case of star formation, where, in different objects, we have seen stars in different stages of their life cycle," Carrasco-Gonzalez says.

"With planets, we haven't been so fortunate, so getting a look at this very early stage in planet formation is extremely valuable."

Stars, such as the Sun and HL Tau, form within clouds of gas that collapse under gravity and clump together inside a swirling disk of gas and dust. Eventually, the dust particles grow into sand, which in turn coagulate to pebble- and rock-sized chunks.

Once these larger chunks gain mass and become protoplanets, they sweep their orbit clear of debris, and usher scraps of gas and dust into rings.

In visible light, HL Tau is partly hidden by the huge dust cloud shrouding it. But the Atacama Large Millimetre/submillimetre Array (ALMA), comprising 66 antennae scattered across the Chajnantor plateau in the Chilean Andes, picks up light with short radio wavelengths (0.32 to 3.6 millimetres) which can travel through dust unimpeded.

In 2014, ALMA snapped an image of HL Tau showing its series of concentric rings separated by gaps – as you would expect a young star beginning to form planets.

But to peer within the inner rings, where the dust cloud was so dense even ALMA's telescopes couldn't see, astronomers needed to look with even longer wavelengths. And for that, they turned to the Karl G Jansky Very Large Array (VLA) – 27 radio telescopes on the Plains of San Agustin, Mexico. In 2014 and 2015, it looked at seven-millimetre wavelengths.

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Image of star HL Tau taken with the Atacama Large Millimetre/submillimetre Array, and right, an image of a dust clump taken by the Very Large Telescope.

The VLA's images revealed a distinct clump of dust close to the centre of the disk.

This clump, astonomers calculated, is made of grains as large as a centimetre wide. All up, it contains around three to eight times the mass of Earth.

This region is where Earth-like planets would likely form.

"These VLA observations are the most sensitive and show the most detail of any yet made of HL Tau's disk at these longer wavelengths," says Claire Chandler, from the National Radio Astronomy Observatory in the US and study co-author.

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