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New observations made by ALMA have revealed unpublished details of the protoplanetary disk surrounding a very young star, in which the possible positions of planets being formed can be distinguished. In practice it is an image of a solar system during its earliest phases.

to scrutinize the Universe. ALMA (Atacama Large Millimeter / submillimeter Array) is a system of 66 high precision parabolic antennas, operating at sub-millimeter and millimeter wavelengths in a range between 0.3 and 9.6 mm. Its main part consists of 50 antennas of 12 meters in diameter, which work together as a single telescope - an interferometer. Another compact network of 4 12-meter radio telescopes and 12 7-meter telescopes completes the set. The antennas are distributed on the Andean plateau of Chajnantor at a height of about 5, 000 meters and are located at distances that can vary from 150 meters to 16 km. ALMA works with unprecedented resolution and sensitivity, with up to ten times sharper vision than the Hubble Space Telescope.

Image Composite image of the young HL Tauri star and its surroundings obtained from ALMA data (enlarged in the upper right box) and from the Hubble space telescope (rest of the image). This is the first image of ALMA for which the sharpness of the image is superior to that normally achieved by Hubble. | ALMA / ESA / Hubble / NASA / Judy Schmidt

The newborn Solar System. Now, to test ALMA in its almost final configuration, its antennas have been aimed at HL Tauri, a very young star located about 450 light-years away from us and surrounded by a disk of gas and dust. The observation surpassed all expectations and revealed unexpectedly minute details in the disk of matter that remained in orbit around the star after its birth, showing a series of brilliant concentric rings separated by discontinuity. The structures are seen at a resolution equal to only five times the distance between the Earth and the Sun. This corresponds to an angular resolution of only 35 thousandths of a second of arc.

The morphology of the disc is almost certainly caused by the presence of young planetary bodies that are forming. This is surprising because young stars like HL Tauri were not expected to have a large number of planetary bodies capable of producing the structures seen in this image. This star has an age that does not exceed one million years, yet the disk appears crowded with planets in formation, a structure much more developed than one would expect given the age of the system.

The young stars like HL Tauri are born in the clouds of gas and dust that collapse under the effect of their gravity, forming dense and warm nuclei that at a certain point, reached temperatures of the order of ten million degrees, allow the triggering of thermonuclear fusion reactions of hydrogen and light up to become young stars. These in turn are initially wrapped in the remaining gas and dust, which sooner or later settles into a disk, known as a protoplanetary disk.

Due to the many collisions, the dust particles stick to each other, growing into lumps up to the size of grains of sand or pebbles. Ultimately, asteroids, comets and even planets can form on the disk. The young planets tear the disc and create rings, gaps and holes like those seen in the structures now observed by ALMA. At visible wavelengths HL Tauri is hidden inside a massive shell of dust and gas. ALMA observes at much greater wavelengths that allow it to penetrate the cloud and study its central regions.

Image Image of the protoplanetary disk surrounding the young star HL Tauri. The observations reveal substructures within the disk that have never been seen before and also the possible positions of the planets that are being formed in the dark areas within the system. This image shows the various structures identified in the HL Tauri system. | ALMA (ESO / NAOJ / NRAO)

planetary formation. The study of protoplanetary disks is fundamental to understand how the Earth was formed in the Solar System. Observing the first stages of planetary formation around HL Tauri could show us how our planetary system appeared more than four billion years ago when it formed.

Most of what we know today about planetary education is based on theories. Images at this level of detail so far came from computer simulations or artistic drawings. This high-resolution HL Tauri image demonstrates what ALMA can do when it exploits the broader configuration and starts a new era of our exploration of star and planet formation. The image obtained by ALMA suggests that the process of planet formation is faster than previously thought, an observation that will revolutionize theories on planetary formation.