Astronomers observe three iron rings in planet-forming disk

The origin of the Earth and the solar system inspires scientists and the public. By studying the current state of our planet and other objects in the solar system, researchers have created a detailed picture of the conditions under which they evolved from a disk of dust and gas surrounding the nascent sun years ago. is approximately 4.5 billion years old.

Thanks to stunning advances in star and planet formation research aimed at distant celestial objects, we can now study conditions in the environments around young stars and compare them to those derived from the early solar system. An international team of researchers led by József Varga of the Konkoly Observatory in Budapest, Hungary, did just that using the European Southern Observatory’s (ESO) Very Large Telescope Interferometer (VLTI). . They observed the planet-forming disk of the young star HD 144432, about 500 light years away.

“By studying the distribution of dust in the innermost region of the disk, we detected for the first time a complex structure in which dust accumulates in three concentric rings in such an environment,” explains Roy van Boekel. He is a scientist at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and co-author of the underlying research article that will appear in the journal Astronomy and astrophysics.

“This region corresponds to the area where the rocky planets of the solar system were formed,” adds van Boekel. Relative to the solar system, the first ring around HD 144432 is in the orbit of Mercury and the second is close to the trajectory of Mars. Additionally, the third ring roughly corresponds to the orbit of Jupiter.

So far, astronomers have discovered such configurations mainly on larger scales, corresponding to areas beyond where Saturn orbits the sun. Ring systems in the disks around young stars usually indicate that planets are forming in the gaps as they accumulate dust and gas in their path.

However, HD 144432 is the first example of such a complex ring system so close to its host star. It occurs in an area rich in dust, the building block of rocky planets like Earth. Assuming that the rings indicate the presence of two planets forming in space, astronomers estimated that their masses roughly resembled that of Jupiter.

Conditions may be similar to those in the early solar system

Astronomers determined the composition of the dust across the disk up to a separation from the central star that corresponds to the distance between Jupiter and the sun. What they discovered is very familiar to scientists who study Earth and the rocky planets of the Solar System: various silicates (metal-silicon-oxygen compounds) and other minerals found in the Earth’s crust and mantle, and perhaps be metallic iron like that of Mercury and Earth. nuclei. If confirmed, this study would be the first to discover iron in a planet-forming disk.

“So far, astronomers have explained observations of dusty disks with a mixture of carbon and silicate dust, materials we see almost everywhere in the universe,” says van Boekel. However, from a chemical point of view, a mixture of iron and silicate is more plausible for the warm, inner regions of the disk.

Indeed, the chemical model that Varga, the lead author of the underlying research paper, applied to the data yields better-fit results by introducing iron instead of carbon.

Additionally, the dust observed in disk HD 144432 can be as hot as 1,800 Kelvin (about 1,500 degrees Celsius) at the inner edge and as moderate as 300 Kelvin (about 25 degrees Celsius) further out. Minerals and iron melt and recondense, often as crystals, in hot regions near the star.

In turn, the carbon grains would not survive the heat and would instead be present as carbon monoxide or carbon dioxide. However, carbon could still be an important constituent of the solid particles of the cold outer disk, which the observations made for this study do not allow us to trace.

Dust rich in iron and poor in carbon would also adapt perfectly to the conditions of the solar system. Mercury and Earth are iron-rich planets, while Earth has relatively little carbon. “We think that the HD 144432 disk could be very similar to the early solar system that supplied a lot of iron to the rocky planets we know today,” says van Boekel. “Our study could be another example showing that the composition of our solar system can be quite typical.”

Interferometry solves small details

Recovery of the results was only possible with exceptionally high resolution observations, as provided by the VLTI. By combining the four 8.2 meter VLT telescopes at ESO’s Paranal Observatory, they can resolve details as if astronomers were using a telescope with a 200 meter diameter primary mirror. Varga, van Boekel and their collaborators obtained data using three instruments to obtain broad wavelength coverage ranging from 1.6 to 13 micrometers, representing infrared light.

MPIA provided essential technological elements for two devices, GRAVITY and the Medium Aperture Multi-Infrared Spectroscopic Experiment (MATISSE). One of MATISSE’s main goals is to study the rocky areas of planet-forming disks around young stars. “By examining the inner regions of protoplanetary disks around stars, we aim to explore the origin of the different minerals contained in the disk, minerals that will later form the solid components of planets like Earth,” explains Thomas Henning, director and director of the MPIA. co-PI of the MATISSE instrument.

However, producing interferometer images like those we are used to obtaining from single telescopes is not simple and takes a long time. A more efficient use of valuable observation time to decipher object structure is to compare sparse data to models of potential target configurations. In the case of HD 144432, a three-ring structure best represents the data.

How common are structured, iron-rich planet-forming disks?

Besides the Solar System, HD 144432 appears to provide another example of planets forming in an iron-rich environment. But astronomers won’t stop there.

“We still have a few promising candidates waiting for the VLTI to examine them more closely,” emphasizes van Boekel. In previous observations, the team discovered a number of disks around young stars that indicate configurations worth revisiting. However, they will reveal their detailed structure and chemistry using the latest VLTI instruments. Ultimately, astronomers may be able to determine whether planets typically form in dusty, iron-rich disks near their parent stars.