Planet building material in the mirror of three rings

Artist’s impression of the disk structure around the star HD 144432 with evidence of silicates and iron in the rings. ©Jenry

Analytical look into a “planetary cradle”: Astronomers have discovered three rings in the inner region of a planet-forming disk around a young star. Apparently two planets have already formed in the gaps from the material that still forms the rings. References to their composition indicate that they contain silicates and possibly iron. The inner disk area could therefore be similar to the one that produced the rocky planets in our solar system over four billion years ago, say the researchers.

How did planetary systems like our own form? Astronomers have been trying to gain more and more insight into this question for a long time. The fundamental processes are now considered to be clear: young stars are initially formed through the gravitational collapse of dense, cold gas clouds. The remaining material then accumulates around them in the form of disks, in which planets can then form. Such circumstellar disks have already been discovered around some young stars. In some cases it is also apparent that planets have already formed: gaps in a circumstellar disk suggest that an orbiting planet in this area has collected the material.

So far, however, such structures have mostly been discovered in the more outer regions of planet-forming disks, which correspond to the zone beyond Saturn's orbit around the Sun. But now a team of astronomers led by József Varga from the Konkoly Observatory in Budapest has identified structures in a circumstellar disk that are close to the host star. The target was the system of the young star HD 144432, which is about 500 light-years away from us. The interferometer was used on the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. This instrument, called VLTI for short, bundles the observation data from four telescopes and thus enables high resolution of astronomical structures. The team's results are based on observations with a broad wavelength coverage in the infrared range of light.

Three rings - two planets in the gaps

“By studying the dust distribution in the innermost region of the disk, we discovered for the first time a complex structure in which the dust forms three rings in such an environment,” says co-author Roy van Boekel from the Max Planck Institute for Astronomy in Heidelberg. “This region corresponds to the zone in which the rocky planets in the solar system were formed,” emphasizes van Boekel. Specifically, the data analyzes show that the first ring around HD 144432 would lie within the orbit of Mercury and the second in the area of ​​the orbit of Mars. Only the third one is at a distance that would roughly correspond to the orbit of our Jupiter. As the scientists explain, the existence of the three rings suggests that two planets formed in the two gaps. This also means: They were created from the material in the inner area of ​​the circumstellar disk of HD 144432, from which the three rings are also formed.

Evidence of silicates and iron

As the team reports, they were also able to draw conclusions about the characteristics and composition of these disc structures based on their data. Spectral signatures of the radiation from the rings and specially developed modeling provided the crucial information. The analysis results suggested that various silicates characterize the dust in the rings, as are also typical for the material of rocky planets. More significant, however, is the indication of metallic iron, such as that present in the Earth's core, because this component has not previously been detected in circumstellar disks. “Astronomers have previously explained observations of dusty disks as a mixture of carbon and silicate dust – materials that we see almost everywhere in the universe,” explains van Boekel. However, in the case of the inner ring material of HD 144432, the researchers' modeling approach produced more conclusive results when iron was taken into account instead of carbon.

A mixture of iron and silicates also seems plausible for the hot, inner disk regions, the researchers emphasize. The minerals and iron could melt there and then condense into crystalline structures. Under these conditions, however, carbon would not occur solidly, but rather as gaseous carbon monoxide or carbon dioxide, the scientists explain. Iron-rich and carbon-poor dust as a building material would also fit what is known about the composition of the rocky planets in our solar system. “We think that the disk of HD 144432 could be very similar to the early solar system, which supplied today's rocky planets with a lot of iron,” says van Boekel. “We suspect that our study is another example that the composition of our solar system is quite typical.”

The team of astronomers now wants to stay on the ball: They plan to target the inner regions of the circumstellar disks in other stars. In this way, they hope to be able to confirm the trace of iron and expand on the results. Van Boekel concludes: “We still have a few promising candidates who are waiting for the VLTI to take a closer look at them.”

Source: Max Planck Institute for Astronomy, specialist article: Astronomy and Astrophysics, doi: 10.1051/0004-6361/202347535

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