Planetary Formation

Three Iron Rings Around A Star Show Where Planets are Forming

Researchers using the ESO’s Very Large Telescope Interferometer (VLTI) have found three iron rings around a young star about 500 light-years away. The rings indicate that planets are forming. What can these rings tell us about how Earth and the other planets in our Solar System formed?

One of the driving questions for humanity is how our home planet formed. Studying our Solar System has led to a partial understanding. Scientists piece together their understanding of our Solar System by studying Earth, examining asteroids, and exploring Mars and even meteorites that came from Mars. But it’s the study of other young solar systems that will take us further because they give us a glimpse of how things were about 4.5 billion years ago when our system formed.

In new research, an international team of researchers used the VLTI and its Matisse spectrometer to study the planet-forming disk around the young Herbig Ae star HD 144432. Their research is titled “Mid-infrared evidence for iron-rich dust in the multi-ringed inner disk of HD 144432.” It’s published in the journal Astronomy and Astrophysics, and the lead author is József Varga from the Konkoly Observatory in Budapest, Hungary.

After a star forms, leftover material forms a rotating disk around the star called a protoplanetary disk. Out of this disk, planets form. But exactly how they form, especially rocky ones like Earth, is still a detailed question awaiting a more detailed answer. It starts with studying the dust in the interior regions of the protoplanetary disk where rocky planets form.

“When studying the dust distribution in the disk’s innermost region, we detected for the first time a complex structure in which dust piles up in three concentric rings in such an environment,” said study co-author Roy van Boekel, a scientist at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. “That region corresponds to the zone where the rocky planets formed in the solar system.”

Rocky planets form in the inner, warmer regions of a solar system close to the star. They can take tens of millions of years to form. The larger, gaseous planets like Jupiter and Saturn in our system form further away, and so do ice giants like Uranus and Neptune. It should be noted, however, that in some cases, planets can migrate to different locations.

The three rings around HD 144432 are roughly in the same region where the rocky planets in our Solar System formed. The first ring lies within Mercury’s orbit, the second ring is close to Mars, and the third ring corresponds roughly to Jupiter’s orbit.

“Our best-fit model has three disk zones with ring-like structures at 0.15, 1.3, and 4.1 au.,” the authors write in their research. This means the structure contains two gaps, at about 0.9 au and about 3 au. These gaps are carved out by still-forming rocky planets, according to the authors. “Assuming that the dark regions in the disk at ~0.9 au and at ~3 au are gaps opened by planets, we estimate the masses of the putative gap-opening planets to be around a Jupiter mass.”

This sketch from the study shows the three dusty ring regions and the gaps between them where planets are likely forming. Inside the gaps are two planets about the same mass as Jupiter. Image Credit: Varga et al. 2024.

This isn’t the first time astronomers have found a complex ring structure like this. But they usually correspond to where Saturn orbits, well beyond where rocky planets formed in our Solar System. Finding one this close to a star leads to the next question. What are the rings made of?

To find that out, the researchers compared their data to known models of dust surface brightness. They find the model that best fits their data. In this case, it’s a three-ringed structure including iron.

This figure from the study illustrates some of the work behind the results. Each panel is a best-fit brightness model image at various data wavelengths. The researchers ran several simulations to fit the data, some with dust and some including iron dust. These ones include iron. The grey circles indicate the approximate beam size of the VLTI, the telescope that captured the data. Image Credit: Varga et al. 2024.

The main component of the dust is no surprise to scientists. It contains silicates, compounds containing silica, oxygen, and metals. About 95% of Earth’s crust is made of silicates. But intriguingly, the scientists also identified iron in the dust.

“To identify the dust component responsible for the infrared continuum emission, we explore two cases for the dust composition, one with a silicate+iron mixture and the other with a silicate+carbon one,” the authors write in their paper. “We find that the iron-rich model provides a better fit to the spectral energy distribution.”

It’ll take more research to provide stronger confirmation of these results. But if they are confirmed, they’re important. This will be the first time scientists have identified iron in the protoplanetary disk around a young star. “Astronomers have thus far explained the observations of dusty disks with a mixture of carbon and silicate dust, materials that we see almost everywhere in the universe,” van Boekel explains.

The region close to the star is much hotter than more distant regions, obviously. The heat provides further confirmation of these results. In the hot environment close to the star, where the temperature reaches 1500 C, iron and minerals melt and often recondense as crystals. Conversely, carbon can’t survive the high temperatures. It would be vapourized and form carbon monoxide or carbon dioxide gas.

These results also line up with what we know about Earth and the rocky planets in our Solar System. Earth is relatively iron-rich and carbon-poor. Mercury is also iron-rich. “We think that the HD 144432 disk may be very similar to the early solar system that provided lots of iron to the rocky planets we know today,” said van Boekel. “Our study may pose as another example showing that the composition of our solar system may be quite typical.”

This table from the research shows some of the elemental compositions for Mercury, Earth, CI chondrites, and two models of the HD 144432 system’s disk zone 2. Image Credit: Varga et al. 2024.

But there’s still more work to be done to strengthen these results. Finding one young solar system that mirrors ours isn’t enough. But the VLTI has shown it can find them. If there are more out there, the VLTI should find them. “Our analysis exemplifies the need for detailed studies of the dust in inner disks with multiwavelength high angular resolution techniques,” the authors write.

Van Boekel and his colleagues have already identified some other solar systems that deserve to be examined with the powerful VLTI and its Matisse spectrometer. “We still have a few promising candidates waiting for the VLTI to take a closer look at,” van Boekel points out.

One day, with more findings like these, we may know for certain that rocky planets, including our own Earth, form close to their stars from iron-rich dust. It seems like we’re inching toward that obvious-seeming conclusion.

Evan Gough

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