We don’t see anyone like us
Ancient observers placed the Earth at the center of the Universe and treated it as a singular, special object. It was natural to think so, because it was hard to imagine that faint dots in the sky could resemble the planet and the warm Sun under our feet. Yet the more people looked up, the more they realized the geocentric view was wrong.
Italian thinker Giordano Bruno challenged this view in a bold way. Building on Copernicus’s idea that the Earth moves around the Sun, Bruno drew far-reaching conclusions about the nature of the cosmos. He argued that the Universe is infinite, not bounded by a celestial sphere, that the Sun is just one star among many, and that celestial matter is essentially the same as earthly matter, implying that worlds like ours exist across the cosmos.
Bruno’s method was not scientific in the modern sense, yet it relied on ancient mysticism and philosophy. Despite this, his ideas influenced contemporary thinkers and helped spark the Scientific Revolution. Interestingly, his execution came not for his cosmology but for rejecting Christian dogmas—he doubted the divinity of Christ and the afterlife, and he equated God with the material Universe.
Bruno’s positions align with modern astrophysics in key results: the Solar System is not unique but one example among many. By studying nearby star systems, scientists hope to find counterparts to our own system at different stages of evolution. So far, astronomers have not found a protoplanetary disk with iron-rich regions everywhere they look, a detail that matters for the origin of rocky worlds.
How did the Solar System and its protoplanetary disk come to be?
The Solar System, like others, formed from a cloud of gas and dust around 4.5 billion years ago. This cloud was mostly hydrogen, with heavier elements created by earlier generations of stars. Gravity pulled the cloud together and it began to spin, as angular momentum dictated. As the material contracted, a hot, dense center formed and the outer regions stretched outward into a protoplanetary disk. When the center reached enough temperature and pressure, nuclear fusion started, giving birth to a star.
Iron stands out as one of the most common elements on rocky planets. It makes up about a third of the Earth’s mass and concentrates in the core. Other heavy elements such as magnesium, silicon, and oxygen also play major roles. This specific mix arose because the early disk was uneven, with rocky material favored in the inner regions closer to the Sun, while hydrogen and gas dominated farther out. The inner system gathered iron-rich material that would form the cores of terrestrial planets.
Thus, if astronomers cannot find rocky planets with substantial iron in other systems, it could cast doubt on the idea that our Solar System is typical. Yet current evidence continues to support the view that rocky planets with iron cores are common outcomes of disk evolution.
Three concentric rings
Researchers from the Max Planck Institute for Astronomy examined star system HD 144432 A at a considerable distance from Earth and suggested it resembles a young version of our own solar system. Modern telescopes can directly image only the brightest exoplanets; they cannot photograph the subtle inner regions of protoplanetary disks. Instead, scientists analyze the system’s light across a spectrum using spectrometers, with the Very Large Telescope in Chile serving as a principal instrument.
Data collected between 2019 and 2022 were compared with physical models to determine what a planetary system would need to look like to produce the observed radiation. The results show a disk with clearly visible rings separated by gaps, hinting at planet formation processes.
“When we mapped the dust in the inner disk, we found a complex structure in which dust forms three concentric rings. This region corresponds to where rocky planets begin to form in a solar system”, explained a co-author of the study, Roy van Boeckel.
The first ring lies at the distance of Mercury, the second between Earth and Mars, and the third near the orbit of Jupiter. Previously, similar rings appeared mainly in distant parts of systems, beyond Uranus. Scientists believe the gaps result from two giant planets clearing gas and dust from their orbits. Based on gap width, those planets would have masses close to Jupiter, though alternative explanations exist that do not require clear gaps.
Iron Kingdom
For scientists focused on terrestrial planets, the shed light on the disk’s composition matters far more than giant planets. The dust is rich in metal-silicate-oxygen components and likely contains metallic iron—a signature compatible with rocky worlds.
For many years, astronomers thought protoplanetary disks were dominated by carbon silicate dust. Yet chemical models now suggest iron-rich material is a natural component of the hot inner disk, producing more accurate results when iron replaces carbon in the model.
The disk is not uniform. In a region corresponding to about 1.3 times the Earth’s orbital radius, crystalline material concentrates, notably forsterite and enstatite. This pattern stems from a strong temperature gradient: inner regions reach around 1500 degrees Celsius, while outer parts stay near 25 degrees. The heat melts material and helps it form crystals, and high temperatures reduce solid carbon content as it oxidizes.
Researchers suggest that the disk around HD 144432 may resemble an early Solar System that left iron-rich residues on rocky planets. The work adds to the view that the Solar System’s composition could be fairly ordinary after all.
In the near future, scientists plan to apply the same approach to other disks, including several that appear even closer in their likeness to the Solar System than HD 144432.