Scientists have long wondered where the materials that formed Earth came from. Our planet is in the inner Solar System. Yet, many thought that 6 to 40 percent of its building blocks came from beyond Jupiter, in the outer Solar System.
This idea was key to explaining how Earth got its water and other volatile substances. It suggested that materials moved between the outer and inner Solar System as Earth was forming. New research now challenges this long-held view.
Earth's Building Blocks
Planetary scientists Paolo Sossi and Dan Bower from ETH Zurich looked at existing measurements of isotope ratios. They compared these from various meteorites, including samples linked to Mars and the asteroid Vesta, with Earth's isotopic makeup.
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Start Your News DetoxIsotopes are like sibling atoms of the same element. They have the same number of protons but different masses due to different numbers of neutrons.
Using a new way to analyze this data, the researchers found something surprising. Their results show that Earth's material came entirely from the inner Solar System.
Material from the outer Solar System makes up less than two percent of Earth's total mass, and possibly none at all. The study was published in Nature Astronomy.
Sossi stated that their calculations clearly show Earth's building material comes from a single source. Bower added that they were "truly astonished" to find Earth is made entirely of inner Solar System material, different from any mix of known meteorites.
The team based their analysis on measurements from ten different isotope systems found in meteorites. Earlier studies usually focused on only two.
Sossi noted their studies are like "data science experiments." He said they used statistical calculations rarely seen in geochemistry, even though they are powerful tools.
Meteorite Isotopes Tell a Story
For decades, scientists have used isotopes in meteorites to track where celestial objects formed. This includes their region in the Solar System. For many years, oxygen isotopes were the main tool for this.
This changed in the early 2010s. An American researcher showed that isotopes of other elements, like chromium and titanium, could also reveal where meteorites formed.
This discovery helped scientists divide meteorites into two main groups. Non-carbonaceous meteorites formed only in the inner Solar System. Carbonaceous meteorites have more water and carbon and came from the outer Solar System.
The new analysis suggests Earth is made entirely from non-carbonaceous material. The researchers found no proof of material exchange between the inner and outer Solar System.
This means Earth likely grew in a stable environment. It slowly gathered smaller nearby planetary bodies as it developed. It also means most volatile substances, including water, must have already been in the inner Solar System.
Jupiter's Role in Dividing the Solar System
Why did the young Solar System have two distinct material zones? Scientists believe Jupiter played a big part. As the gas giant grew quickly, its huge gravity created a gap in the protoplanetary disc around the young Sun. These discs are ring-shaped and made of gas and dust, where planets are born.
Jupiter likely acted as a barrier. It limited the movement of material from the outer Solar System into the inner region. Until now, scientists didn't know how effective this barrier was.
The new analysis suggests very little material from beyond Jupiter reached the area where Earth formed.
Bower emphasized that their calculations are very strong. They rely only on the data, not on physical assumptions that are not yet fully understood.
The results also show that Earth's material is similar to that of Mars and Vesta.
What About Mercury and Venus?
The researchers think Mercury and Venus might follow the same pattern as Earth, Mars, and Vesta.
Paolo Sossi said they can theoretically predict the composition of these two planets based on their analysis.
However, this prediction cannot be tested directly yet. Scientists do not have rock samples from Mercury or Venus to analyze in the same way.
A New Question About Earth's Water
Sossi stated that their results offer new insights into how Earth and other rocky planets formed.
The findings also bring up a major new question. If Earth didn't get much water-rich material from the outer Solar System, how did enough water exist in the hot inner Solar System to create Earth's oceans?
Sossi and his team plan to investigate this question next. They also want to see if similar processes happen in planetary systems around other stars.
Sossi noted that the scientific discussion about Earth's building blocks is far from over, despite these new findings.
Deep Dive & References
Homogeneous accretion of the Earth in the inner Solar System - Nature Astronomy, 2026
