Earth Formed Entirely From Inner Solar System, Study Finds

A new ETH Zurich isotope analysis concludes Earth's building blocks came entirely from the inner Solar System, upending the idea that water-rich outer material arrived from beyond Jupiter.

Planetary scientists Paolo Sossi and Dan Bower of ETH Zurich published a study in Nature Astronomy arguing that Earth formed almost entirely from material in the inner Solar System. For years, many researchers proposed that 6 to 40 percent of Earth's building material arrived from the outer Solar System beyond Jupiter, a key idea for explaining how our planet acquired water and other volatiles. The new analysis instead finds that outer Solar System material accounts for less than two percent of Earth's total mass, and possibly none at all. "Our calculations make it clear: the building material of the Earth originates from a single material reservoir," Sossi said. Bower added, "We were truly astonished to find that the Earth is composed entirely of material from the inner Solar System distinct from any combination of existing meteorites." The News International also covered the findings, noting the study shifts away from the long-held belief in substantial outer-system delivery.

The result is a genuine reversal. Earlier models treated Earth as a mixed planet, with water-rich carbonaceous material from beyond Jupiter delivering a meaningful share of its mass and volatiles. According to ScienceDaily, the new analysis found no evidence for the previously proposed exchange of material between the inner and outer Solar System reservoirs, suggesting Earth instead grew in a relatively stable environment by gradually incorporating smaller nearby bodies. That also implies volatile substances, including water, must already have existed in the hot inner Solar System — something scientists had generally thought unlikely. The News International reports that the team's conclusion rests on ten different isotope systems, far more than earlier studies that typically focused on only two, such as oxygen isotopes. The division of meteorites into non-carbonaceous (inner Solar System) and carbonaceous (outer Solar System) groups dates to work in the early 2010s showing chromium and titanium isotopes could also reveal formation regions.

Sossi and Bower compared existing isotope-ratio measurements from a wide range of meteorites — including samples associated with Mars and the asteroid Vesta — with Earth's isotopic composition. ScienceDaily reports they used statistical methods rarely applied in geochemistry, and that Bower emphasized the calculations "rely solely on the data itself, not on physical assumptions, as these are not yet fully understood." The results show Earth's material composition resembles that of Mars and Vesta, and that Earth is made entirely of non-carbonaceous material. Why two distinct reservoirs existed at all? Scientists think Jupiter, growing rapidly as a gas giant, carved a gap in the protoplanetary disc around the young Sun. Its enormous gravity appears to have acted as a barrier, limiting how much outer Solar System material ever mixed inward — and the new analysis suggests very little actually reached the region where Earth formed. The News International attributed the same conclusion to the pair's ten-isotope analysis.

The findings raise a major new question: if Earth did not receive large amounts of water-rich material from the outer Solar System, scientists must explain how enough water existed in the hot inner Solar System to eventually fill Earth's oceans. Sossi and his team plan to investigate that next, and also want to determine whether similar processes could occur in planetary systems around other stars. According to ScienceDaily, the researchers suspect Mercury and Venus fit the same pattern as Earth, Mars and Vesta, and say they can theoretically predict those planets' compositions — though without rock samples from either planet, the prediction cannot yet be directly tested. Sossi told ScienceDaily that "the scientific discourse over the building blocks of Earth is far from over, despite the new findings," predicting many heated debates ahead with Bower about Earth and its neighboring planets. The study, "Homogeneous accretion of the Earth in the inner Solar System," appears in Nature Astronomy.