This idea has major implications for understanding how Earth acquired volatile substances such as water. If material traveled inward from the outer solar system, it must have crossed between the distant and inner regions of the solar system while Earth was forming.

New research is now challenging that scenario.

“We were really surprised too.”

Planetary scientists Paolo Sossi and Dan Bower of ETH Zurich compared Earth’s isotopic composition with existing measurements from a wide range of meteorites, including samples associated with Mars and the asteroid Vesta.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, giving them different masses. Their composition can provide clues about where planetary materials formed.

Earth’s building materials may have come entirely from the inner solar system

Using a new method to analyze isotope data, the researchers reached a surprising conclusion: the material that formed Earth came entirely from the inner solar system.

Material from beyond Jupiter appears to account for less than 2% of Earth’s total mass and may not have contributed at all. The study was published in Nature Astronomy.

“Our calculations reveal that Earth’s building materials originate from a single material reservoir,” Sossi says.

Bower adds: “We were really surprised to find that the Earth is made up entirely of material from the interior of our solar system, which is different from the existing meteorite mix.”

The ETH Zurich team based its analysis on existing measurements from 10 different isotope systems found in meteorites. Earlier studies typically focused on only two isotope systems.

“Our research is actually a data science experiment,” Sossi says. “We performed statistical calculations that are powerful tools but rarely used in geochemistry.”

How meteorite isotopes reveal where planets formed

For decades, scientists have used meteorite isotopes to investigate where objects formed, including which regions of the solar system they came from. For many years, oxygen isotopes were the primary tool used to determine their origin.

That changed in the early 2010s, when American researchers showed that isotopes of other elements, including chromium and titanium, could also reveal where meteorites formed.

This discovery allowed scientists to classify meteorites into two broad groups:

• Non-carbonaceous meteorites: These formed in the inner solar system.
• Carbonaceous meteorites: These contain more water and carbon and originated in the outer solar system.

According to the new analysis, Earth is made entirely of non-carbonaceous materials. The researchers found no evidence of significant mass exchange between the inner and outer solar system reservoirs, as previously proposed.

The findings suggest that Earth grew in a relatively stable environment, gradually incorporating smaller neighboring planetary bodies as it developed. They also indicate that most volatile substances, including water, may have already existed within the inner solar system.

Jupiter may have divided the young solar system

Why did the young solar system contain two distinct reservoirs of matter?

Scientists believe Jupiter may have played a major role. As the gas giant grew rapidly, its immense gravity created a gap in the protoplanetary disk surrounding the young Sun. These disks are made of gas and dust and are where planets form.

Jupiter may have acted as a barrier, restricting the movement of material from the outer solar system into the region where Earth formed. Until now, however, scientists did not know how effective that barrier was.

The new analysis suggests that very little material from beyond Jupiter reached Earth’s planetary neighborhood.

“Our calculations are very robust and rely only on the data themselves, not on physical assumptions, which are not yet fully understood,” Bower emphasizes.

The results also indicate that Earth’s material composition is similar to that of Mars and Vesta.

Could Mercury and Venus share Earth’s origin?

Researchers believe Mercury and Venus may fit the same pattern as Earth, Mars and Vesta.

“Based on our analysis, we can theoretically predict the composition of these two planets,” Sossi says.

However, scientists cannot yet directly test that prediction. They do not currently have rock samples from Mercury or Venus that can be analyzed in the same way.

If Earth formed nearby, where did its water come from?

“Our findings shed new light on the formation history of Earth and other rocky planets,” Sossi says.

The findings also raise a major 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 create Earth’s oceans.

Sossi and his team plan to investigate that question next. They also want to determine whether similar processes occur in planetary systems around other stars.

“But until then, Dan and I will have many heated discussions about the material composition of Earth and its neighboring planets, because despite new discoveries, the scientific debate over what Earth is made of is far from over,” Sossi says.