Disappeared world: Before the Earth was formed, there was already a moon- to Mars-sized protoplanet in the inner solar system. This celestial body, which was later destroyed, could be the origin of the exotic Angrit meteorites, as analyzes using a new method now suggest. Accordingly, the minerals in these meteorites must have been formed under high pressure inside a large, ancient celestial body. There may even have been an alternative way of planet formation back then, as researchers report.
The early solar system was a dangerous place: larger chunks of gas and dust formed in the rotating disk, constantly colliding with each other and accreting together to form planetary building blocks. Unstable orbits and gravitational turbulence caused violent collisions between planetesimals and even large protoplanets – Earth’s moon was also formed at that time by the collision of the young Earth with the protoplanet Theia.
The Enigma of Angrite
Astronomers have long suspected that a very rare type of meteorite could have its origins in an early, previously unrecognized protoplanet in the solar system. The so-called angrites are among the oldest known meteorites in our planetary system; they were formed just a few million years after the first solid materials crystallized in the protoplanetary disk of the young sun.
This makes the Angrites fundamentally different chemically from Earth, Mars and other rocky planets and asteroids in the inner solar system. They contain only a small amount of silicate and small amounts of alkali metals, show traces of magmatic processes and have special isotope ratios. To date, no asteroid is known from which these meteorites could have come. Instead, planetary researchers suspect that the only 68 angrites known so far could be debris from a celestial body the size of a moon or even a planet.
“A wealth of geochemical, isotopic and paleomagnetic data suggests that the angrite parent body was sufficiently large to partially melt and form a magma ocean and a metallic iron-nickel core,” explain Aaron Bell of the University of Colorado at Boulder and his colleagues.
Silicate crystals reveal the conditions of formation
But how large this hypothetical Angrit mother body was is a matter of debate. To answer this question, Bell and his team have now examined one of the rare angrites using a new method. “We have developed, tested and applied a novel clinopyroxene liquid geobarometer,” they explain. This measuring instrument makes it possible to reconstruct the pressure and temperature conditions under which certain calcium- and aluminum-containing silicate crystals, the clinopyroxenes, were formed. These silicates also occur in small quantities in angrites.
The NWA 12774 meteorite, which was only found in the Sahara in 2019, served as the test object. It is a small piece of rock weighing around 450 grams. The interior of this angrite consists of a fine-grained, dark matrix in which crystals of olivine and pyroxene are embedded. Bell and his team analyzed a sample of this angrine using their new measurement method.
As big as Earth’s moon or Mars
The result: “The CaTs liquid barometer delivers an average crystallization pressure of more than 17 kilobars for NWA 12774,” report the researchers. “This print shows that the Angrit source body must have been large, with a minimum radius of around 1000 kilometers.” The relatively well-preserved sharp edges of the clinopyroxene crystals in the meteorite also suggest that this rock material did not stay at great depths for long and was transported to the surface relatively quickly, as Bell and his team report.
According to the researchers, it could even be that the angrites were formed in a layer just a few hundred kilometers deep of an even larger original celestial body. “Then this would increase the minimum radius of this Angrit parent body to 1800 kilometers,” they write. The prehistoric planetary building block from which these meteorites come could have been about the size of the Earth’s moon or perhaps even Mars.
Did planet formation happen differently at first?
“Our barometric measurements are the first unambiguous evidence that supports the hypothesis of a large Angrite source body,” state Bell and his colleagues. “According to this, the Angrites come from a protoplanet that was destroyed in a catastrophe in the earliest stages of development of the inner solar system.” This protoplanet, which was destroyed at the time, must have existed before Earth, Mars and the other planets.
“This Angrite source body consisted of material that was fundamentally different from the ingredients for Earth and Mars,” says Bell. “The existence of this early protoplanet therefore suggests a distinct, separate formation path for protoplanets in this early era.” Only when the protoplanet was destroyed did our home planet begin to form. The Earth may even contain unrecognized debris from this long-vanished celestial body. So far, however, the Angrit meteorites are the only witnesses to this primeval catastrophe.
Source: Aaron Bell (University of Colorado, Boulder) et al., Earth and Planetary Science Letters, 2026; doi: 10.1016/j.epsl.2026.120029