A lost planet the size of the Moon left its chemistry behind in a Sahara meteorite

Among the 80,000 or so meteorites cataloged on Earth, only 68 belong to a family called angrites. What makes them unusual is not just their rarity. It is their chemistry: they contain almost none of the silica that makes up most of the rocky material in the inner solar system, including Earth and Mars. Where angrites came from has been an open question for decades. A new analysis of one — a specimen called NWA 12774, recovered from the Sahara Desert in 2019 — has produced the clearest answer yet: from inside a world roughly the size of Earth’s Moon that has since ceased to exist.

The mineral crystals inside NWA 12774 could only have formed under pressures that would not fit inside any known asteroid. Researchers at the University of Colorado Boulder, led by geoscientist Aaron Bell, calculated that the meteorite’s aluminum-rich clinopyroxene crystals required at least 17.5 kilobars of pressure during formation. The floor of the Mariana Trench, the deepest point in Earth’s oceans, generates roughly 1 kilobar. What produced the conditions recorded in NWA 12774 was not a trench but a planet.

How they measured a vanished world

The technique Bell’s team used is called geobarometry — reading mineral chemistry as a record of the pressure at which it crystallized. Clinopyroxene changes its aluminum content predictably depending on how deep inside a planetary body it forms: more aluminum means higher pressure. By analyzing the exact mineral ratios in NWA 12774 and modeling the pressure conditions required to produce them, the researchers reconstructed the formation depth, and from there the minimum size of the body it came from.

The crystallization had to occur far enough below a surface that the overlying mass weighed enough to generate 17.5 kilobars. Only a body with a radius of at least 1,000 kilometers could produce that internal pressure through its own gravity. The fact that NWA 12774’s crystals preserved sharp edges and intact chemical gradients told the team that the meteorite formed in the shallower layers of such a body, meaning the planet’s total size was larger still. The study estimates a radius potentially reaching 1,800 kilometers.

What makes angrites chemically unlike everything else

Angrites do not fit on any known planetary family tree. Earth, Mars, and the Moon share broadly similar silica-rich chemistry, consistent with formation from the same general region of the early solar nebula. Angrites contain almost none of that silica. As Bell stated in the study, the materials that formed the angrite parent body are fundamentally different from the ingredients of Earth and Mars. Their chemical signature points to a body that assembled from a distinct reservoir of solar system material.

For comparison, the angrite parent body would have had a roughly Moon-scale volume but built from chemistry that has no obvious descendant in the present solar system. That distinction matters for reconstructing how many planetary embryos were competing in the early solar system and how completely the record of those embryos has since been erased.

How big was it — and where did it go?

The estimated 1,000–1,800 km radius puts the angrite parent body in the size range of Pluto (~1,190 km) or Earth’s Moon (~1,737 km), well below Mars at 3,300 km but far too large to be classified as an asteroid. A body of this size would have developed a differentiated interior: a metallic core, a mantle, and a crust. It was a full planetary embryo.

What destroyed it is unconfirmed. The most plausible explanation is a catastrophic collision during the early solar system’s period of heavy bombardment. Its debris may have scattered into what became the asteroid belt. All 68 known angrites — ancient, chemically distinct — are likely fragments of this same world. “It’s incredible to think there was once a world this large,” Bell said. “We only know it existed because a few fragments of it happened to land on Earth.”

What this doesn’t settle

The study establishes a minimum size, not a confirmed diameter or interior model. The 17.5 kilobar lower bound comes from the aluminum content threshold observed in NWA 12774; the actual parent body could have been larger if this meteorite formed at shallower depths relative to its surface. The paper also does not identify where in the solar nebula the angrite parent body originally formed, nor does it resolve whether its silica-poor chemistry reflects a distinct formation zone or post-accretion alteration. Only a handful of angrites have received geobarometric analysis; the population of protoplanets they represent may be larger and more chemically diverse than the current dataset suggests.

Common questions about the lost protoplanet

What is an angrite meteorite?

Angrites are among the rarest and oldest meteorite types — only 68 known examples among more than 80,000 catalogued specimens. They formed within a few million years of the Sun’s birth and carry chemistry that matches no known surviving planet. NWA 12774 provides the strongest parent-body size estimate to date.

How do scientists calculate the size of a planet that no longer exists?

The technique is called geobarometry. Certain minerals, including clinopyroxene, alter their chemical composition depending on the pressure at which they crystallize. By measuring that composition in a meteorite sample and comparing it against calibrated standards, scientists can calculate the minimum pressure required and, from that, the minimum planetary size needed to produce it.

Could material from this lost protoplanet be inside Earth today?

Possibly. During the solar system’s violent early phase, material from destroyed planetary embryos was regularly incorporated into the growing terrestrial planets. Earth’s bulk composition likely includes contributions from worlds that no longer exist as distinct bodies.

Are there more unknown protoplanets like the angrite parent body?

Almost certainly. Planetary formation models predict that dozens of embryos competed for material in the early inner solar system; the four rocky planets are the survivors. Bell noted that many unanalyzed meteorites may carry signatures of other lost worlds.

If geobarometric analysis of the remaining angrite collection confirms they all share one parent body, it will constrain how many Moon-to-Mars-scale embryos the early inner solar system produced — and how completely a catastrophic impact can erase a planet, leaving behind only 68 rocks on a world that formed 4.5 billion years later.

Reference: Bell et al., “High-pressure clinopyroxene in Northwest Africa 12774 and new geobarometric evidence for a planetary embryo-sized angrite parent body,” Earth and Planetary Science Letters, 2026. DOI: 10.1016/j.epsl.2026.120029

Tags: astronomy, Aaron Bell, angrite, geobarometry, NWA 12774, protoplanet