Scientists Found a Piece of a Lost Planet in the Sahara Desert

A 4.5-billion-year-old meteorite recovered from the Sahara Desert in 2019 may be a fragment of a planet that no longer exists — one that formed and was destroyed before Earth was fully assembled. A new study published in the journal Earth and Planetary Science Letters concludes that the space rock, known as NWA 12774, likely originated in the deep interior of a massive protoplanet at least the size of Earth's moon and possibly approaching the size of Mars, shattered during the violent collisions of the early solar system.

The finding is significant because no physical evidence of this kind of large early world has ever been recovered before. Most of what scientists know about the first few million years of planetary formation comes from computer models and astronomical observations. This meteorite appears to be a physical remnant of that history.

A Rock Unlike Almost Any Other

NWA 12774 is an angrite, one of the rarest categories of meteorites on Earth. Of the roughly 80,000 meteorites ever cataloged, only 68 are angrites. These rocks contain almost no silica, the mineral that makes up the bulk of every terrestrial planet in our solar system. Because silica is so common and angrites have so little of it, scientists long assumed angrites came from small asteroids rather than full-sized worlds.

But NWA 12774 contains clinopyroxene, a mineral crystal typically found in Earth's crust and mantle. More telling, its clinopyroxene crystals are unusually rich in aluminum — a chemical signature that indicates the crystals formed under enormous pressure, far more than any asteroid interior could generate.

A team led by Aaron Bell, an Earth scientist at the University of Colorado Boulder, spent about a year developing a computer model to calculate exactly how much pressure was required to form those crystals. The answer was at least 17.5 kilobars. For comparison, the pressure at the bottom of the Mariana Trench, the deepest point on Earth, is just one kilobar. This meteorite could only have formed deep inside a world far larger than any known asteroid.

How Big Was the Parent Body?

The pressure calculations placed the parent body at a minimum of 1,200 miles across. But the crystals themselves suggest even that figure is too small. The clinopyroxene crystals in NWA 12774 are too sharp-edged to have survived in a deep planetary interior, where heat and pressure would have smoothed or recrystallized them. That means NWA 12774 likely formed closer to the surface of its parent world, which requires a larger total body to generate the same pressure at shallower depths. Working through those constraints, Bell's team calculated that the parent world was probably at least 2,200 miles across and could have approached the size of Mars, which spans about 4,200 miles in diameter.

“This means that, within four million years of the solar system’s formation, you’re making things that are the size of the moon,” said Francois Tissot, a geochemist at the California Institute of Technology who was not involved in the study, to Scientific American. “It’s a very, very rapid formation timescale.”

What Happened to It?

The researchers do not know exactly how the protoplanet was destroyed. The most likely explanation is that it was shattered during the intense collisions that characterized the early solar system, the same era during which the Moon is believed to have formed from debris following a massive impact on early Earth. The fragments of this lost world may have been scattered across the inner solar system and incorporated into Earth and other rocky planets over time.

“Meteorites are essentially a library of information about the formation and evolution of the early solar system,” Bell said in the university’s press release. “Angrites, in particular, preserve a record of processes that occurred at the very beginning of planetary formation.” He added: “There are many meteorites sitting in drawers that haven’t been thoroughly studied, so there were likely more of these protoplanets we don’t know about.”

Rocks as Records

The study adds to a growing body of evidence that the early solar system was far more dynamic than the relatively stable arrangement of planets visible today. Researchers at the Smithsonian and elsewhere have noted that meteorites already recovered on Earth are still yielding surprises under closer examination.

NWA 12774 spent billions of years crossing space before landing in the Sahara. It waited there until 2019 for someone to pick it up. The answers it contained had been there the whole time.

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