Four-and-a-half billion years ago, a massive world, possibly as big as the moon or even Mars, orbited our sun before crashing into another celestial body and shattering into rubble.
Now, in a paper published in the journal Earth and Planetary Science Letters , scientists report the first definitive evidence that this lost planetary embryo, or protoplanet, existed. Its unique geological makeup challenges long-held assumptions about how planets evolve.
"It's incredible to think there was once a world this large," said Aaron Bell , an assistant research professor in the Department of Earth Science at the University of Colorado Boulder. "We only know it existed because a few fragments of it happened to land on Earth. These meteorites preserved evidence of a completely different pathway through which early planets developed."
What gave away the lost world's secret was a piece of its debris uncovered on Earth in the Sahara Desert, known as the Northwest Africa (NWA) 12774 angrite meteorite.
Angrites are among the oldest known volcanic rocks in the solar system, forming within just a few million years after the solar system began about 4.56 billion years ago. They are also exceptionally rare. Out of more than 80,000 meteorites discovered on Earth, only 68 are angrites.
What makes angrites especially puzzling is their chemistry. Unlike Earth, Mars and other rocky planets, angrites contain very little silicon dioxide, or silica, which is a major ingredient in nearly every known terrestrial planet in the solar system.
For that reason, scientists thought angrites must always come from an asteroid, something with a radius of less than 200 kilometers (124 miles).
When Bell and his colleagues were studying NWA 12774, they found the meteorite contained clinopyroxene, a mineral crystal commonly found in Earth's crust and mantle. In particular, NWA 12774's clinopyroxene was exceptionally rich in aluminum, a telltale sign that the rock formed under enormous pressure deep underground.
The researchers then reconstructed the pressure conditions that might have been present for NWA 12774 to form.
To their surprise, the aluminum-rich clinopyroxene needed at least 17.5 kilobars of pressure. For comparison, the crushing pressure at the bottom of the Mariana Trench, the deepest point on Earth, is only around 1 kilobar.
That level of pressure could not have existed inside a small asteroid. Instead, the calculations suggested that the body where angrites came from must have been at least 1,000 kilometers (621 miles) in radius.
Other clues in the meteorite pointed to an even more striking possibility. The crystals inside NWA 12774 still preserved sharp edges and delicate chemical patterns that would have been erased if they formed deep underground. This suggested that the crystals likely formed at relatively shallow depths inside the parent body, so the world had to be even larger.
Under that scenario, the angrite parent body might have stretched beyond 1,800 kilometers (1118 miles) in radius, making it comparable in size to Earth's moon and possibly approaching a Mars-sized world, which has a radius of 3300 kilometers (2050 miles).
"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," Bell said.
It remains unclear how the protoplanet met its end. One possibility is that a catastrophic event in the early solar system shattered it, with its fragments later become the building blocks of other terrestrial planets, including Earth.
"The materials that formed the angrite parent body are fundamentally different from the ingredients of Earth and Mars. It points to a distinct and separate evolutionary path in planetary formation in the early history of our solar system," Bell said.
