More than 4.5 billion years ago, a huge world, potentially as large as the moon or even Mars, traveled around the young Sun before a violent collision shattered it into pieces.
Scientists now say they have found the first direct evidence that this long-lost planetary embryo, known as a protoplanet, once existed. The discovery, described in the journal Earth and Planetary Science Letters, points to a type of planetary evolution unlike anything previously recognized.
"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."
Rare Meteorite Holds Clues to a Lost World
The breakthrough came from a meteorite discovered in the Sahara Desert called Northwest Africa (NWA) 12774, an angrite meteorite.
Angrites are among the oldest volcanic rocks known in the solar system. They formed only a few million years after the solar system emerged about 4.56 billion years ago. They are also extremely uncommon. Of the more than 80,000 meteorites found on Earth, just 68 belong to the angrite group.
These meteorites have long puzzled scientists because of their unusual composition. Compared with Earth, Mars, and other rocky planets, angrites contain very little silicon dioxide, or silica, a key ingredient in nearly every known terrestrial planet.
Because of that chemistry, researchers had assumed angrites originated from small asteroids with radii of less than 200 kilometers (124 miles).
Evidence of Extreme Pressure Deep Inside a Planet
While examining NWA 12774, Bell and his colleagues identified clinopyroxene, a mineral commonly found in Earth's crust and mantle. The clinopyroxene in this meteorite contained exceptionally high levels of aluminum, an important clue that it formed under intense pressure deep within a larger body.
The team then calculated the conditions required to produce the mineral.
Their results were surprising. The aluminum-rich clinopyroxene would have needed at least 17.5 kilobars of pressure to form. By comparison, pressure at the bottom of the Mariana Trench, the deepest location on Earth, reaches only about 1 kilobar.
Such extreme pressure could not have existed inside a small asteroid. Instead, the findings indicated that the parent body of the angrites must have had a radius of at least 1,000 kilometers (621 miles).
A World Comparable to the Moon
Additional evidence suggested an even larger source.
The crystals within NWA 12774 still retain sharp edges and delicate chemical features. If they had formed deep inside a massive world, those details would likely have been erased over time. Their preservation suggests the crystals formed relatively close to the surface of the parent body.
If that interpretation is correct, the original world would have needed to be much larger than the minimum estimate.
According to the researchers, the angrite parent body may have exceeded 1,800 kilometers (1118 miles) in radius. That would place it in the same size range as Earth's moon and potentially approaching the scale of Mars, 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.
A Different Path for Planet Formation
Scientists still do not know exactly what happened to this ancient world. One possibility is that it was destroyed during a major collision in the chaotic early solar system. Its fragments may later have become part of other rocky 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.
