A Meteorite Fell to Earth From a Planet That Doesn't Exist
Today's moons, planets, and dwarf planets formed from a proto-planetery disk surrounding our Sun 4.6 billion years ago.
However, not all of those celestial bodies survived, and a new analysis of a the meteorite NWA 15915 shows that it likely originated from a world that was similar to Mercury in its composition—but not Mercury itself.
These rare meteorites, which make up only 0.2 percent of meteorites broadly, provide an incredibly rare window into the chaotic machinations of the early Solar System.
Today, the Solar System appears as a neatly organized set-up of eight (possibly nine?) planets orbiting the Sun. But flash back more than four billion years ago—as planets formed from our star's proto-planetary disk—and things were much more chaotic. Our own planet, for example, had a devastating rendezvous with its sister protoplanet, and remnants of that interaction can still be found deep in the Earth's lower mantle.
Meteorites littered across the planet can also contain details of this tumultuous period of the early Solar System, and a new analysis of one such meteorite—known as Northwest Africa (NWA) 15915—shows that this 2.84-kilogram (6.26 pound) rock could be a piece of a long-dead planet that likely formed in a Mercury-like environment. This new analysis (led by Jennifer Mitchell at the University of Minnesota) looked at the coarse-grained achondrite that was originally discovered in Algeria in 2023, and found that the rock didn't fit within other meteorite groups or planetary bodies.
This is particularly puzzling, as a vast majority of meteorites can be traced to some well-known parent body—whether that be the Moon, Mars, or other asteroids. In fact, according to New Scientist, only 0.2 percent of meteorites are outliers. And NWA 15915 fits this atypical description.
To better understand the origins of this meteorite, Mitchell and her team used an electron microscope and spectroscopic instruments to study its composition. They found that it was different from Mercury's makeup, but that it formed in a similarly low-oxygen environment (like Mercury did). Mitchell reported the results at the Lunar and Planetary Sciences Conference in Texas last week, and also submitted a meeting brief with preliminary findings.
'Our initial analysis of NWA 15915 indicates an interesting magmatic and post-crystallization history,' the meeting brief reads. 'The magnetic properties of NWA 15915 show evidence of Fe-metal, daubréelite, and troilite. At present, our work supports the view of a large differentiated body in the inner Solar System.'
As New Scientist explains, this meteorite has a strange mix of magnetic metal-rich minerals, and because these crystals were relatively large, they cooled slowly. Both of these attributes suggest a Mercury-like origin.
Although finding rocks from long-dead worlds is a rarity, it isn't an impossibility. In some models, the early Solar System contained at least 50 to 100 Moon-to-Mars-sized protoplanets. While many of these coalesced into the planets we know today (like Theia's violent combination with Earth roughly four billion years ago), others (such as the dwarf planets Ceres, Pallas, and Vesta) survived mostly intact. So, while several of these worlds may have been destroyed billions of years ago, pieces of them still survive—on Earth, on other planets, or spread throughout the Main Asteroid and Kuiper belts.
Mitchell reiterates that this is only a preliminary finding, but it provides an incredible opportunity to learn more about the forgotten worlds of our early Solar System.
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