Every now and then our planet gets a postcard from the unknown.

Maybe it's a meteorite that lights up the night sky for an electric instant. Or maybe it's dazzling debris from some passing comet.

And scientists spend years puzzling over the mild-mannered pebbles that remain. Sure, the sender isn't completely unknown. Most meteorites hail from our solar system. They often have a lot to tell us about what makes up our solar system and how it was formed.

But when an asteroid rumbled into our atmosphere on Oct. 7, 2008, it lit up not only the night sky but a scientific inquiry that would span years. When it first barreled into our airspace, the comet weighed 80 tons before breaking into countless smaller shards that pelted northern Sudan.

Knowing we don't get these kinds of visitors very often, scientists scrambled to collect some 600 of these pieces. They're classified as ureilites, a rare stellar stone dating back to the earliest days of our solar system.

And did we mention they contains diamonds?

Still, the return address on these diamond-pocked parcels, dubbed Almahata Sitta, remained a mystery. That is, until researchers from the École Polytechnique Fédérale de Lausanne in Switzerland made an astonishing discovery: These diamonds didn't just hail from our solar system, but from a world that no longer exists.

Their findings, published in the journal Nature Communications, suggest the Almahata Sitta are postcards from a ghost planet — a world no bigger than Mars, but smaller than Mercury that may have existed 5 billion years ago.

These so-called "lost planets" once formed an early version of our solar system, before colliding violently with each other to form Mercury, Venus, Earth and Mars. But scientists could find nary a trace of the proto-planets — until these relics crashed literally in our backyard.

Researchers concluded that diamonds inside the fragments were formed inside a planet that no longer exists. Peter Jenniskens/SETI Institute/NASA Ames

After probing the crystals that line the diamonds — electrons were transmitted through each specimen to create an image — researchers noted that the diamonds formed under intense pressure. It was the kind of pressure only a planet sized somewhere between Mars and Mercury could exert.

Their conclusion? These diamonds are hard evidence that proto-planets existed and are a sparkling validation of the Protoplanet Hypothesis.

"This is the first compelling evidence for such a large body that has since disappeared," researchers noted in the study. "This study provides convincing evidence that the ureilite parent body was one such large 'lost' planet before it was destroyed by collisions."

But before that planet came to a violent end, it may have gotten a message out — a priceless postcard that could reshape our understanding of the solar system.