Tons of space dust rain down on Earth every day. Most of it comes from asteroids and comets in our own solar system — the debris from countless collisions. But some of it comes from outside the solar system. Millions of years ago, in fact, our planet was bombarded by debris from exploding stars — material that might have caused changes in Earth’s climate.

Scientists have found this debris in ocean sediments, and in samples of Moon dust gathered by Apollo astronauts.

In particular, they’ve discovered tiny bits of a radioactive form of iron. It’s produced by exploding stars, but it decays fairly quickly, so any that’s found on Earth must have arrived within the last few million years.

The first samples were discovered in ocean sediments in 1999. More were found later in samples from the Moon. And earlier this year, a team reported finding more iron in sediments from the Atlantic, Pacific, and Indian oceans.

There were high concentrations in samples from about 1.5 million and 2.3 million years ago, with a fainter concentration about 8 million years ago. That suggests that Earth was bombarded with debris from three different supernovae.

The more recent concentrations correspond to a time when Earth’s climate grew cooler, while the older one corresponds to a time of big changes in life on our planet. Cosmic rays from the explosions may have spurred cloud formation, altering the climate — dramatic changes caused by exploding stars.

Script by Damond Benningfield