The radioactive leftovers of recent star explosions close to Earth have been found in space for the first time. The rare material, called iron-60, was detected by a NASA spacecraft over the past 17 years, according to a new study published today in Science. Iron-60 is known to form in supernovae, giant explosions that occur when stars die. The metal’s presence in the space around Earth adds to the theory that multiple supernovae have occurred relatively close to our Solar System in the last few million years, raining down iron-60 on Earth.

Iron-60 has been found deep within the oceans' crusts on Earth, and on the Moon. Just recently, samples of lunar soil brought back by Apollo astronauts were found to contain the radioactive substance. But its presence in our planetary system has baffled scientists for a long time. Iron-60 has a half-life of 2.6 million years — the length of time it takes for half of a sample of the material to decay. That means the metal couldn't have formed during the early years of Earth 4.5 billion years ago. The iron-60 must have come from a different source: outer space.

The most likely explanation researchers have come up with is supernovae explosions. When a star runs out of fuel, its core collapses and produces a variety of different elements — including the radioactive iron-60. These materials then shoot out at high speeds through the Universe when the star explodes. Two recent studies published in Nature have speculated about the supernovae that might be responsible for Earth's iron-60 reserve. One paper suggested that a series of star explosions brought iron-60 to our planet, and the other paper pinpointed that about half of that metal came from two stars that exploded 2 millions years ago about 300 light years from Earth.

An artist rendering of NASA's ACE spacecraft. (NASA)

This new study further strengthens the argument for multiple nearby supernovae by offering a direct detection of the stars' ejected material traveling through space. "It all fits together very nicely," said study author Martin Israel, a physicist at Washington University in St. Louis. "It’s all consistent." The metal was detected by NASA's ACE spacecraft, a probe that launched 19 years ago to measure solar particles and high-speed cosmic rays traveling through space. Martin and his colleagues combed through the past 17 years of ACE's data, looking for evidence of iron-60. They found that 15 iron-60 nuclei had encounter the ACE spacecraft during the 17-year time period, a rate of about one element per year.

That may seem like a small amount, but Israel says it's enough of a detection to confirm that these particles came from recent, nearby supernovae. "The fact that we’re seeing iron-60 at all means the cosmic rays must have been accelerated not terribly long ago," said Israel. He also noted that the researchers waited until they had enough data to prove the detections weren't some sort of glitch. "We didn’t have to wait exactly 17 years, but we had to wait until enough accumulated to convince people these weren’t some kind of malfunction."

Based on these detections, Israel and his team believe that at least two recent supernovae are to blame for what the ACE spacecraft found. One star exploded, creating the iron-60; then a blast wave from a second supernova accelerated the metal up to near light speed, according to Israel. The researchers think these stars were likely located in a nearby star cluster called the Scorpius-Centaurus Association, which is more than 380 light years from Earth.

Though all these recent iron-60 studies support the same supernova theory, there is one key difference between the radioactive metal found in space and the one found in the deep-sea crusts. The iron-60 on Earth is trapped inside dust particles that were likely ejected directly from multiple supernovae, according Anton Wallner, a physicist at Australian National University and a study author of one of the recent Nature papers. That means the supernovae that created the iron-60 found in space are different from the supernovae that are responsible for the metal's presence deep within the ocean floor.

But Wallner says today's study still fits with all the recent reports that have come out about the radioactive metal. "Although these iron-60 particles have a different origin, all new detections point to the same scenario," said Wallner. "They all agree with the scenario of several supernova explosions in the last few million year nearby Earth."