Exploding stars may have seeded the Earth with star dust 2 million years ago
Imagine standing on Earth a little more than 2 million years ago.
It’s nighttime. You’re looking up at the sky, marveling at the beauty of the star-filled night unobscured by artificial light pollution, when suddenly you spot an incredibly bright looking star where only a dim object glowed before.
The star is so bright, in fact, that it matches the intensity of the moon. As the sun rises the next morning, you notice that the star is still visible during the day.
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Over the course of a couple of weeks, however, the star’s brightness fades and a far dimmer star shines in its place. You're left wondering what you just saw.
What you would have witnessed is the rise and fall of a supernova -- the explosion of a massive, dying star that has run out of fuel and is shedding its element-rich outer layers to space.
Scientists haven’t found any stars in Earth’s cosmic neighborhood that could explode as supernovas anytime soon. However, they have observed them in other solar systems.
A team of researchers has discovered some evidence that a little more than 2 million years ago at least one supernova exploded not far from Earth, potentially delivering star dust -- essentially its stellar guts -- down to Earth.
This possible explosion from a specific location may explain why the element iron-60 -- which is produced from supernova explosions -- can be found in ocean core samples around the world, according to two new studies published in the journal Nature this week.
Stellar iron in the oceans
Scientists have known about the existence of iron-60 for years, since it was found in the ocean’s crust.
The only problem is, iron-60 shouldn’t be there.
The element is less stable than the more common form of iron found on Earth, so it degrades faster.
In fact, all of the iron-60 that may have been native to Earth and on the planet through the more than 4 billion years of Earth’s history, should already be gone. The iron isotope has a half-life of just 2.6 million years.
This means that it takes 2.6 million years for half of the iron-60 atoms to disintegrate, so all of them would decay in a little more than 5 million years.
So where does the present-day trove of iron-60 come from?
Today, scientists think this iron isotope likely comes from supernovas which burst forth little bits of the isotope out into the cosmos, eventually hitting Earth and embedding it within Earth's crust.
One of the studies in Nature this week shows that iron-60 can be found in crust and sediment samples from the Indian, Pacific and Atlantic oceans.
“We cover basically all of the major oceans, and demonstrate that by finding iron-60 in all those samples that the signal must be global,” Anton Wallner, a co-author of one of the studies and an astronomer at Australian National University, said in a Nature podcast.
“It must come from a source therefore which is outside of the solar system and the most likely source for iron-60 is massive stars.”
Pinpointing a stellar explosion
Another team of scientists looked at the abundances of iron-60 and tried to work backwards.
The researchers set out to find the stars that exploded millions of years ago which delivered the iron-60 to the planet.
Today, there are no clusters in the immediate vicinity of Earth that may have produced the supernovas, but by looking back in time at where star clusters were millions of years ago, the team of scientists think they found a possible cluster that was close enough to produce the explosions.
The team looked at how the star cluster may have moved through the galaxy, allowing them to see where supernovas could have exploded and then match that to the iron-60 abundances in the ocean.
“We find, which is very nice, that we get a peak of enhancement of iron-60 at around 2.2 million years ago,” Dieter Breitschwerdt, an astronomer at Berlin Institute of Technology in Germany and co-author of the other study, told Mashable.
This iron-60 "enhancement" could indicate that at least one relatively nearby supernova exploded at that time.
The supernovas that produced the iron-60 were probably about 300 light-years from the solar system, quite close in cosmic terms, the study concludes.
Lingering questions
But while these results are intriguing, we can’t get too excited quite yet.
“The modeling is very sophisticated,” Dan Milisavljevic, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was unaffiliated with these studies, told Mashable via email.
“Breitschwerdt et al. use many variables as input that have been observationally constrained or theoretically estimated. The big picture is there. But... there are many links in the chain of reasoning that can potentially be broken,” he cautioned.
For instance, we don’t actually know how much iron-60 is produced and sent out into the universe when stars explode.
Breitschwerdt's study is by no means proof that this particular star cluster sparked these supernovas, but it is a compelling proof-of-concept for this kind of work, Milisavljevic said.
“[The] Bottom line is that in a study like this where there are many parameters to adjust (especially when predicting events that took place millions of years ago) we can never robustly prove (or disprove) the results,” Milisavljevic said.
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Miriam Kramer worked as a staff writer for Space.com for about 2.5 years before joining Mashable to cover all things outer space. She took a ride in weightlessness on a zero-gravity flight and watched rockets launch to space from places around the United States. Miriam received her Master's degree in science, health and environmental reporting from New York University in 2012, and she originally hails from Knoxville, Tennessee. Follow Miriam on Twitter at @mirikramer.
