Astronomers have proposed a rather uncomfortable past for our solar system and our planet — as well as an alternative explanation for a radioactive anomaly on Earth.

Something strange happened to our solar system a few million years ago. Every time scientists analyze Antarctic ice cores, deep sea sediments, ferromanganese crusts, or even samples of lunar rock, they find something that shouldn’t be there: a radioactive isotope of iron, known as iron-60.

The isotope is not produced on Earth or the Moon, and its half-life is 2.6 million years — way too short for it to be a leftover of the solar system’s formative years. Careful dating showed something even more intriguing. Twice — around 2.5 million years ago and possibly also around 7 million years ago — significantly more iron-60 came to Earth than usual.

Radioactive iron-60, which has a half-life of 2.6 million years, suddenly increased some 2.6 million years ago and has been declining since. A second, weaker spike of iron-60 started some 7 million years ago. These measurements of iron-60 come from the ferro-manganese deep-ocean crust.
Fields & Wallner / Annual Review of Nuclear and Particle Science 2023

The radioactive isotope is a known debris product of supernovae, and it’s possible two supernovae exploded near our solar system at these times. But that explanation presents a quandary: The supernovae must have exploded close enough to deliver a sufficiently large amount of debris to Earth, and such debris would have posed a threat to life. Yet there are no records indicating major mass extinction events within the last few tens of millions of years.  

On June 10th in Nature Astronomy, Merav Opher (Boston University), Avi Loeb (Harvard), and Josh Peek (Space Telescope Science Institute) proposed a different idea: What if iron-60 peaked not because of specific supernovae but because something temporarily weakened the magnetic structure that protects the solar system?

The heliosphere is a giant magnetic bubble that shields Earth and the other planets from the charged particles that permeate the space between stars, known as the interstellar medium. Despite its name, though, the heliosphere isn’t a sphere — the Sun’s motion through the interstellar medium causes it to have a comet-like shape, compressed in the direction of movement, with a longer tail extending behind it.

The constant flow of the solar wind currently pushes the closer boundary of this magnetic structure far beyond Pluto’s orbit (which extends at most 49 astronomical units, or a.u., from the Sun), out to around 120 a.u. The Voyager 1 and 2 probes crossed this outer boundary of the heliosphere in 2012 and 2018, respectively.(In the other direction, the heliosphere’s boundary is much farther away.)

Illustration showing position of Voyager 1 and Voyager 2
NASA's Voyager 1 and Voyager 2 probes have passed outside of the heliosphere, a protective, comet-shaped structure that extends well past Pluto's orbit. The heliosphere shown here is simplified for artistic purposes; the real heliosphere has a more turbulent boundary.
NASA / JPL-Caltech

The interstellar medium contains vast clouds of dense, cold dust, and the nearest of these clouds currently lie several dozen light-years away. In the past, Opher and her colleagues propose, one of them might have encountered the solar system, collapsing the heliosphere to a fraction of its current size.

The consequences for our planet may have been dramatic. “Earth and all the planets were exposed to massive amounts of hydrogen, increased radiation, and interstellar dust,” Opher explains. Global temperatures may have dropped, starting a prolonged cold phase — perhaps even causing the most recent ice age.

Dust clouds in Lynx
These observations show cold, dense clouds in the Lynx constellation by their radio emission. These clouds are currently moving straight away from the solar system.
Opher et al. / Nature Astronomy 2024

One particular ribbon-shape set of clouds, lying in the direction of the constellation Lynx, caught the attention of the astronomers. It seems to be moving straight away from us and may have hit us just about 2 million years ago. Using a computer model, Opher and her team calculated the effect of such a collision on the heliosphere. Reproducing the cloud’s cold, dense dust in the simulation squeezed the heliosphere from today’s 120 a.u. to only about 0.22 a.u. – well within Earth’s and even Mercury’s orbits around the Sun.

“It seems that Earth was fully exposed to the interstellar medium for three-quarters of a year each orbit,” says Opher, due to the heliosphere’s comet-shaped appearance.

Heliosphere shape
Simulations show side (left) and top (right) views of the shape of the heliosphere upon encountering the cold, dense Lynx cloud. Earth's orbit around the Sun is plotted in red.
Opher et al. / Nature Astronomy 2024

“The idea is interesting and provocative,” comments Brian Fields (University of Illinois, Urbana-Champagne). Fields, who is not part of Opher’s team, was among the scientists who, back in the 1990s, proposed that supernovae created the then-newly discovered iron-60 peaks.

There are caveats, however. For one, the likelihood that the proposed collision ever happened is low, simply because we don’t yet know the exact direction of the cloud’s motion. A measurement of its motion in the plane of the sky would help narrow down whether it actually crossed paths with the solar system in the past.

Second, each time iron-60 peaked, it did so for a long time — about 1 million years.The Lynx cloud appears only thick enough for a 10,000-year-long collision. Additional observations will help measure its size. “We don't really know the extent of the cold cloud,” Opher cautions.

A final caveat is that there appear to be two iron-60 peaks, not just one. Two nearby supernovae can create two spikes in iron-60; to reproduce multiples spikes in the cloud scenario requires two clouds or perhaps just one with a large hole in it.

“In either scenario, it is necessary that the iron-60 be made recently and nearby,” Fields says. And regardless which scenario pans out, it’s a good thing to have multiple ideas at play, he adds. “Working out the different consequences for these scenarios, and developing tests to discriminate them, will sharpen the field and motivate future theoretical and experimental work,” he notes. “This is a healthy way for science to progress.”


Image of Warren-Odom


June 21, 2024 at 5:09 pm

"I've looked at clouds from both sides now,
From inside, outside, and still somehow
It's clouds' illusions I recall;
I really don't know clouds at all."

(with apologies to Joni Mitchell)

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