Environment Life

NASA finds the Sun’s ancient journey through Milky Way may have frozen & then warmed Earth

Fahad Redha

September 2, 2026
6 min read

NASA finds the Sun’s ancient journey through Milky Way may have frozen & then warmed Earth

For most of human history, the Sun has been treated as a constant, a fixed source of light and heat that Earth simply orbits. Two new NASA-funded studies suggest that treatment has been wrong for most of Earth’s existence, and that the Sun’s own turbulent past, both where it has travelled and how violently it once behaved, has left fingerprints on Earth’s climate that scientists are only now learning to read.

The first study comes from NASA’s SHIELD center, one of the agency’s DRIVE Science Centers, and was published on 21 August in the Annual Review of Astronomy and Astrophysics. It tackles a question that sounds more like plasma physics than climate science: where has the solar system actually been?

The entire solar system sits inside a bubble called the heliosphere, an envelope of charged particles streaming outward from the Sun in every direction, extending roughly 120 astronomical units (the distance between the Earth and the Sun) in its leading edge. Merav Opher, SHIELD’s principal investigator at Boston University, and her colleagues built computer simulations to reverse-engineer the heliosphere’s path through the galaxy over the Sun’s 4.6-billion-year life. Their simulations found that the Sun has ploughed through dense, freezing clouds of interstellar gas and dust at least three times in just the past few million years.

The pressure from these cold clouds, the researchers found, may have been strong enough to crush the heliosphere down to a size smaller than Earth’s own orbit. On each occasion, that would have left Earth’s atmosphere sitting directly outside the Sun’s shield, exposed to the raw interstellar medium rather than the Sun’s own protective wind. The modelled encounters cluster around three windows: roughly 2 to 3 million years ago, 6 to 7 million years ago, and 13 to 14 million years ago.

Those dates are not simply a product of the simulation. Elements associated with interstellar dust have turned up in deep-sea sediment cores, in Antarctic snow, and in lunar samples dating to the same periods, giving Opher’s team physical evidence to test their reconstruction against. In the simulations, flooding Earth’s atmosphere with dense galactic hydrogen raised atmospheric water vapour and disturbed the upper atmosphere in ways that eventually reached conditions at the surface. Opher’s paper argues this could help explain some of the stepwise cooling patterns already visible in the geological record, including possible contributions to past ice ages, though the exact mechanisms remain a subject of ongoing debate among researchers in the field.

SHIELD’s longer-term ambition is to build what its scientists call a digital twin of the heliosphere, a detailed working model of how the Sun’s bubble behaves when it meets different galactic environments. That model, NASA says, could eventually help identify which other star systems in the galaxy are capable of shielding their own planets well enough to sustain life.

The second study looks not at where the Sun has travelled but at how it behaved when it was young, addressing a much older puzzle. Roughly three billion years ago, the Sun burned at only 70% of its current brightness. By the physics of sunlight alone, early Earth should have been a frozen ball of rock. Geological evidence says otherwise: liquid water existed on Earth’s surface long before the Sun brightened to anything like its current output. Scientists call this contradiction the Faint Young Sun paradox, and it has resisted a clean explanation for decades.

Vladimir Airapetian, a scientist at NASA’s Goddard Space Flight Center, and his collaborators looked for an answer in the behaviour of other young, Sun-like stars observed by NASA’s retired Kepler space telescope. Those stars, still in their stellar infancy, throw off enormous superflares far more often than mature stars like the Sun do today. Airapetian’s theory is that the young Sun did the same, and that the resulting storm of high-energy particles may have driven chemical reactions in Earth’s early atmosphere.

To test it, his team built a sealed chamber holding a gas mixture meant to resemble early Earth’s atmosphere, molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide, then bombarded it with protons to mimic the particles thrown off by a solar superflare. The bombardment produced nitrous oxide, a greenhouse gas roughly 300 times more potent than carbon dioxide. Some of that gas would not have survived long, since the young Sun’s intense ultraviolet radiation would have broken part of it back down into nitrogen and oxygen. The research was published in Astrophysical Journal Letters.

Even so, the team’s simulations found that surviving just 10% of the nitrous oxide produced in the lab experiment would have been enough to warm Earth’s equatorial regions to around 41 degrees Fahrenheit, five degrees Celsius, comfortably above the freezing point of water. That smaller surviving concentration may have done double duty. Research elsewhere has found that temperatures only slightly above freezing can actually favour the assembly of complex amino acid chains more effectively than warmer conditions, meaning the same violent solar activity that kept early Earth from freezing solid may also have nudged its chemistry toward the conditions that eventually produced life.

Read together, the two studies replace a simple picture of the Sun as a stable background presence with a more turbulent one. Earth’s climate history turns out to be entangled with things that have nothing to do with Earth itself: which patch of the galaxy the solar system happened to be passing through, and how aggressively a much younger Sun once behaved. Earth was never an isolated system quietly making its own weather. It has spent its entire history answering to a star, and to a galaxy, that were never as constant as they looked from the ground.

This is one of those NASA stories that reframes something you assumed was permanent, in this case the Sun itself, and EyeOnLondon will be watching for the next output from SHIELD’s digital twin modelling project, which is designed to keep testing exactly this kind of claim.

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