Sun’s cosmic journeys may have shaped Earth’s climate
A Telescope Built for Another Mission
The
Kepler space telescope was never designed to study our own Sun. Launched in 2009, its mission was to stare at a patch of sky containing more than 150,000 stars, hunting for the telltale dimming that reveals planets passing in front of their hosts. It found thousands of exoplanets, transforming our understanding of planetary systems across the galaxy. But the data it gathered contained secrets closer to home, waiting for a different kind of analysis.
Among the stars Kepler observed were young Sun-like stars — “toddler” stars, as researchers call them. These stars are prone to throwing fits. The data shows they regularly erupt with massive superflares, flinging high-energy particles in all directions on a daily basis. [1] This behavior is a far cry from our Sun’s current temperament. Today, our Sun produces a superflare perhaps once in a millennium, if that often. The young versions of solar twins were different creatures entirely, violent and unpredictable.
Vladimir Airapetian, a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, saw something profound in this stellar tantrum. If our young Sun was like these other stars, he proposed, the barrage of high-energy solar particles could have triggered chemical reactions that were key to warming early Earth. This idea would eventually challenge a long-standing mystery known as the Faint Young Sun paradox, a puzzle that has confounded planetary scientists for decades.
The paradox is stark in its simplicity. Three billion years ago, the young Sun was 70% as bright as it is today. Under these dimmer conditions, Earth should have been frozen solid, a ball of ice orbiting a weak star. Yet geological evidence shows stable liquid water already existed long before that. A balmy Earth under a cooler, dimmer Sun — the contradiction seems impossible. The numbers simply do not add up under any conventional model.
Rewriting the Climate Model
The standard model of Earth’s climate has long focused on internal factors. Scientists looking to explain periods of warming and cooling examined orbital changes, greenhouse gases, and ice cover. These factors certainly matter, and they explain much of what we observe in the geological record. But new research suggests they may not tell the whole story. Something external, something far larger than our planet itself, may have been pulling the strings.
That something is the heliosphere, the massive bubble created by our Sun that envelops our entire solar system. Just as our planet is encased by an atmosphere, so our entire solar system is encased inside a kind of “atmosphere” created by the Sun. This protective bubble is formed by a continuous solar wind of charged particles streaming out from the Sun in all directions. It extends far beyond Pluto, shielding every planet in our system from the harsh environment of interstellar space.
Our heliosphere orbits around the center of our galaxy, the Milky Way. Throughout the Sun’s 4.6-billion-year existence, our heliosphere has traversed various regions within our galaxy. Most of the time, these journeys have been uneventful. But in a paper published on Aug. 21 in Annual Review of Astronomy and Astrophysics, researchers at NASA’s SHIELD center used computer modeling to reverse-engineer the path of the heliosphere through our galaxy. [2] The results revealed that the environments it passed through may have triggered changes on Earth.
Merav Opher, SHIELD’s principal investigator at Boston University, and her team ran simulations that showed the Sun has encountered frigid expanses of gas and dust at least three different times in the past few million years. [3] In these instances, massive interstellar “cold clouds” pushed against the heliosphere to such an extent that it shrank to smaller than Earth’s orbit. Our planet was stranded outside the Sun’s protective shield, exposed to a completely different cosmic environment.
These exposures occurred approximately 2 to 3 million years ago, 6 to 7 million years ago, and 13 to 14 million years ago. During these windows, Earth’s atmosphere would have been subjected to totally different surroundings. The simulation results match geologic evidence: Elements prevalent in interstellar dust appear in deep-sea sediment core samples, Antarctic snow, and lunar samples during these exact timelines. The fingerprints of these cosmic encounters are literally written in the rocks of our planet.
The Limits of What We Can Measure
The SHIELD center is one of several that NASA funds to unlock the next generation of heliospheric research. As a DRIVE Science Center, SHIELD builds a team of researchers with differing expertise, approaches, and opinions. Their goal is to develop a model, or “digital twin,” of the heliosphere that helps reveal how it interacts with its surroundings, including dense interstellar clouds. This digital twin must account for events that happened millions of years ago, which presents a fundamental challenge. Parallel efforts at the University of California, Berkeley’s Space Sciences Laboratory and the Max Planck Institute for Solar System Research are developing complementary models of heliospheric dynamics, each testing different assumptions about interstellar cloud density and solar wind pressure.
Computer models are only as good as the data that feeds them. The simulations that revealed these heliosphere collapse events depend on our understanding of both the Sun’s path through the galaxy and the structure of interstellar clouds. Both of these factors carry significant uncertainty. We cannot directly observe the heliosphere as it was millions of years ago, nor can we measure the exact density and temperature of clouds that have long since dispersed.
The same limitation applies to Airapetian’s work on the Faint Young Sun paradox. His team simulated early Earth’s atmosphere in a sealed chamber, mixing molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide. They then fired protons into the mixture, simulating the onslaught of particles from superflares. This proton bombardment triggered several changes including the production of nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide. The research was published in Astrophysical Journal Letters. [4]
This nitrous oxide could help Earth hold onto heat, but not all of it would last. The young Sun’s intense ultraviolet radiation would break some of it down, splitting the molecule back into nitrogen and oxygen. But even if only 10% of the nitrous observed in the experiment survived, Airapetian’s team’s computer simulations confirmed, it would still warm Earth’s equatorial regions to about 41 degrees Fahrenheit, or 5 degrees Celsius, above water’s freezing point. This smaller amount of nitrous could even accelerate prebiotic synthesis: just-above-freezing temperatures have been found to be more efficient for building complex chains of amino acids than warmer temperatures.
Current measurement technology cannot directly observe the ancient heliosphere or the early Sun’s superflare activity. Researchers rely on indirect evidence — the composition of deep-sea sediments, the behavior of distant young stars, the chemistry of laboratory simulations. Each piece of evidence constrains the models a little more, but the gaps remain significant. The heliosphere collapse events remain simulations, not observations. The superflare-driven warming remains a hypothesis, not a proven mechanism.
Understanding our unique, habitable solar system will help unravel the mysteries of life’s evolution on Earth and potentially uncover other habitable star systems. Together, these two studies show that the Sun can lead to surprising implications for Earth. While our planet stands alone in many ways, it was formed and has always existed as part of a star-planet system. Understanding that unique relationship promises new insights about both Earth and the star that sustains it.