Ever wondered how our own Earth came to be, and why some planets out there in the cosmos might be denser or lighter than others? It's all tied to the cosmic clock—when stars lived, died, and sprinkled their elemental gifts into the galaxy.
Buckle up, because a groundbreaking study from scientists at the University of Nevada, Las Vegas (UNLV), teamed up with experts from Israel's Open University, is flipping our understanding of planet formation on its head. For the first time, they've modeled how the exact timing of events in our galaxy's history directly shapes the makeup and density of planets, including our own Earth. Published in the prestigious Astrophysical Journal Letters under the title "Effect of Galactic Chemical Evolution on Exoplanet Properties," this research reveals that the life cycles of stars nearby play a crucial role in crafting worlds. And get this—but here's where it gets controversial: it suggests that the building blocks for life on planets like ours didn't just magically appear all at once, potentially challenging our ideas about how common habitable worlds really are.
Let's break this down for beginners. Think of galactic chemical evolution as the slow, cooking process of our Milky Way galaxy, where stars are born, fuse elements in their cores, and eventually explode or fade away, releasing those elements into space. These elements—like oxygen, silicon, iron, and nickel—become the raw materials for planets. But stars aren't all alike in their timelines. High-mass stars, those hefty giants, burn bright and fast, typically exploding after just 10 million years. When they do, they fling out lighter elements such as oxygen, silicon, and magnesium, which often end up forming the outer rocky layers of planets—what we call the mantle.
On the flip side, low-mass stars, the more modest ones, chug along for billions of years before they release heavier elements like iron and nickel. These are the key ingredients that sink to the core of a planet, making it denser. The timing of when these stars contribute to the mix around a forming solar system is everything. If a planet assembles when both types of stars have had a chance to enrich the surrounding gas and dust disk with their elements, it results in a rich variety of materials. Planets that form mainly from the debris of short-lived high-mass stars tend to have larger mantles and smaller cores, making them less dense overall. But if there's time for those patient low-mass stars to add in iron and nickel, the cores grow bigger, boosting the planet's density.
Take Earth as an example—it's a younger rocky planet in galactic terms, so it benefited from a good mix of elements, leading to its solid core and active geology. Older planets, formed earlier when high-mass stars dominated, might be fluffier, with less heavy stuff packed in. This model, developed by lead author Jason Steffen, an associate professor in UNLV's Department of Physics and Astronomy, ties directly into why we see such differences. "Materials that go into making planets are formed inside of stars that have different lifetimes," Steffen explains. And it doesn't stop there—these variations even hint at why life-supporting ingredients arrive in stages, not instantaneously.
And this is the part most people miss: the research team's journey to this model was like piecing together a galactic puzzle. Over the past decade, they'd been building software for various niche projects, but it wasn't until new observations came in that they saw the full picture. "It was like having the solution in hand, waiting for the right problem," Steffen says. With just a small tweak to their code, they created the first fully integrated simulation that traces planet formation from star birth, through element creation and stellar explosions, all the way to collisions, world-building, and internal structures. It's a comprehensive look that accounts for galactic history's impact.
One of the biggest takeaways? The ingredients for life—think complex molecules, water-forming elements, and the right balance of minerals—show up at different points in the galaxy's timeline. "A lot of the elements needed for a habitable planet, and for living organisms, are made available at different times throughout galactic history," Steffen notes. This means the conditions for life might not kick in right away in a young solar system. For instance, if a planet forms too early, it might lack the iron for a magnetic field to shield against radiation, making habitability trickier.
But here's the spark for debate: Does this mean life is rarer than we think, dependent on precise cosmic timing, or is the universe more forgiving, with multiple chances as stars evolve? Some might argue this model supports a deterministic view of planetary evolution, where only systems with the right "schedule" produce Earth-like worlds. Others could counter that it opens up exciting possibilities for diverse alien biospheres on planets with unique elemental cocktails. What do you think—could this timing factor explain why we haven't found more evidence of extraterrestrial life yet, or is it just another piece of the puzzle?
Share your thoughts in the comments: Do you agree that galactic chemical evolution makes life a matter of good timing, or disagree and believe other factors dominate? Let's discuss!
For more details, check out the full paper: Jason H. Steffen et al, Effect of Galactic Chemical Evolution on Exoplanet Properties, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/ae0457 (https://dx.doi.org/10.3847/2041-8213/ae0457)
Citation: Planet formation depends on when it happens: New model shows why (2025, October 17) retrieved 17 October 2025 from https://phys.org/news/2025-10-planet-formation.html
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