US scientists' model show how timing of planet formation determines planetary composition and density

UNLV scientists have created the first comprehensive model showing how the timing of planet formation throughout galactic history determines planetary composition and density, revealing why older rocky planets are less dense than Earth. The research demonstrates that planets forming early had access only to lighter elements from short-lived massive stars, while later planets like Earth inherited heavier core-forming elements from long-lived stellar sources.

Timing, as they say, is everything. That's true whether you're catching a flight, planting a garden, or apparently, forming a planet. A groundbreaking study from UNLV scientists reveals that when planets form during galactic history fundamentally determines what they're made of – and potentially whether they can support life.

The research, represents the first time scientists have comprehensively modeled how planetary composition and density depend on the timing of formation. And the answer involves something beautifully cosmic: the life and death cycles of nearby stars acting as celestial construction suppliers.

"Materials that go into making planets are formed inside of stars that have different lifetimes," explains Jason Steffen, associate professor with UNLV's Department of Physics and Astronomy and the paper's lead author. "These findings help explain why older, rocky planets are less dense than younger planets like Earth, and also suggest that the necessary ingredients for life didn't arrive all at once."

Think of it like building a house, but your suppliers deliver materials on wildly different schedules. Some arrive in days, others take decades. The structure you ultimately build depends heavily on what's available when you start construction.

Planets face a similar constraint. All the basic elements making up rocky worlds – oxygen, silicon, iron, nickel, magnesium – are forged inside stars. Planets are essentially constructed from the debris of dying stars scattered across space. But stars die on vastly different timelines, creating a staggered delivery system for planetary building blocks.

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High-mass stars burn bright and die young, typically within ten million years. When these stellar giants explode in supernovae, they scatter lighter elements like oxygen, silicon, and magnesium throughout surrounding space. These materials generally form the outer layers and mantles of rocky planets.

Low-mass stars take the opposite approach, living for billions of years before releasing heavier elements like iron and nickel – crucial components for planetary cores. It's a cosmic waiting game that fundamentally affects what kinds of planets can form when.

Planets forming in solar systems where both high-mass and low-mass stars had sufficient time to contribute materials end up with a richer variety of elements. Early planets forming primarily from high-mass star debris tend to have larger mantles but smaller cores, because the heavy core-forming elements simply weren't available yet.

Later in galactic history, when low-mass stars have had time to contribute abundant iron and nickel, planets can develop larger, denser cores. This explains why older rocky planets discovered by astronomers are generally less dense than younger worlds like Earth.

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The UNLV team, collaborating with scientists from the Open University of Israel, didn't set out specifically to solve this puzzle. Over the past decade, they'd created various software models for niche projects. Only recently did they realize they possessed all the necessary pieces to create the first fully integrated planet formation model of this kind.

"It was like having the solution in hand, waiting for the right problem," Steffen says. "When the new observations were published, we realized we could model the full system with just a small addition of code at the beginning."

Their simulation tracks the complete life cycle of planet formation – from star birth and element synthesis through stellar explosions, material collisions, planet assembly, and internal planetary structure. It's essentially a time-lapse video of planetary construction spanning billions of years.

The implications extend beyond understanding planetary architecture. This research suggests that conditions suitable for life don't appear immediately after galaxy formation. The elements required for habitable planets and living organisms become available gradually throughout galactic history.

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"One implication of these findings is that the conditions for life don't start immediately," Steffen notes. "A lot of the elements needed for a habitable planet, and for living organisms, are made available at different times throughout galactic history."

This means early galaxies likely contained planets fundamentally different from Earth – worlds with different compositions, different densities, and potentially different capacities to support life as we understand it. The universe needed time to mature, building up its inventory of heavy elements through successive generations of stellar death.

For Earth specifically, our planet formed roughly four and a half billion years ago, late enough in galactic history that both high-mass and low-mass stars had contributed their elemental gifts to the solar nebula. We inherited a balanced mix of light mantle materials and heavy core elements, creating a planet with just the right composition to develop plate tectonics, magnetic fields, and ultimately, conditions suitable for life.

The research also helps explain observed patterns in exoplanet populations. As astronomers discover more planets orbiting distant stars, they're finding that planetary characteristics correlate with stellar age and metallicity. This UNLV model provides the theoretical framework explaining those observations.

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Understanding planetary formation timing matters for the search for extraterrestrial life. If certain elements necessary for biology only become abundant after billions of years of stellar evolution, then habitable worlds might be more common around younger stars than older ones – or vice versa, depending on which specific elements prove most crucial.

The universe, it turns out, is still learning how to make planets optimized for life. And timing, as always, is everything.