A new study led by UNLV scientists sheds light on how planets, including Earth, formed in our galaxy – and why the life and death of nearby stars are an important piece of the puzzle. 

In a paper published Sept. 23 in the Astrophysical Journal Letters, researchers at UNLV in collaboration with scientists from the Open University of Israel for the first time modeled details about how the timing of planet formation in the history of the galaxy affects planetary composition and density. 

“Materials that go into making planets are formed inside of stars that have different lifetimes,” says Jason Steffen, associate professor with the UNLV 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 the Earth, and also suggest that the necessary ingredients for life didn’t arrive all at once.”

Timing is Everything in Planetary Construction

All the basic elements that make up planets – like oxygen, silicon, iron, and nickel – are formed inside stars. Planets are essentially built from the debris of dying stars, but the stars die on vastly different timelines which can influence the structure of forming planets as a result. 

High-mass stars burn out relatively quickly, typically within 10 million years, and when they explode they scatter lighter elements like oxygen, silicon, and magnesium into space. These materials are generally what make up the outer layers of rocky planets. 

Low-mass stars live for billions of years and release heavier elements like iron and nickel, key elements for the formation of planetary cores.

Planets forming in solar systems where both high-mass and low-mass stars had time to contribute materials to the planetary disk will contain a greater variety of those elements. Those forming from the evolution and death of high-mass stars tend to have larger mantles and smaller cores. When time is allowed for low-mass stars to contribute heavier elements with greater abundance, such as iron and nickel, planet cores are larger.  

Over the last decade, the research team had created software models for various niche projects, but only recently realized that it had all the 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. When the new observations were published, we realized we could model the full system with just a small addition of code at the beginning,” says Steffen.

This simulation tracks the entire life cycle of planet formation from star birth and element synthesis to explosions, collisions, planet formation, and the planetary internal structure.

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

Publication Details

The paper, “Effect of Galactic Chemical Evolution on Exoplanet Properties,” was published September 23, 2025 in the Astrophysical Journal Letters. In addition to Steffen, collaborators include Cody Shakespeare and Robert Royer with the Nevada Center for Astrophysics and UNLV Department of Physics and Astronomy; and David Rice and Allona Vazan with the Astrophysics Research Center at The Open University of Israel.