The age of a galaxy plays a crucial role in determining the types of planets it can form, according to new research published in The Astrophysical Journal Letters. The study, led by Jason Steffen, an associate professor at the University of Nevada Las Vegas, delves into how the chemical composition of a galaxy evolves over time as stars live and die, dispersing elements necessary for planet formation.
Initially, the Universe was composed primarily of hydrogen and helium, the lightest elements created during the Big Bang. As stars form and undergo nucleosynthesis, they produce heavier elements, collectively referred to as “metals” in astronomical terms. These elements are then released into interstellar space, allowing for the formation of rocky planets like Earth. The study highlights that the rise in metallicity over time within a galaxy influences the variety of planets formed, including the characteristics of rocky planets.
Steffen noted, “Materials that go into making planets are formed inside of stars that have different lifetimes.” This variation in stellar lifetimes affects the composition of planets. For example, high-mass stars produce heavier elements like oxygen, silicon, and magnesium within a few million years, enriching their surroundings. In contrast, lower-mass stars generate lighter metals, but their longer lifespans mean they contribute these materials later.
This difference in timing and elemental composition leads to significant variations in planet density. The research indicates that older rocky planets tend to be less dense than younger ones, such as Earth. The authors explain that early planet formation occurs with higher abundances of elements from high-mass stars, resulting in larger mantles and smaller cores. Conversely, the later addition of elements from lower-mass stars increases core sizes relative to mantles.
Graphical data from the study illustrates how the chemical composition of a galaxy shifts over time, affecting the ratios of essential elements. For instance, the ratio of iron to magnesium is crucial in predicting the relative sizes of a planet”s crust and mantle, while the magnesium to silicon ratio impacts the type of volcanic rock present on a planet. A planet with a thick crust may inhibit volcanic activity and plate tectonics, features believed to be vital for habitability.
The research suggests that early planets have higher magnesium to silicon ratios, making them less favorable for supporting life. Furthermore, the findings indicate that planets forming around early-generation stars in the Milky Way would have lower iron content, leading to smaller cores and potentially less protection from cosmic radiation.
Steffen concludes, “One implication of these findings is that the conditions for life don”t start immediately.” The essential elements needed for a habitable rocky planet become available at different stages throughout a galaxy”s development. As our understanding of rocky exoplanets advances, future missions like JWST and PLATO will provide deeper insights into these planetary properties.
Overall, this research underscores the complexity of cosmochemistry and its influence on planet formation. The evolution of a galaxy significantly shapes the characteristics of the planets that emerge, suggesting that denser, more habitable planets are more likely to form as galaxies age.
