Are there places in the galaxy that are better suited for habitability? A new study suggests that the Sun’s trek through the Milky Way may contribute additional complexity to the search for life in the galaxy.
Galactic Habitable Zones
Illustration of a star system’s habitable zone. Too close to the star, shown in red, the planet will be too hot to support life, and farther away, the planet is too cold. The zone in the middle, shown in green, is the distance from the star that is just the right temperature for a planet to host liquid water, a crucial ingredient of life. Click to enlarge. [NASA]
In crowded areas and active star-forming regions, energetic events like supernovae can subject their neighboring star systems to harmful radiation. In the galaxy’s outskirts, there has not been enough chemical enrichment to provide the materials necessary for life. This implies that our solar system resides in a galactic habitable zone, and this region may host other life-harboring systems.
But in the search for habitable planetary systems, it is not sufficient to only consider a star’s current position in the galaxy. As evidenced by its chemical makeup, the Sun likely formed closer to the galactic center and migrated outwards to its current orbital radius, experiencing a wide variety of conditions as it moved through the galaxy. While the Sun’s current orbit is suitable for life, could its migration history have influenced the solar system’s habitability?
Simulation results showing how different models of galactic central bars and spiral arms transport stars over time. The top panel shows that for a steady model, stars are not significantly transported away from their birth location. The bottom panel shows that for models with dynamic changes in both the central bar and the spiral arms, this causes sufficient migration to explain the Sun’s migration pathway from an inner radius to its current location. Click to enlarge. [Baba et al 2024]
Migration Through the Milky Way
To understand the drivers and conditions of the Sun’s migration, Junichi Baba (Kagoshima University; National Astronomical Observatory of Japan) and collaborators perform simulations of various models of the galaxy. The authors find that dynamic changes in the Milky Way’s central bar and its spiral arms are sufficient to transport the Sun from its origin to its current position in the galaxy.
Though the Milky Way’s bar and arms drive the Sun’s migration through the galaxy, the specific path the Sun takes depends on whether or not it becomes trapped in corotation resonance with the central bar — orbiting the galactic center at the same speed as the bar’s rotation. The authors test a trapped and untrapped migration path and find that these scenarios produce significantly different migration pathways. In the trapped scenario, the Sun’s orbital radius oscillates more drastically, while in the untrapped case the Sun more gradually shifts to its current position. These pathways subsequently expose the solar system to different environments within the galaxy.
Sustaining Life
Histories of environmental changes surrounding the Sun along its potential orbits. The orange line shows the path of a trapped orbit, where the Sun experiences more dramatic environmental changes than an untrapped orbit (shown in green). Click to enlarge. [Baba et al 2024]
This study highlights the importance of considering how star systems have migrated through the galaxy over their histories. Their current galactic environment is not the only influence on their ability to harbor life. Thus, as the search for life continues in exoplanet systems, tracing the orbital paths these systems have trudged is crucial for understanding their habitability.
Citation
“Solar System Migration Points to a Renewed Concept: Galactic Habitable Orbits,” Junichi Baba et al 2024 ApJL 976 L29. doi:10.3847/2041-8213/ad9260