What can the Galactic Habitable Zone (GHZ), which is a galaxy’s region where complex life is hypothesized to be able to evolve, teach scientists about finding the correct stars that could have habitable planets? This is what a recent study accepted for publication in Astronomy & Astrophysics hopes to address as an international team of researchers investigated a connection between the migration of stars, commonly called stellar migration, and what this could mean for finding habitable planets within our galaxy. This study has the potential to help scientists better understand the astrophysical parameters for finding habitable worlds beyond Earth and even life as we know it.
For the study, the researchers used a series of computer models to simulate how stellar migration could influence the location and parameters of the GHZ. The models included scenarios both with and without stellar migration to ascertain the statistical probabilities for terrestrial (rocky) planets forming around stars throughout the galaxy. The researchers also used a chemical evolution model to ascertain the formation and evolution of our galaxy, specifically regarding its thickness.
In the end, the researchers found that stellar migration influences the formation of habitable planets within the outer regions of the galaxy. This is because stellar migration results in a redistribution of stars throughout the galaxy, with the team estimating a 5 times greater likelihood of stellar migration resulting in stars hosting habitable planets compared to a lack of stellar migration. Additionally, the team found that gas giant planets could influence the formation of terrestrial planets within the inner regions of the galaxy.
The paper notes in its conclusions, “In this study, we have significantly expanded the exploration of the parameter space defining the Galactic Habitable Zone, compared to previous analyses present in literature. Our findings are particularly relevant in the context of upcoming space missions, such as the ESA [European Space Agency] PLAnetary Transits and Oscillations of Stars (PLATO), the ESA Ariel space mission and Large Interferometer For Exoplanets (LIFE). These missions will deliver unprecedented data on planetary properties, orbital architectures, and atmospheric compositions.”
The notion of the GHZ builds off the longstanding idea of the stellar habitable zone (HZ), which is the specific distance a planet must orbit its star for liquid water to exist on its surface, which was first introduced in the 1950s. Like all scientific notions, the idea of a GHZ has evolved over time since it was first introduced in the 1980s, but the overarching idea is this region is comprised of heavier elements (i.e., iron, silicon, and oxygen) that are used to form terrestrial planets like Earth. As this study notes, the exact size of the GHZ is still being debated, but the consensus in the scientific community is that the GHZ does not exist in the center of the galaxy, as this region hosts countless supernovae and other celestial events that would limit habitable planets from forming.
As the study notes, there are several ESA missions in the pipeline whose goals will be to expand our knowledge of both how and where to find life beyond Earth. For example, the PLATO mission, which is slated to launch in December 2026, will have the goal of scanning one million stars to observe and identify exoplanets that cross in front of them, known as a transit, and is one of the most common methods for discovering exoplanets to date.
The Ariel mission, which is slated to launch in 2029, will have the goal of observing at least 1,000 confirmed exoplanets to learn more about their chemical and heat compositions. Finally, the LIFE mission has was started in 2017 with the goal of studying the atmospheres of terrestrial exoplanets to identify potential signs of life known as biomarkers.
What new discoveries about the GHZ and stellar migration will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!