Milky Way Worlds: A High-Resolution Look at Our Galaxy’s Exoplanets
Padois and collaborators set out to answer a big question: What kinds of planets should exist throughout the Milky Way, including places we cannot observe yet? Because today’s telescopes detect only a small portion of the Galaxy’s planets, mostly those close to the Sun, the authors combine realistic galaxy simulations with modern models of planet formation to predict what the rest of the Milky Way might be hiding. Using the NIHAO-UHD suite of cosmological simulations, they recreate the local “solar neighborhood” and later explore other Galactic environments such as the inner disk, outer disk, and regions high above the plane. Their goal is to build a flexible framework that can also help estimate what future missions (like PLATO or the Roman Space Telescope) might discover.
Building the Stellar and Planetary Populations
The authors begin with simulated “star particles,” each representing thousands of stars formed together in the model galaxy. They convert these particles into individual F-, G-, K-, and M-type stars using well-tested stellar evolution tracks. To keep the model grounded in real astrophysics, they assign stars properties such as mass, age, and metallicity. Planets are then added based on known occurrence rate trends, how likely different stars are to host Earth-like planets, super-Earths/Neptunes, or giant planets. These rates depend heavily on stellar mass and metallicity. After determining how many planets each star should have, the team assigns each world a mass and orbital period using distributions derived from both observations (especially the NASA Exoplanet Archive) and planet-formation simulations. This approach ensures the population features familiar categories such as hot Jupiters, cold Jupiters, and small rocky planets, even though the model does not attempt to reproduce detailed orbital architectures.
The Solar Neighborhood in the Simulation
Within the model galaxy’s solar neighborhood, roughly the same location as the Sun, the authors generate more than eight million stars and assign planets to the single-star systems. This produces about 22 million planets, most of which are Earth-like or super-Earth/Neptunes. Their simulated local planet population ends up being 52.5% Earth-like, 44% super-Earth/Neptunes, and 3.5% giant planets. Many of these planets orbit M-dwarfs, since such stars are the most common in the Galaxy, even though real surveys underdetect them. By comparing the temperatures of the host stars and the energy received by each planet, they evaluate how many fall within the circumstellar habitable zone, defined here using an “optimistic” set of limits based on Venus and Mars. They find that roughly 23% of their planets lie within these habitable-zone boundaries.
Testing the Model with Kepler’s Observations
To see how realistic their simulated planets are, the authors recreate what the Kepler Space Telescope would detect if it observed their simulated galaxy. They select stars in the part of the simulated sky matching Kepler’s field, account for dust extinction, and apply Kepler’s magnitude limits. After simulating each planet’s probability of transiting and estimating its signal-to-noise ratio, they identify which worlds Kepler would have been able to detect. The resulting “detected” population looks broadly similar to the real set of Kepler planets, especially in how the planets are distributed in orbital period and radius. However, the simulation detects too many planets per star, and it overpredicts the number of planets around F-type stars and evolved red-giant stars. The authors attribute these differences to specific limitations in the galaxy simulation, such as an overabundance of young stars and a thicker disk than the true Milky Way, as well as simplifications in their exoplanet population model.
Planet Populations Across the Galaxy
Extending beyond the solar neighborhood, the team investigates how planet populations vary in different Galactic environments. By examining regions ranging from the inner disk to the outskirts and the high-|Z| locations above and below the plane, they find that metallicity plays the dominant role. Regions near the Galactic center, richer in heavy elements, tend to host more giant planets, while the outer disk, which is more metal-poor, hosts fewer. Despite these environmental differences, the overall proportions of Earth-like, super-Earth/Neptunian, and giant planets remain surprisingly consistent when the host galaxy resembles the Milky Way in mass and structure.
Comparing Different Simulated Galaxies and Looking Ahead
Finally, the authors repeat their analysis for all six galaxies in the NIHAO-UHD simulation suite. Though each galaxy has its own formation history and structure, the planet populations remain broadly similar if the galaxy itself is Milky-Way-like. This suggests that their method is robust and can be used to forecast the science return of future exoplanet missions. The paper ends with a discussion of limitations and future improvements, including plans to incorporate planets around binary stars, better treatment of orbital dynamics, and more sophisticated modelling of planetary radii and atmospheric loss.
Source: Padois
