Scientists study where and when stars and their planets were born in the Galaxy to understand how planets form. They used data to estimate the ages and chemical makeup of stars with and without planets. The results show that stars with planets are younger, richer in metals, and were born closer to the Galaxy's center compared to stars without planets. Stars with bigger planets tend to have even more metals and were born even closer. Planet formation becomes less common farther from the Galaxy's center, especially for big planets. Over time, planets have formed farther out as the Galaxy evolved, following its chemical changes.
Scientists have discovered over 5,500 exoplanets—planets that orbit stars outside our solar system. These exoplanets come in many forms, from massive gas giants to smaller rocky planets similar to Earth. One of the biggest mysteries in astronomy is understanding where these planets originally formed in the Milky Way and how their formation is influenced by their birthplaces. A recent study by Teixeira and colleagues aimed to answer this question by tracing the birth locations of stars and their planets within the galaxy. This research provides valuable insights into how the conditions of the galaxy shape the formation of planets over time.
To determine where stars and their planets were born, the researchers used data from different sources. They collected photometric data, which measures the brightness and color of stars, spectroscopic data, which reveals the chemical makeup of stars, and astrometric data, which tracks their movements across the sky. By combining all this information, scientists were able to estimate the ages of stars and calculate their original positions in the galaxy when they first formed. This process allowed them to determine the Galactic birth radius (rₐₛₜ), which is the distance from the center of the Milky Way where a star and its planets were initially created.
The study found some interesting differences between stars that have planets and those that do not. One major finding was that stars hosting planets tend to have higher amounts of metals, such as iron. In astronomy, the term "metals" refers to any element heavier than hydrogen and helium. This suggests that planets, especially large ones like Jupiter, are more likely to form around stars that contain a higher concentration of heavy elements. This is because metals play a crucial role in the process of planet formation by providing the building blocks needed to create solid cores.
Another significant discovery was that stars with planets are generally younger than stars without planets. This finding indicates that planet formation has become more common in the galaxy's recent history, possibly due to an increase in the availability of heavy elements. Over billions of years, exploding stars have enriched the Milky Way with these essential elements, creating better conditions for planets to form. As a result, younger stars are more likely to host planetary systems compared to their older counterparts.
The research also showed that stars with planets were born closer to the center of the galaxy, where the concentration of metals is higher. In contrast, stars without planets were found to have formed farther away from the galactic center, in regions with fewer heavy elements. This suggests that the inner parts of the Milky Way provide a more favorable environment for planet formation, while the outer regions have conditions that make it harder for planets to develop.
An interesting aspect of the study focused on the differences between high-mass and low-mass planets. The researchers found that high-mass planets, such as gas giants, tend to form around stars with higher metallicity and smaller birth radii. On the other hand, low-mass planets, like rocky planets, can form in a wider range of environments and are not as dependent on the presence of heavy elements. This means that while gas giants are mostly found closer to the galactic center, smaller planets can be found throughout the Milky Way.
Understanding the birthplaces of stars and their planets is important because it helps scientists learn more about the overall process of planet formation. The location where a star is born determines the materials available for planet formation, and these materials directly impact the types of planets that can form around that star. The study also sheds light on how the galaxy has evolved over time and how these changes have influenced the formation of planets in different regions.
One of the key findings of the study is that the formation of high-mass planets has become more efficient over time. As the galaxy became richer in metals, the conditions for forming gas giants improved, and these planets started appearing in larger numbers across different parts of the Milky Way. In contrast, low-mass planets have been forming steadily over time, with their formation peaking between 4 and 8 billion years ago. This suggests that certain periods in the galaxy’s history were more favorable for the formation of specific types of planets.
The study also found that stars without detected planets tend to form farther from the galactic center as time goes on. This indicates that the conditions necessary for planet formation were not always present in the outer regions of the Milky Way. However, as the galaxy continued to evolve and spread heavy elements to these distant areas, the chances of planet formation may have gradually increased.
These findings align with existing theories that suggest the Milky Way formed from the inside out. This means that the central regions of the galaxy formed first and accumulated heavy elements earlier than the outer regions. As a result, more planets were able to form closer to the center, while the outer regions took longer to develop the necessary conditions for planet formation. This idea is further supported by the observation that younger stars tend to have more metals and are more likely to host planets.
The study by Teixeira and colleagues also highlights the importance of considering radial migration—the movement of stars across the galaxy over time. Stars do not stay in their birthplaces forever; instead, they gradually move around due to interactions with other stars and the gravitational pull of the Milky Way. This movement means that the planets we observe today are not necessarily located where they originally formed. Understanding how stars move across the galaxy helps scientists piece together a more complete picture of planet formation.
Looking ahead, future missions such as PLATO, an upcoming space telescope designed to study exoplanets, will provide even more accurate data on the ages of stars. This will allow scientists to refine their models and gain a better understanding of the relationship between a star’s age, metallicity, and its likelihood of hosting planets. With more precise data, researchers will be able to trace the origins of planets even more accurately and uncover new insights into the formation and evolution of planetary systems across the Milky Way.
In conclusion, the study offers important insights into the galactic-scale factors that influence planet formation. It shows that stars with planets are younger, richer in metals, and were likely born closer to the center of the galaxy. It also highlights the connection between a star's birthplace and its potential to host high-mass or low-mass planets. These findings contribute to our understanding of how planets form and evolve over time and provide valuable clues about our own solar system's place within the broader context of the Milky Way. As our knowledge continues to expand, we move closer to answering fundamental questions about the origins of planets and the possibility of finding other Earth-like worlds in the galaxy.
Reference: Teixeira, J., Adibekyan, V. and Bossini, D. (2025), Where in the Milky Way Do Exoplanets Preferentially Form?. Astron. Nachr. e20240076. https://doi.org/10.1002/asna.20240076
Technical terms
- Exoplanets – Planets that orbit stars outside our solar system.
- Galactic birth radius (rₐₛₜ) – The distance from the center of the Milky Way where a star and its planets were originally formed.
- Metallicity ([Fe/H]) – The amount of heavy elements (like iron) in a star compared to hydrogen, which helps in planet formation.
- Photometric data – Information about a star’s brightness and color, used to understand its properties.
- Spectroscopic data – Data obtained by analyzing the light from a star to find out what it is made of.
- Astrometric data – Measurements of a star’s movement and position in the sky over time.
- Galactocentric distance – How far an object is from the center of the Milky Way galaxy.
- Formation efficiency – How easily planets can form around a star based on available materials and conditions.
- Core accretion model – A theory that explains how planets form by gradually collecting gas and dust.
- Radial migration – The movement of stars from their original birthplaces to different locations in the galaxy over time.
- Interstellar medium (ISM) – The gas and dust spread across the galaxy, which stars and planets form from.
- Supernova (SNe) – A powerful explosion that happens when a star dies, releasing heavy elements into space.
- Galactic chemical evolution – How the elements in the Milky Way change and spread over time, influencing star and planet formation.
- Negative metallicity gradient – The decrease in metal content as you move farther away from the center of the galaxy.
- Radial velocity method – A technique used to detect planets by measuring small changes in a star’s movement caused by the pull of an orbiting planet.