Astronomers have identified three common characteristics of stars that host planets.

So far, only a few thousand exoplanets have been discovered. This is a tiny number relative to the scale of the Galaxy, which contains hundreds of billions of stars. Additionally, most of these worlds can be considered as being in our immediate vicinity — in the "residential area" of the Milky Way. However, recent observations have allowed us to identify two exoplanets located near the galactic center, about 27,000 light-years away from Earth. Nonetheless, the majority of observed worlds are situated within just a few thousand light-years around us. It’s important to note that our Galaxy extends for about a hundred thousand light-years.

Nevertheless, there are already enough known exoplanets for statistical analysis and to form a general understanding of what influences their formation in the Universe. Among the nearest stars, there are quite a few that have no planets detected. Of course, this does not guarantee that they are absent, but it does suggest that the presence of planets is not a given for every star.

A team of astrophysicists from Portugal and Italy recently decided to compile and analyze this exoplanet data, comparing three parameters: the age of the parent star, its chemical composition, and its distance from the galactic center. They used two samples: firstly, 670 stars within approximately six thousand light-years from us, which include 872 planets, and secondly, 938 stars within about one and a half thousand light-years, representing the closest ones. Interestingly, 818 of these nearest stars are classified as planetless, meaning they are solitary. The remaining 120 stars host a total of 177 worlds.

The age and chemical composition of stars are generally determined by their light: the "brightness," or luminosity, indicates the temperature, which helps to understand the "life stage" of the star. The chemical elements in a star can be identified by examining its spectrum, as each element emits light in a unique way.

Planet-holding stars were divided into three categories: those with giant worlds, small planets like Earth, and those with exclusively tiny worlds.

The findings from this extensive comparison were shared in a paper, available on the Cornell University preprint server (USA). It turned out that, in general, stars that host planets are younger, richer in heavy elements, and closer to the galactic center.

Interestingly, stars that only have low-mass planets are older than all other "family" stars. Overall, the average age of stars with planets is limited to about six to seven billion years. It’s worth noting that the Universe is 13.8 billion years old. This suggests that planets did not start to form immediately, nor did they arise simultaneously with the first stars; the majority of world-holding stars are already from later generations.

It seems that by the time stars with planets appeared, their predecessors had enriched the Universe with a variety of chemical elements through nuclear fusion, explosions, and collisions. This diversity in the cosmos led to the formation of gas-dust clouds — the future "stellar nurseries" and simultaneously "cradles of planets."

In any case, the new study shows that stars with planets are indeed significantly more "metal-rich" than solitary stars. It should be noted that in astronomy, "metals" refer to all elements heavier than hydrogen and helium.

Finally, the analysis allowed for the determination of the so-called galactic birth radius for different stars — the radius around the center of the galaxy where most of these stars form. It was found that stars with super-Jupiters and other very massive planets are surprisingly concentrated within a radius of 18-19 thousand light-years from the center of the Milky Way, while stars with smaller worlds are distributed somewhat more broadly.