Nearly 30% of stars "consume" Earth-like planets: what's happening in space?

In recent years, astronomers have developed methods to measure the metal content in stars with remarkable accuracy. The term "metals" in astronomy refers to chemical elements heavier than hydrogen and helium. Related stars are born in the same vast molecular cloud, implying they should have similar metal content or metallicity. However, researchers have found that some related stars exhibit significantly distinct chemical compositions. The authors of the study suggest that the metal pollution in stars occurs because they absorb nearby rocky planets, similar to Earth, which are rich in metals. The research has been published on the arXiv preprint server, as reported by ScienceAlert.

In space, there exists a class of planets that have extremely short orbital periods. They are located very close to their stars and complete an orbit around them in just a few hours. The chemical composition of these rocky planets resembles that of Earth, and their diameters rarely exceed twice the size of our planet. It is not precisely known how such planets form. They may have migrated from a more distant part of their stellar system or could be remnants of a larger planet that lost its atmosphere due to stellar radiation.

However, such planets are exceedingly rare. They orbit only about 0.5% of stars similar to the Sun. These planets possess very high temperatures and molten surfaces.

According to the study, approximately 3% to 30% of Sun-like stars have engulfed rocky planets with masses ranging from 1 to 10 times that of Earth. Consequently, they have enriched these stars with a significant amount of metals.

The authors of the study proposed three main scenarios illustrating how stars may consume rocky planets with very short orbital periods.

• The first scenario is called high eccentricity migration. In this scenario, a planet ends up very close to its star and has a high eccentricity, meaning its orbit is elongated. Due to its proximity to the star and its gravitational pull, the planet quickly loses its eccentricity and attains a circular orbit.
• The second scenario is known as low eccentricity migration. In this case, the planet moves toward its star more slowly. Low eccentricity migration occurs in compact systems with three or more planets, which helps mitigate its eccentricity.
• The third scenario involves a companion planet with a short orbital period that uses its gravity to place the latter in orbital resonance. The planet's movement toward the star concludes when it exits resonance.

Modeling indicates that the second scenario is more probable, where the planet consumed by the star is part of a compact multi-planet system. The study shows that such planets are engulfed by stars within 100 million to 1 billion years after their formation.

At the same time, astronomers note that signs of metal pollution in stars may fade over time. This means it is possible that over 30% of Sun-like stars have consumed rocky planets. Additionally, modeling results suggest that only one planet with a very short orbital period may form around a star, indicating that only one planet could be absorbed by the star.