About a third of stars contain the remains of Earth-like planets, observations and modeling show

In recent years, astronomers have learned to measure the metal content of stars with extreme precision. It quickly became clear that even stars from the same molecular cloud have strong differences in chemical composition, which can only be explained by contamination from external sources. Earth-like planets with ultrashort orbits were called such a source and this was proven using models.

An artistic representation of a star with a planet in an ultra-short orbit. Image source: NASA

Related stars are born in the same giant molecular cloud (GMC), although they are not necessarily binary systems. Such stars would be expected to have very similar metallicities, although no GMC cloud is completely uniform and small differences are common among stars that form in a common star-forming region. But when differences are nevertheless noticeable, there must be some other explanation for this.

The new study, titled “Metal Contamination of Sun-Like Stars from Ultra-Short Period Planetary Disintegrations,” suggests that rocky planets are the source of the discrepancies being detected. The authors are scientists from Northwestern University and Cornell University. The study has been uploaded to arxiv.org and submitted to the AAS network of journals.

Ultrashort-period exoplanets (USPs) orbit their stars very closely and typically complete an orbit in just a few hours. They have a composition similar to that of Earth, and rarely have a radius greater than twice that of Earth. Their origin is not entirely clear. They could have formed further away and then migrated closer to their star, or they could have been the remnants of much larger planets that lost their atmosphere due to stellar radiation.

One problem is that USP class planets are found in about 0.5% of Sun-like stars. On the other hand, taking into account the differences in the metallicity of stars “from the same basket,” such planets can simply quickly be absorbed by their stars and, therefore, pollute them with metals.

«Short-period exoplanets are potentially vulnerable to tidal disruption and absorption by their host stars,” the authors write. Research shows that between 3% and 30% of sun-like main sequence stars (FGKs) have gobbled up rocky planets with masses ranging from 1 to 10 Earth masses.

The authors developed a model to predict the number of USPs produced and the time required for their uptake. The proposed model can reproduce both the observed low occurrence of USPs in Sun-like stars and their metallicity from pollution. The model shows that absorption most often occurs in compact multiplanetary systems and, often, this occurs during the migration of planets during the transition from a more elongated orbit to a close circular one. In general, the model predicts that a planet will be consumed by a star between 100 million and 1 billion years after formation.