The formation of a new exoplanet can cause chemical discrepancies in paired stars

Co-paired stars, or stars that travel together, can provide insights into processes that other stars can’t. Differences in their brightness, orbits, and chemical composition can hint at different features, and scientists are beginning to exploit them.

A new paper by researchers in Australia, China, the U.S. and Europe, posted to the arXiv preprint server, has analyzed data to determine whether one of those features—specifically the depletion of particular elements in a star—could be a sign that it has formed a planet, or that it ate one.

The short answer is that the formation of a planet probably causes it. However, the data and methodology used to reach that conclusion are worth exploring. The underlying data set consisted of 125 co-moving star pairs captured in the Complete Census of Co-moving Pairs of Stars (C3PO), one of the more memorable contrived initialisms astronomers have come up with.

Importantly, each of those pairs had a difference in chemical composition between the two stars. With that base dataset, the researchers also collected data on the same set of stars using Magellan, Keck, and the Very Large Telescope.

The co-moving stars selected for the study didn’t just have chemical differences but showed significant differences in magnetic activity. Specifically, those that lacked “refractory elements” had much higher levels of magnetic activity than those with a regular amount. In this context, refractory elements mean elements with a “condensation temperature” of more than 900 Kelvin. To keep with the theme of further explanation, in this context, condensation temperature is the temperature at which at least 50% of the elements transition from a gaseous state into a solid one.

Elements with high condensation temperature (i.e., refractory elements), like iron, titanium, and aluminum, can solidify relatively close to the star, whereas “volatile elements” (i.e., those with condensation temperatures below 900 K), like carbon and oxygen, can solidify further away from the star. The authors found that decreases in the chemical abundance of a particular refractory element were positively correlated with increases in magnetic activity levels. Conversely, low volatile element abundance had a much smaller impact on the magnetic readings of its star.

It’s important to note that the condensation temperature, not just the atomic number, has this sort of impact, though refractory elements commonly have a higher atomic number than volatiles. Also, it seems the star’s age also has an impact, with younger stars exhibiting more magnetic activity, even compared to older stars with the same amount of chemical abundance.

This theory that decreased refractory elements lead to higher magnetic activity has an interesting corollary. Since planets can bind refractory material, stars that host planets are more likely to have higher magnetic activity levels. The actual mechanisms for this increase in magnetic activity are still unresolved.

Still, the paper suggests two potential causes: Star–planetary interactions, even those caused by gravitational forces, could affect the star’s magnetic field. Also, the star might be more efficient at contracting during its pre-main-sequence phase if there aren’t any refractory elements to hold it back, causing it to have a more active magnetic field.

The authors ruled out several other potential causes for these magnetic discrepancies. One important feature was the use of co-moving stars, who are assumed to be the same age. That eliminates the potential of a galactic chemical evolution that would change a star’s makeup based on when / where they were “born.” It also lowers the risk that “mixing” going on inside the stars themselves could have an appreciable impact on their magnetic activity, since both parts of the pair would be subject to similar forces.

Finally, the stars’ activity cycles could potentially affect the magnetic forces. Still, they found no correlation between the activity cycle and the amount of materials with high condensation temperature in the star itself, making it an unlikely candidate.

Further work would include looking for other evidence of the proposed planets in the co-moving systems and collecting data about stellar rotation to rule that out as a cause. For now, though, this paper adds to our understanding of what kind of formation processes these early stars undergo. There will undoubtedly be more of those to discover.