Iron-rich stars host planets on closer orbits than their iron-poor siblings, astronomers find. The results could help reveal how planets form.

The more iron a star contains, the closer its planet’s orbit. And astronomers aren’t quite sure why.

Robert Wilson, a graduate student at the University of Virginia, announced the puzzling result at a meeting of the American Astronomical Society in Washington, D.C.

Stars are mostly hydrogen and helium, with just a smattering of heavier elements. Since stars forge heavy elements in their core, the ones we see on the surface come from previous generations of stars. The longer a star’s lineage, the more such elements enrich it (or pollute it, depending on your point of view). The heaviest element a star can make is iron, so its abundance serves as a proxy for the presence of all the other elements in the star, or in astro-speak, the star’s metallicity.

Planets form out of the same natal gas as their parent star. So a star’s high metallicity is a sign that its planets came together within metal-enriched gas. Previous studies have found that metallicity plays a role in planet formation — but astronomers don’t yet understand how the connection works.

Wilson studied metallicity’s effect on planet formation using data on 282 candidates discovered by the exoplanet-hunting Kepler mission. The Sloan 2.5-meter telescope in New Mexico took spectra of these systems as part of the APOGEE program, revealing each star’s iron abundance.

To Wilson’s surprise, the stars richest in iron host planets on scorchingly close orbits, while stars with lower iron abundances have planets on farther-out orbits. The results point to different formation histories for the two types of planets.

A clear line divides the two groups of planets: iron-rich stars host planets with orbits of 8 days or less, while the farther-out planets circle their iron-poor stars on periods longer than 8 days. Yet the two sets of stars aren’t all that different from each other — the ones labeled iron-rich have only 25% more iron than those labeled iron-poor.

“That’s like adding five-eighths of a teaspoon of salt into a cupcake recipe that calls for half a teaspoon, among all its other ingredients,” Wilson says. When baking a planet, it turns out, even a small difference in the metallicity of a planet’s natal cloud can have surprisingly strong effects on its formation.

But how? Wilson suspects that higher-metallicity gas makes for flatter planet-forming disks. The presence of heavy elements helps gas in the planetary disk cool and collapse to the centerline — like someone forgot the baking powder when making pancakes. Thinner disks make it easier for forming planets to migrate inward, closer to the star.

The next step will be an astronomer’s version of America’s Test Kitchen: Wilson is working with theorists to cook up stars and their planet-forming disks within different metallicity environments to see if they can reproduce the same iron-rich/iron-poor divide.