Iron-rich stars keep planets close to home

Small differences in the amount of iron in a star may herald substantial differences in the type of planetary systems that form around it, say scientists at a meeting of the American Astronomical Society (AAS) in National Harbor, Maryland. 

Specifically, reports Robert Wilson, a graduate student in astronomy at The University of Virginia, stars containing higher levels of iron are more likely to have planets with orbital periods of 8.5 days or less. 

Just as the Earth has a shorter “year” than Mars and a longer one than Venus, short orbital periods mean that these worlds lie closer to their stars — and are correspondingly more sunbaked — than worlds with longer orbital periods. 

The find came by comparing exoplanet data from NASA’s Kepler space telescope with spectral data from a ground-based project called the Apache Point Observatory Galactic Evolution Experiment (APOGEE), conducted by the Sloan Digital Sky Survey. 

Kepler has found thousands of exoplanets via the tiny dimming of starlight occurring whenever one passes in front of its star. These light variations also allow scientists to determine the planet’s basic characteristics, such as its size and orbital period. APOGEE studies the stars themselves, taking spectra that allow astronomers to determine such things as the amount of iron they contain.

It turns out not to take much difference in iron content to affect how many close-in planets circle each of these suns. As little as a 25% difference in is enough to have an effect, Wilson says, even though iron is only a tiny fraction of any given star’s mass.

“This is really quite surprising,” he says. 

“That’s like adding five-eighths of a teaspoon of salt into a cupcake recipe that calls for half a teaspoon of salt, among all its other ingredients,” he adds. “I’d still eat that cupcake. That really shows us how even small differences in stellar composition can have profound impacts on planetary systems.”

Not that Wilson believes the iron in a star directly affects how its planets form. Rather, he says, the star’s iron content is a marker for how much iron was in the protoplanetary disk from which both it and its planets condensed.

That is, something about forming in a protoplanetary disk that contains relatively large amounts of iron appears to create solar systems with a larger likelihood of close-in planets. 

One way this might occur is if, for some reason, planets form closer to their stars in metal-rich protoplanetary disks. Another is if the heavier materials in the disk somehow create drag that makes planets, formed farther out, more likely to migrate inward. “We are working with people to run simulations to try and find out,” Wilson says. 

It is also possible that there are two different planetary formation processes at work. “Mechanism one might work at distances further from the star, whereas mechanism two would be more dominant at [closer] distances,” he says.

Rachel Beaton, a postdoctoral researcher at Princeton University, US, also uses APOGEE to study iron content in stars, although her specialty is variable stars, not exoplanet systems. But in both cases, she tells the AAS meeting, the iron itself might not be the magic ingredient. 

Astronomers, she says, often use iron as a proxy for a star’s total metal content. “It’s really just the first tip of the iceberg of the chemical content,” she explains. “I think what will be really interesting is what the other elements are doing.” 

Brad Tucker, an astrophysicist at Australian National University, wasn’t at the AAS meeting and hasn’t seen the data. “But if they really have found two populations of planets related to metallicity, I think this is a very interesting find,” he says.

“There are a number of new projects coming online to simultaneously measure the properties of the star and the planets in various systems, and this shows there is a lot of promise and exciting things to learn.” 

Meanwhile, Wilson notes that this appears to be one more factor playing into the search for habitable exoplanets. If you want to find planets that aren’t vastly too hot for life, he says, “your best bet is to look around a star with iron content similar to the Sun”. 

“A star with Sun-like iron content will host warm planets, while stars relatively enhanced in iron will host hot planets, very close in.”