Astronomers identify signature of Earth-eating stars

What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? 

A spectrum taken of the Sun is shown above. 

The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. 

By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. 

The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. 

Credit: N.A.Sharp, NOAO /NSO /Kitt Peak FTS /AURA /NSF

Some Sun-like stars are 'Earth-eaters.' During their development they ingest large amounts of the rocky material from which 'terrestrial' planets like Earth, Mars and Venus are made.

Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a model that estimates the effect that such a diet has on a star's chemical composition and has used it to analyze a pair of twin stars that both have their own planets.

The results of the study were published online May 7 in the Astrophysical Journal.

"Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets," said Vanderbilt Professor of Astronomy Keivan Stassun, who supervised the study.

"We can actually see the signature predicted by our model, in detail, element by element."

This ability will add substantially to astronomers' understanding of the process of planet formation as well as assist in the ongoing search for Earth-like exoplanets, according to the astronomers.

First, some background: Stars consist of more than 98 percent hydrogen and helium. All the other elements make up less than 2 percent of their mass.

Astronomers have arbitrarily defined all the elements heavier than hydrogen and helium as metals and have coined the term "metallicity" to refer to the ratio of the relative abundance of iron to hydrogen in a star's chemical makeup.


Since the mid-1990's, when astronomers developed the capability to detect extrasolar planets in large numbers, there have been several studies that attempt to link star metallicity with planet formation.

In one such study, researchers at Los Alamos National Laboratory argued that stars with high metallicity are more likely to develop planetary systems than those with low metallicity.

Another study concluded that hot Jupiter-sized planets are found predominantly circling stars with high metallicity while smaller planets are found circling stars with a wide range of metal content.

Building on the work of coauthor Simon Schuler of the University of Tampa, Mack took this type of analysis a step further by looking at the abundance of 15 specific elements relative to that of the Sun.

He was particularly interested in elements like aluminum, silicon, calcium and iron that have melting points higher than 1,200 degrees Fahrenheit (600 degrees Celsius) because these are the refractory materials that serve as building blocks for Earth-like planets.

More information: Paper: iopscience.iop.org/0004-637X/787/2/98/ On Arxiv: arxiv.org/abs/1404.1967