'Earth-Eating' Stars Provide Clues to Planet Formation

Some Sun-like stars are known for their ability to ingest large amounts of the rocky material from which "terrestrial" planets like Earth, Mars and Venus are made. New research on these "Earth-eater" stars may provide scientists with clues about the formation of planets as well as assist in the ongoing search for Earth-like exoplanets, according to astronomers.

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

Several past studies have believed there was a link between planet formation and a star's "metallacity" - the ratio of the relative abundance of iron to hydrogen in a star's chemical makeup. Stars consist of more than 98 percent hydrogen and helium and all other elements (arbitrarily defined as metals) are less than two percent of its mass.

Mack looked at the abundance of 15 specific elements relative to that of the Sun, focusing on elements like aluminum, silicon, calcium and iron that have melting points higher than 1,200 degrees Fahrenheit (600 degrees Celsius) - these are the refractory materials that serve as building blocks for Earth-like planets.

Applying the technique to two stars - designated as HD 20781 and HD 20782 - with theoretically the same chemical composition, Mack and colleagues found that the relative abundance of the aforementioned refractory elements was much higher than that of the Sun. Also, the higher the melting temperature of a particular element, the more of it there was.

These findings support the theory that such Sun-like stars literally eat Earth-like planets.

They calculated that each of the twins would have had to consume an additional 10-20 Earth-masses of rocky material to produce the observed chemical signatures.

"With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect," Mack explained in a statement.