Solar systems have been known to devour their offspring. Some sun-like stars ingest large amounts of the rocky material from which terrestrial planets such as Earth, Mars and Venus are made. Scientists hope that studying stars with a bad case of the munchies will deepen their understanding about planet formation.
Up until now, locating this type of solar system has been a tricky process. But Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a method that relies on spectral analysis to identify stars with a mineral-heavy diet.
"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 supervising astronomy professor Keivan Stassun in a university statement. "After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail, element by element."
This approach is based on the fact that aside from helium and hydrogen, the remaining elements that comprise a star account for less than 2% of its 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.
Ever since astronomers developed the capability to detect exoplanets, they've been trying to unravel the cosmic alchemy that links star metallicity with planet formation. For instance, in one study, researchers at the Los Alamos National Laboratory argued that stars with high metallicity are more likely to develop planetary systems. Another study concluded that hot Jupiter-sized planets are found predominantly circling stars with high metallicity.
Mack built upon research conducted by Simon Schuler, a professor of physics and astronomy at the University of Tampa, who had expanded the examination of stars' chemical composition beyond their iron content. 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, because they serve as building blocks for Earth-like planets.
Mack, Schuler and Stassun applied this technique to a planet-hosting binary star pair designated HD 20781 and HD 20782. Both stars likely condensed out of the same cloud of dust and gas, so both should have started with the same chemical compositions.
The two stars are G-class dwarf stars similar to the sun. One star is orbited closely by two Neptune-size planets, while the other possesses a single Jupiter-size planet that follows a highly eccentric orbit. When the astronomers analyzed the spectrum of the two stars, they discovered that the relative abundance of planet-building elements was significantly higher than that of our sun. They also found that the higher the melting temperature of a particular element, the higher was its abundance — a trend that serves as a compelling signature of the ingestion of Earth-like rocky material. They calculated that each of the twins would have had to consume an additional 10 to 20 Earth-masses of rocky material to produce these chemical signatures. Specifically, the star with the Jupiter-sized planet appears to have swallowed an extra 10 Earth masses, while the star with the two Neptune-sized planets scarfed down an additional 20.
This new model suggests that a G-class star with levels of 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. As Mack explains:
"Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter. The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets.
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."
You can read the study online at the Astrophysical Journal. Or, if you'd like to hear from the astronomers themselves and see some awesome space art, check out the video above, where Stassun observes somewhat ominously: "Our work suggests that the broader question we've been trying to answer for several decades now — 'How common are Earth-like worlds around sun-like stars?' — is sort of the wrong question. Rather the more relevant question may be, 'Of all the rocky planets that a sun-like star makes, how many of them survived as opposed to diving into the host star?"