The Search for Earth-Like Planets

Earth-eating star illustration


Some sun-like stars are “earth-eaters.” During their development they ingest large amounts of rocky material from which “terrestrial” planets like Earth, Mars and Venus are made.

Trey Mack, a Vanderbilt graduate student in astronomy, has developed a model that estimates the effect 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,” says Professor of Astronomy Keivan Stassun, who supervised the study. This ability adds to our understanding of planet formation and will assist in the search for Earth-like exoplanets.

Stars consist of more than 98 percent hydrogen and helium. 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 iron to hydrogen in a star’s makeup.

Since the mid-1990s, when astronomers developed the capability to detect extrasolar planets in large numbers, studies have attempted to link star metallicity with planet formation. In one study, researchers at 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 while smaller planets are found circling stars with a wide range of metal content.

Building on the work of co-author Simon Schuler of the University of Tampa, Mack looked at the abundance of 15 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 these refractory materials serve as building blocks for Earth-like planets.

Mack, Schuler and Stassun applied this technique to the binary pair designated HD 20781 and HD 20782. Both stars should have condensed out of the same cloud of dust and gas; both, therefore, should have started with the same chemical compositions.

One star is orbited closely by two Neptune-size planets. The other possesses a single Jupiter-size planet that follows a highly eccentric orbit. The difference in their planetary systems makes the two stars ideal for studying the connection between exoplanets and chemical composition of their stellar hosts.

The astronomers found the abundance of refractory elements to be significantly higher than that of the sun. The higher the melting temperature of a particular element, the higher its abundance—a trend that serves as a compelling signature of the ingestion of Earth-like rocky material. They calculated each twin would have had to consume an additional 10 to 20 Earth-masses of rocky material to produce its chemical signatures.

The results support the proposition that a star’s chemical composition and the nature of its planetary system are linked.

“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter,” says Mack. “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.”

Following this logic it’s unlikely that either binary twin possesses terrestrial planets. At one twin, the two Neptune-size planets orbit it at one-third the distance between Earth and the sun. At the other twin, the Jupiter-size planet spends a lot of time in the outer reaches of the planetary system, but its orbit also brings it close to the star. Astronomers speculate that the star with the Neptune-size planets ingested more terrestrial material than its twin because the two planets were more efficient at pushing material into their star than the single, Jupiter-size planet was at pushing material into its star.

“When we find stars with similar chemical signatures, we are able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets,” says Mack. “And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own.”

The research was supported by National Science Foundation grants.

Watch a video with Mack and Stassun: