Some stars ingest material from rocky planets like Earth, and now astronomers have a way to study the effect of such a diet on a star’s chemical composition.
The result is a new modeling technique that may help scientists identify Earth-like exoplanets.
To test the model, they focused on a pair of twin stars that both have their own planets. Both stars—designated HD 20781 and HD 20782—are G-class dwarf stars similar to the Sun and both should have condensed out of the same cloud of dust and gas—so they 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 make the two stars ideal for studying the connection between exoplanets and the chemical composition of their stellar hosts.
How the new method works
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.
Some scientists have argued that stars with high metallicity are more likely to develop planetary systems than those with low metallicity. One study suggested 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.
In the current study, Trey Mack, a graduate student in astronomy at Vanderbilt University, 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.
What the twin stars show
When Mack and colleagues analyzed the spectrum of the two twin stars, they found that the relative abundance of refractory elements was significantly higher than that of the 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 the 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.
A ‘stamp’ that we can detect
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 is unlikely that either of the binary twins possesses terrestrial planets.
At one twin, the two Neptune-sized planets are orbiting the star quite closely, at one-third the distance between the Earth and the Sun. At the other twin, the Jupiter-sized planet spends a lot of time in the outer reaches of the planetary system but it’s eccentric orbit also brings it in extremely close to the star.
The astronomers speculate that the reason the star with the two Neptune-size planets ingested more terrestrial material than its twin was because the two planets were more efficient at pushing material into their star than the single Jupiter-sized planet was at pushing material into its star.
If the chemical signature of G-class stars that swallow rocky planets proves to be universal, “when we find stars with similar chemical signatures, we will be 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.”
“This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask,” says Keivan Stassun, an astronomy professor who supervised the study. “The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?”
The National Science Foundation supported the study, which was published in the Astrophysical Journal.