Scientists seem to have come up with a new parlor game – how many ways can we potentially discover exoplanets? The two most common methods, the transit method and the Doppler method, each have their own problems. Alternative methods are beginning to sprout, and a new one was recently suggested by Jacob Nibauer, a student in the Department of Physics and Astronomy at the University of Pennsylvania. His suggestion: look at the chemical composition of a star. And his results after analyzing data from around 1,500 stars hold some surprises.
Spectroscopy enables scientists to collect data directly on the chemical composition of stars. Mr. Nibauer’s method took into account that stars and planets arise from the same nebula material. Given that the chemical makeup of this material can be estimated before a star is formed, if the star itself lacks some of the material used to make rocky planets, it’s a pretty strong indicator that rocky planets actually exist would orbit this star.
UT video discussing some possibilities for types of rocky exoplanets.
To prove this theory, Mr. Nibauer used data from APOGEE-2, part of the Sloan Digital Sky Survey, and focused on 5 different elements that predominate in rocky planets whose chemical composition was included in the APOGEE-2 data. He then used a statistical tool called Bayesian analysis to categorize the star types in the dataset into a regular category in which the star still has the expected amount of “refractory” (ie, rock-forming) elements that would be expected from the nebula cloud , or an “exhausted” category where concentrations are lower than expected.
Interestingly, the data showed that most of the stars in the study were actually sun-like in chemical composition and fell into the “discharged” category due to their lack of refractory material. Previous studies of the chemical makeup of stars showed the Sun as an outlier, but they may have been skewed as they used some of the Sun’s own properties as a sorting mechanism. But the method of categorizing the two groups before analyzing the sun and then placing our closest star in the appropriately categorized group is a much more unbiased approach.
Data from the study shows stars from the study (orange) and the ratios of iron to hydrogen and for each of the five elements in the study.
Photo credit: Jacob Nibauer
Despite removing this bias, there are still many unanswered questions in this research. So far there has been no clear evidence that “exhausted” stars are associated with rocky planets more than unexhausted ones. In addition, even 1,500 stars are a relatively small sample size given the total number of stars in the galaxy. As more data is collected on both exoplanets themselves and the chemical signature of stars, a clearer picture will emerge of the relationship, if any, between the presence of these rock-forming minerals and that of rocky planets in these extrasolar systems.
Learn more:
UPenn – connection between the chemical composition of a star and the formation of planets
The Astrophysical Journal – Statistics of the chemical composition of solar analog stars and connections to planet formation
UT – What are stars made of?
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