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Study Finds No Clear Correlation between Compositions of Giant Exoplanets and Their Host Stars

There isn’t a strong correlation between gas giant and host star compositions when it comes to elements heavier than hydrogen and helium, according to new research.

Study Finds No Clear Correlation between Compositions of Giant Exoplanets and Their Host Stars
This illustration shows a gas giant exoplanet orbiting its parent star. Image credit: NASA / ESA / STScI / L. Calcada.

In their youths, stars are surrounded by a protoplanetary disk — a rotating disk of gas and dust from which planets are born.

Astronomers have long wondered how much a star’s makeup determines the raw material from which planets are constructed — a question that is easier to probe now that we know the galaxy is teeming with exoplanets.

“Understanding the relationship between the chemical composition of a star and its planets could help shed light on the planetary formation process,” said Dr. Johanna Teske, an astronomer at the Observatories of the Carnegie Institution for Science.

“Previous work looked at the relationship between the presence of planets and how much iron exists in the host star, but we wanted to expand that to include the heavy element content of the planets themselves, and to look at more than just iron,” added Daniel Thorngren, an astronomer at the Université de Montréal.

In the study, the researchers compared the bulk heavy element content of 24 cool giant planets to the abundances of carbon, oxygen, magnesium, silicon, iron, and nickel in their 20 host stars.

They were surprised to find that there is an absence of a clear correlation between the amount of heavy elements in these giant planets and the amount of these planet-forming elements in their host stars.

So how can astronomers explain the established trend that stars rich in heavy elements are more likely to host gas giant planets?

“Unraveling this discrepancy could reveal new details about the planet formation process,” said Dr. Jonathan Fortney, a scientist in the Department of Astronomy and Astrophysics at the University of California, Santa Cruz.

“For example, what other factors are contributing to a baby planet’s composition as it forms? Perhaps its location in the disk and how far it is from any neighbors. More work is necessary to answer these crucial questions.”

One clue may come from the astronomers’ combined results bundling the heavy elements into groupings that reflect their characteristics.

They saw a tentative correlation between a planet’s heavy elements and its host star’s relative abundance of carbon and oxygen, which are called volatile elements, versus the rest of the elements included in this study, which fall into the group called refractory elements.

These terms refer to the elements’ low boiling points in the case of the refractory elements. Volatile elements may represent an ice-rich planetary composition, whereas refractory elements may indicate a rocky composition.

“I’m excited to explore this tentative result further, and hopefully add more information to our understanding of the relationships between star and planetary compositions from upcoming missions like NASA’s James Webb Space Telescope, which will be able to measure elements in exoplanet atmospheres,” Dr. Teske said.

The study was published in the Astronomical Journal.

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Johanna K. Teske et al. 2019. Do Metal-rich Stars Make Metal-rich Planets? New Insights on Giant Planet Formation from Host Star Abundances. AJ 158, 239; doi: 10.3847/1538-3881/ab4f79

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