Compact Multi-Planet Systems are More Common around Metal-Poor Stars, Astronomers Says

A team of U.S. astronomers has discovered that small planetary systems with multiple planets are more likely to form around stars that have lower amounts of ‘metals’ (elements heavier than helium, including iron, silicon, magnesium, and carbon) than our own Sun.

An illustration of a compact, multi-planet system. Image credit: Michael S. Helfenbein / Yale University.

For the study, Yale University researcher John Michael Brewer and colleagues looked at 716 stars and 1,148 surrounding planets.

“‘Metals’ are all elements that make up the rocks in small, rocky planets. We used iron as a proxy,” Dr. Brewer said.

An abundance of compact, multi-planet systems around metal-poor stars suggests several things

“First, it may indicate that there are many more of these systems than previously assumed,” he said.

“Until recently, research instruments have not had the necessary precision to detect smaller planets and instead focused on detecting larger planets. Now, with the advent of technology such as the Extreme Precision Spectrometer (EXPRES), we will be able to find smaller planets.”

In addition, our study suggests that small planetary systems may be the earliest type of planetary system, making them an ideal place to search for life on other planets.”

“Low-metallicity stars have been around a lot longer. That’s where we’ll find the first planets that formed.”

The same team demonstrated in 2005 that higher metallicity in stars increased the probability of forming large, Jupiter-like planets.

This provided strong support to the core-accretion model for gas giant planet formation and established this as the leading mechanism for planet formation.

Understanding the formation of smaller planets has been more elusive.

“Our surprising result, that compact systems of multiple, small planets are more likely around lower metallicity stars suggests a new, important clue in understanding the most common type of planetary system in our Milky Way Galaxy,” said Dr. Songhu Wang, also of Yale University.

According to the team, another tantalizing possibility to explore is the connection between iron and silicon in the birth of planets.

The new study shows a high silicon-to-iron ratio in stars with lower metallicity.

“Silicon could be the secret ingredient,” said Yale Professor Debra Fischer.

“The ratio of silicon to iron is acting as a thermostat for planet formation. As the ratio increases, nature is dialing up the formation of small, rocky planets.”

The results were published in the Astrophysical Journal Letters (arXiv.org preprint).

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John M. Brewer et al. 2018. Compact Multi-planet Systems are More Common around Metal-Poor Hosts. ApJL 867, L3; doi: 10.3847/2041-8213/aae710

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