Astronomers and geoscientists from the United States and Brazil have joined forces to study the mix of major rock-building elements in exoplanet-hosting stars, and to consider what this reveals about the exoplanets.
An artist’s rendition of interior compositions of planets around Kepler-102 and Kepler-407. The picture shows what minerals are likely to occur several different depths. An exoplanet in orbit around Kepler-102 is Earth-like, dominated by olivine minerals, whereas a planet around Kepler-407 is dominated by garnet, so less likely to have plate tectonics. Image credit: Robin Dienel / Carnegie Institution.
“Our study combines new observations of stars with new models of planetary interiors,” said Dr. Johanna Teske, a researcher in the Department of Terrestrial Magnetism at the Carnegie Institution in Washington, DC, and the Carnegie Observatories in Pasadena, CA.
“We want to better understand the diversity of small, rocky exoplanet composition and structure — how likely are they to have plate tectonics or magnetic fields?”
Earth-sized planets have been found around many stars — but ‘Earth-sized’ does not necessarily mean Earth-like.
Some of these Earth-sized planets have been found orbiting stars with chemical compositions quite different from our Sun, and those differences in chemistry could have important consequences.
The study focuses on 90 Kepler stars that show evidence of hosting rocky planets, and which have also been surveyed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE).
“We report on our measurements of the major rock-building elements O, Mg, Si and Fe for the host stars of known small planets using high-resolution spectra from the APOGEE survey,” the authors said.
“We discuss the role that these abundance ratios play in the mantle mineralogy and core formation and speculate on the consequences for habitability of rocky exoplanets.”
In particular, Dr. Teske and co-authors presented planetary systems around Kepler-102 and Kepler-407.
Kepler-102 is about 389 light-years away. It is slightly less luminous than the Sun and hosts five known planets.
Kepler-407 is a star almost identical in mass to the Sun. It is approximately 1,065 light-years away and hosts at least two planets, one with a mass less than three Earth masses.
“Looking at these two exoplanet systems in particular, we determined that Kepler-102 is like the Sun, but Kepler-407 has a lot more silicon,” Dr. Teske said.
To understand what a lot more silicon might mean for the planets around Kepler-407, astronomers turned to geophysicists for help.
Team member Dr. Cayman Unterborn, a researcher at Arizona State University, ran computer models of planet formation.
“We took the star compositions found by APOGEE and modeled how the elements condensed into planets in our models,” Dr. Unterborn said.
“We found that the planet around Kepler-407 would likely be rich in the mineral garnet.”
“The planet around Kepler-102 is probably rich in olivine, like Earth.”
“That seemingly-small difference in minerals might have major consequences for these planets.”
Garnet is a stiffer mineral than olivine, so it flows more slowly.
“This means that a garnet planet like one around Kepler-407 would be much less likely to have long-term plate tectonics,” Dr. Unterborn said.
“To sustain plate tectonics over geologic timescales, a planet must have the right mineral composition.”
Plate tectonics is believed to be essential for life on Earth, because of how volcanoes and ocean ridges recycle elements between Earth’s crust and mantle. This recycling regulates the composition of our atmosphere.
“Without these geological processes, life may not have had the chance to evolve on Earth,” said team member Dr. Wendy Panero, from the Ohio State University.
“Determining the likelihood of such geological processes on other planets will help distinguish which ones are the best targets for future missions searching for signs of life.”
“If we’re looking for a needle, why not start in the sewing box?”
The next step in the team’s research is to extend this study to all of the stars observed by APOGEE that host small planets. That extension would allow astronomers to map out a wider range of planet compositions and structures to find those most likely to be Earth-like in their mineral content.
The researchers reported their findings Jan. 7, 2017 at the 229th Meeting of the American Astronomical Society in Grapevine, Texas.
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Jennifer Johnson et al. 2017. Compositions of Small Planets & Implications for Planetary Dynamics. 229th AAS Meeting, abstract # 413.06
This article is based on a press-release from the Sloan Digital Sky Survey.
