Can garnet planets be habitable?

January 6, 2017
Artist rendition of interior compositions of planets around the stars Kepler 102 and Kepler 407. The picture shows what minerals are likely to occur several different depths. Kepler 102 is Earth-like, dominated by olivine minerals, whereas Kepler 407 is dominated by garnet, so less likely to have plate tectonics. Credit: Robin Dienel, Carnegie DTM

What makes a rocky planet Earth-like? Astronomers and geoscientists have joined forces using data from the Sloan Digital Sky Survey (SDSS) to study the mix of elements in exoplanet host stars, and to consider what this reveals about their planets.

In results presented today at the American Astronomical Society (AAS) meeting in Grapevine, Texas, astronomer Johanna Teske explained, "our study combines new observations of stars with new models of planetary interiors. We want to better understand the diversity of small, rocky exoplanet composition and structure—how likely are they to have or magnetic fields?"

Earth-sized 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.

Astronomers in the Sloan Digital Sky Survey have made these observations using the APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectrograph on the 2.5m Sloan Foundation Telescope at Apache Point Observatory in New Mexico. This instrument collects light in the near-infrared part of the electromagnetic spectrum and disperses it, like a prism, to reveal signatures of different elements in the atmospheres of stars. A fraction of the almost 200,000 stars surveyed by APOGEE overlap with the sample of stars targeted by the NASA Kepler mission, which was designed to find potentially Earth-like planets. The work presented today focuses on ninety Kepler stars that show evidence of hosting rocky planets, and which have also been surveyed by APOGEE.

In particular, Teske and colleagues presented solar systems around the stars Kepler 102 and Kepler 407. Kepler 102 is slightly less luminous than the Sun and has five known planets; Kepler 407 is a star almost identical in mass to the Sun and hosts at least two planets, one with a mass less than 3 Earth masses.

"Looking at these two exoplanet systems in particular," Teske explains, "we determined that Kepler 102 is like the Sun, but Kepler 407 has a lot more silicon."

To understand what a lot more silicon might mean for the planets around Kepler 407, astronomers turned to geophysicists for help. Cayman Unterborn of 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. We found that the planet around Kepler 407, which we called 'Janet," would likely be rich in the mineral garnet. The planet around Kepler 102, which we called 'Olive,' is probably rich in olivine, like Earth."

That seemingly-small difference in minerals might have major consequences for Janet and Olive. Garnet is a stiffer mineral than olivine, so it flows more slowly. Unterborn explains that this means that a garnet planet like Janet would be much less likely to have long-term plate tectonics. "To sustain plate tectonics over geologic timescales, a planet must have the right mineral composition," Unterborn says.

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. Wendy Panero of the School of Earth Sciences at The Ohio State University says that "without these geological processes, life may not have had the chance to evolve on Earth." 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," Panero says, "why not start in the sewing box?"

The next step in the team's research is to extend this study to all of the 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. Teske concludes, "As we've learned more about the Earth, we have learned about how many pieces come together to make it habitable. How often will exoplanets get that lucky?"

Explore further: What kinds of stars form rocky planets?

Related Stories

What kinds of stars form rocky planets?

December 3, 2015

As astronomers continue to find more and more planets around stars beyond our own Sun, they are trying to discover patterns and features that indicate what types of planets are likely to form around different kinds of stars. ...

Kepler marks five years in space

March 7, 2014

(Phys.org) —Five years ago today, on March 6, 2009, NASA's Kepler Space Telescope rocketed into the night skies above Cape Canaveral Air Force Station in Florida to find planets around other stars, called exoplanets, in ...

Kepler's six years in science (and counting)

May 13, 2015

NASA's Kepler spacecraft began hunting for planets outside our solar system on May 12, 2009. From the trove of data collected, we have learned that planets are common, that most sun-like stars have at least one planet and ...

Measuring the mass of a Mars-size exoplanet

June 18, 2015

Determining the size of an Earth-size exoplanet by the amount of starlight it blocks hundreds of light-years away once was the realm of science fiction. Measuring the mass of such a small planet based on its gravity was ...

Eight new planets found in 'Goldilocks' zone

January 6, 2015

Astronomers announced today that they have found eight new planets in the "Goldilocks" zone of their stars, orbiting at a distance where liquid water can exist on the planet's surface. This doubles the number of small planets ...

Recommended for you

Planetary waves, first found on Earth, are discovered on Sun

March 27, 2017

The same kind of large-scale planetary waves that meander through the atmosphere high above Earth's surface may also exist on the Sun, according to a new study led by a scientist at the National Center for Atmospheric Research ...

NASA spacecraft investigate clues in radiation belts

March 27, 2017

High above Earth, two giant rings of energetic particles trapped by the planet's magnetic field create a dynamic and harsh environment that holds many mysteries—and can affect spacecraft traveling around Earth. NASA's Van ...

Stars born in winds from supermassive black holes

March 27, 2017

Observations using ESO's Very Large Telescope have revealed stars forming within powerful outflows of material blasted out from supermassive black holes at the cores of galaxies. These are the first confirmed observations ...

Evidence of giant tsunami on Mars suggests an early ocean

March 27, 2017

(Phys.org)—A team of researchers with members from France, Italy and the U.S. has found what they believe is evidence of a giant tsunami occurring on Mars approximately 3 billion years ago due to an asteroid plunging into ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

thingumbobesquire
not rated yet Jan 07, 2017
The enigma of astrobiology: what are the necessary and sufficient conditions for life? http://thingumbob...obiology

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.