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.Click here for a larger version. Image 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 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.
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 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. 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?”
Source: Sloan Digital Sky Survey (SDSS)
