An artist’s rendering of how the iron content of a star can impact its planets. A normal star (green label) is more likely to host a longer-period planet (green orbit), while an iron-rich star (yellow label) is more likely to host a shorter-period planet (yellow orbit). Image Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration. Image Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration. Click on the image for a larger version
Astronomers with the Sloan Digital Sky Survey (SDSS)
have learned that the chemical composition of a star can exert
unexpected influence on its planetary system — a discovery made possible
by an ongoing SDSS survey of stars seen by NASA’s Kepler spacecraft,
and one that promises to expand our understanding of how extrasolar
planets form and evolve.
“Without these detailed and accurate measurements of the iron content
of stars, we could have never made this measurement,” says Robert
Wilson, a graduate student in astronomy at the University of Virginia
and lead author of the paper announcing the results.
The team presented their results today at the American Astronomical
Society (AAS) meeting in National Harbor, Maryland. Using SDSS data,
they found that stars with higher concentrations of iron tend to host
planets that orbit quite close to their host star — often with orbital
periods of less than about eight days — while stars with less iron tend
to host planets with longer periods that are more distant from their
host star. Further investigation of this effect may help us understand
the full variety of extrasolar planetary systems in our Galaxy, and shed
light on why planets are found where they are.
The story of planets around Sun-like stars began in 1995, when a team
of astronomers discovered a single planet orbiting a Sun-like star 50
light years from Earth. The pace of discovery accelerated in 2009, when
NASA launched the Kepler spacecraft, a space telescope designed to look
for extrasolar planets. During its four-year primary mission, Kepler
monitored thousands of stars at a time, watching for the tiny dimming of
starlight that indicates a planet passing in front its host star. And
because Kepler looked at the same stars for years, it saw their planets
over and over again, and was thus able to measure the time the planet
takes to orbit its star. This information reveals the distance to from
star to planet, with closer planets orbiting faster than farther ones.
Thanks to Kepler’s tireless monitoring, the number of exoplanets with
known orbital periods increased dramatically, from about 400 in 2009 to
more than 3,000 today.
Although Kepler was perfectly designed to spot extrasolar planets, it
was not designed to learn about the chemical compositions of the stars
around which those planets orbit. That knowledge comes from the SDSS’s
Apache Point Observatory Galactic Evolution Experiment (APOGEE), which
has studied hundreds of thousands of stars all over the Milky Way
Galaxy. APOGEE works by collecting a spectrum for each star — a
measurement of how much light the star gives off at different
wavelengths (colors) of light. Because atoms of each chemical element
interact with light in their own characteristic way, a spectrum allows
astronomers to determine not only which elements a star contains, but
also how much — for all elements including the key element iron.
“All Sun-like stars are mostly hydrogen, but some contain more iron
than others,” says Johanna Teske of the Carnegie Institution for
Science, a member of the research team. “The amount of iron a star
contains is an important clue to how it formed and how it will evolve
over its lifetime.”
By combining data from these two sources — planetary orbits from
Kepler and stellar chemistry from APOGEE — astronomers have learned
about the relationships between these “iron-enriched” stars and the
planetary systems they hold.
“We knew that the element enrichment of a star would matter for its
own evolution,” says Teske, “But we were surprised to learn that it
matters for the evolution of its planetary system as well.”
The work presented today builds on previous work, led by Gijs Mulders
of the University of Arizona, using a larger but less precise sample of
spectra from the LAMOST-Kepler project. (LAMOST, the Large-Area
Multi-Object fiber Spectroscopic Telescope, is a Chinese sky survey.)
Mulders and collaborators found a similar trend — closer-in planets
orbiting more iron-rich stars — but did not pin down the critical period
of eight days.
“It is encouraging to see an independent confirmation of the trend we
found in 2016,” says Mulders. “The identification of the critical
period really shows that Kepler is the gift that keeps on giving.”
What is particularly surprising about the new result, Wilson
explained, is that the iron-enriched stars have only about 25 percent
more iron than the others in the sample. “That’s like adding
five-eighths of a teaspoon of salt into a cupcake recipe that calls for
half a teaspoon of salt, among all its other ingredients. I’d still eat
that cupcake,” he says. “That really shows us how even small differences
in stellar composition can have profound impacts on planetary systems.”
But even with this new discovery, astronomers are left with many
unanswered questions about how extrasolar planets form and evolve,
especially planets Earth-sized or slightly larger (“super-Earths”).
Do
iron-rich stars intrinsically form planets with shorter orbits? Or are
planets orbiting iron-rich stars more likely to form farther out and
then migrate to shorter period, closer-in orbits? Wilson and
collaborators hope to work with other astronomers to create new models
of protoplanetary disks to test both of these explanations.
“I’m excited that we still have much to learn about how the chemical
compositions of stars impact their planets, particularly about how small
planets form,” Teske says. “Plus, APOGEE provides many more stellar
chemical abundances besides iron, so there are likely other trends
buried within this rich dataset that we have yet to explore.”
Source: Sloan Digital Sky Survey (SDSS)
About Sloan Digital Sky Survey
Funding for the Sloan Digital Sky Survey IV has been provided by the
Alfred P. Sloan Foundation, the U.S. Department of Energy Office of
Science, and the Participating Institutions. SDSS acknowledges support
and resources from the Center for High-Performance Computing at the
University of Utah. The SDSS web site is www.sdss.org.
SDSS is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS Collaboration including the
Brazilian Participation Group, the Carnegie Institution for Science,
Carnegie Mellon University, the Chilean Participation Group, the French
Participation Group, Harvard-Smithsonian Center for Astrophysics,
Instituto de Astrofísica de Canarias, The Johns Hopkins University,
Kavli Institute for the Physics and Mathematics of the Universe (IPMU) /
University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz
Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für
Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA
Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE),
National Astronomical Observatories of China, New Mexico State
University, New York University, University of Notre Dame, Observatório
Nacional / MCTI, The Ohio State University, Pennsylvania State
University, Shanghai Astronomical Observatory, United Kingdom
Participation Group, Universidad Nacional Autónoma de México, University
of Arizona, University of Colorado Boulder, University of Oxford,
University of Portsmouth, University of Utah, University of Virginia,
University of Washington, University of Wisconsin, Vanderbilt
University, and Yale University.
Contacts
Robert Wilson,
The University
rfw3ev@virginia.edu,
1-434-924-0686
Johanna Teske,
Carnegie Institution for Science, jteske@carnegiescience.edu,
1-202-478-4885,
@johannateske
Karen Masters,
klmasters@haverford.edu,
+44 (0)7590 5266005,
@KarenLMasters
Jordan Raddick,
SDSS Public Information Officer, Johns Hopkins University,
raddick@jhu.edu,
1-410-516-8889,
@raddick
