These animations show approximately 200,000 years
of orbital evolution in the Kepler-223 planetary system. The planets’
interactions with the disk of gas and dust in which they formed caused
their orbits to shrink toward their star over time at differing rates.YouTube version
Sean Mills (left) and Daniel Fabrycky (right),
researchers at the University of Chicago, describe the complex orbital
structure of the Kepler-223 system in a new study. Credits: Nancy Wong/University of Chicago
The four planets of the Kepler-223 star system appeared to have little in common with the planets of our own solar system today. But a new study using data from NASA's Kepler space telescope suggests a possible commonality in the distant past. The Kepler-223 planets orbit their star in the same configuration that Jupiter, Saturn, Uranus and Neptune may have had in the early history of our solar system, before migrating to their current locations.
"Exactly how and where planets form is an outstanding question in
planetary science," said the study's lead author, Sean Mills, a graduate
student in astronomy and astrophysics at the University of Chicago in
Illinois. "Our work essentially tests a model for planet formation for a
type of planet we don't have in our solar system."
The puffy, gaseous planets orbiting Kepler-223, all of which are far
more massive than Earth, orbit close to their star. "That's why there's a
big debate about how they formed, how they got there and why don't we
have an analogous planet in our solar system," Mills said.
Mills and his collaborators used data from Kepler -- its mission is
now known as K2 -- to analyze how the four planets block their stars'
light and change each other's orbits. This information also gave
researchers the planets' sizes and masses. The team performed numerical
simulations of planetary migration that generate this system's current
architecture, similar to the migration suspected for the solar system's
gas giants. These calculations are described in the May 11 Advance
Online edition of Nature.
The orbital configuration of our own solar system seems to have evolved
since its birth 4.6 billion years ago. The four known planets of the
much older Kepler-223 system, however, have maintained a single orbital
configuration for far longer.
Astronomers call the planets of Kepler-223 "sub-Neptunes." They
likely consist of a solid core and an envelope of gas, and they orbit
their star in periods ranging from only seven to 19 days. They are the
most common type of planets known in the galaxy, even though there is
nothing quite like them around our sun.
Kepler-223's planets also are in resonance, meaning
their gravitational influence on each other creates a periodic
relationship between their orbits. Planets are in resonance when, for
example, every time one of them orbits its sun once, the next one goes
around twice. Three of Jupiter's largest moons, where the phenomenon was
discovered, display resonances. Kepler-223 is the first time that four
planets in an extrasolar system have been confirmed to be in resonance.
"This is the most extreme example of this phenomenon," said study
co-author Daniel Fabrycky, an assistant professor of astronomy and
astrophysics at the University of Chicago.
Formation scenarios
The Kepler-223 system provides alternative scenarios for how planets
form and migrate in a planetary system that is different from our own,
said study co-author Howard Isaacson, a research astronomer at the
University of California, Berkeley, and member of the California Planet
Search Team.
"Data from Kepler and the Keck Telescope were absolutely critical in
this regard," Isaacson said. Thanks to observations of Kepler-223 and
other exoplanetary systems, "We now know of systems that are unlike our
sun's solar system, with hot Jupiters, planets closer than Mercury or in
between the size of Earth and Neptune, none of which we see in our
solar system. Other types of planets are very common."
Some stages of planet formation can involve violent processes. But
during other stages, planets can evolve from gaseous disks in a smooth,
gentle way, which is probably what the sub-Neptune planets of Kepler-223
did, Mills said.
"We think that two planets migrate through this disk, get stuck and
then keep migrating together; find a third planet, get stuck, migrate
together; find a fourth planet and get stuck," Mills explained.
That process differs completely from the one that scientists believe
led to the formation of Mercury, Venus, Earth and Mars, which likely
formed in their current orbital locations.
Earth formed from Mars-sized or moon-sized bodies smacking together,
Mills said, in a violent and chaotic process. When planets form this
way, their final orbital periods are not near a resonance.
Substantial movement
But scientists suspect that the solar system's larger, more distant
planets of today -- Jupiter, Saturn, Uranus and Neptune -- moved around
substantially during their formation. They may have been knocked out of
resonances that once resembled those of Kepler-223, possibly after
interacting with numerous asteroids and small planets (planetesimals).
"These resonances are extremely fragile," Fabrycky said. "If bodies
were flying around and hitting each other, then they would have
dislodged the planets from the resonance." But Kepler-223's planets
somehow managed to dodge this scattering of cosmic bodies.
NASA's Ames Research Center in Moffett Field, California, manages the
Kepler and K2 missions for NASA's Science Mission Directorate. NASA's
Jet Propulsion Laboratory in Pasadena, California, managed Kepler
mission development. Ball Aerospace & Technologies Corporation
operates the flight system with support from the Laboratory for
Atmospheric and Space Physics at the University of Colorado at Boulder.
For more information about the Kepler and K2 missions, visit: http://www.nasa.gov/kepler
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Michele Johnson
NASA Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov
Written by Steve Koppes
University of Chicago
773-702-8366
skoppes@uchicago.edu
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Michele Johnson
NASA Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov
Written by Steve Koppes
University of Chicago
773-702-8366
skoppes@uchicago.edu
Editor: Tony Greicius
Source: NASA/Kepler and K2