This illustration compares the eccentric orbit of HR 5183 b to the more circular orbits of the planets in our own solar system. Animation credit: W. M. Keck Observatory/Adam Makarenko. Vimeo
Three times the mass of Jupiter, a first-of-its-kind planet swings around its star on bizarre path
Maunakea, Hawaii – Astronomers have discovered a
planet three times the mass of Jupiter that travels on a long,
egg-shaped path around its star. If this planet were somehow placed into
our own solar system, it would swing from within our asteroid belt to
out beyond Neptune.
Other giant planets with highly elliptical orbits have been found
around other stars, but none of those worlds were located at the very
outer reaches of their star systems like this one.
“This
planet is unlike the planets in our solar system, but more than that, it is
unlike any other exoplanets we have discovered so far,” says Sarah Blunt,
a Caltech graduate student and first author on the new study publishing in The
Astronomical Journal. “Other planets detected far away from their
stars tend to have very low eccentricities, meaning that their orbits are more
circular. The fact that this planet has such a high eccentricity speaks to some
difference in the way that it either formed or evolved relative to the other
planets.”
The planet was discovered using the radial velocity method, a
workhorse of exoplanet discovery that detects new worlds by tracking how
their parent stars “wobble” in response to gravitational tugs from
those planets.
However, analyses of these data usually require observations taken
over a planet’s entire orbital period. For planets orbiting far from
their stars, this can be difficult: a full orbit can take tens or even
hundreds of years.
The California Planet Search, led by Caltech Professor of Astronomy Andrew W. Howard,
is one of the few groups that watches stars over the decades-long
timescales necessary to detect long-period exoplanets using radial
velocity.
The data needed to make the discovery of the new planet were first
provided by W. M. Keck Observatory in Hawaii. In 1997, the team began
using Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES) to
take measurements of the planet’s star, called HR 5183.
“The key was persistence,” said Howard. “Our team followed
this star with Keck Observatory for more than two decades and only saw evidence
for the planet in the past couple years! Without that long-term effort, we
never would have found this planet.”
In
addition to Keck Observatory, the California Planet Search also used the Lick
Observatory in Northern California and the McDonald Observatory in Texas.
The
astronomers have been watching HR 5183 since the 1990s, but do not have data
corresponding to one full orbit of the planet, called HR 5183 b, because it
circles its star roughly every 45 to 100 years. The team instead found the
planet because of its strange orbit.
“This
planet spends most of its time loitering in the outer part of the solar system
in this highly eccentric orbit, then it starts to accelerate in and does a
slingshot around its star,” explains Howard. “We detected this
slingshot motion. We saw the planet come in and now it’s on its way out. That
creates such a distinctive signature that we can be sure that this is a real
planet, even though we haven’t seen a complete orbit.”
The
new findings show that it is possible to use the radial velocity method to make
detections of other far-flung planets without waiting decades. And, the
researchers suggest, looking for more planets like this one could illuminate
the role of giant planets in shaping their solar systems.
Planets take shape out of disks of material left over after stars
form. That means that planets should start off in flat, circular orbits.
For the newly detected planet to be on such an eccentric orbit, it must
have gotten a gravitational kick from some other object.
The most plausible scenario, the researchers propose, is that the
planet once had a neighbor of similar size. When the two planets got
close enough to each other, one pushed the other out of the solar
system, forcing HR 5183 b into a highly eccentric orbit.
“This
newfound planet basically would have come in like a wrecking ball,” says
Howard, “knocking anything in its way out of the system.”
This
discovery demonstrates that our understanding of planets beyond our solar
system is still evolving. Researchers continue to find worlds that are unlike
anything in our solar system or in solar systems we have already discovered.
“Copernicus taught us that Earth is not the center of the solar
system, and as we expanded into discovering other solar systems of
exoplanets, we expected them to be carbon copies of our own solar
system,” Howard explains, “But it’s just been one surprise after another
in this field. This newfound planet is another example of a system that
is not the image of our solar system but has remarkable features that
make our universe incredibly rich in its diversity.”
The study, titled, “Radial Velocity of an Eccentric Jovian World
Orbiting at 18AU,” was funded by the National Science Foundation, NASA,
Tennessee State University and the State of Tennessee, the Beatrice
Watson Parrent Fellowship, the Trottier Family Foundation, and Caltech.
Other Caltech authors include: BJ Fulton, a staff scientist at IPAC;
former postdoctoral scholar Sean Mills (BS ’12); Erik Petigura, a former
postdoctoral scholar now based at UCLA; and Arpita Roy, R.A. & G.B.
Millikan Postdoctoral Scholar in Astronomy.
Source: W.M. Keck Observatory
About HIRES
The High-Resolution Echelle Spectrometer
(HIRES) produces spectra of single objects at very high spectral resolution,
yet covering a wide wavelength range. It does this by separating the light into
many “stripes” of spectra stacked across a mosaic of three large CCD
detectors. HIRES is famous for finding exoplanets. Astronomers also
use HIRES to study important astrophysical phenomena like
distant galaxies and quasars, and find cosmological clues about the structure
of the early universe, just after the Big Bang.
About W.M. Keck Observatory
About W.M. Keck Observatory
The W. M. Keck Observatory telescopes are the most
scientifically productive on Earth. The two, 10-meter optical/infrared
telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced
instruments including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectrometers, and world-leading laser guide star
adaptive optics systems. The data presented herein were obtained at the W. M.
Keck Observatory, which is operated as a scientific partnership among the
California Institute of Technology, the University of California, and the National
Aeronautics and Space Administration. The Observatory was made possible by the
generous financial support of the W. M. Keck Foundation. The authors recognize
and acknowledge the very significant cultural role that the summit of Maunakea
has always had within the Native Hawaiian community. We are most fortunate
to have the opportunity to conduct observations from this mountain.
