Image of the planetary-mass companion VHS 1256-1257 b (bottom right) and its host star (center).
Credit: Gauza, B. et al 2015, MNRAS, 452, 1677-1683
Image of the planetary-mass companion GSC 6214-210 b (bottom) and its host star (top).
Credit: Ireland, M. J. et al 2011, ApJ, 726, 113
Credit: Kraus, A. L. et al. 2014, ApJ, 781, 20
Researchers Measure the Spin Rates of Bodies Thought to be Either Planets or Tiny "Failed" Stars
Maunakea, Hawaii - Taking a picture of an
exoplanet—a planet in a solar system beyond our sun—is no easy task. The
light of a planet's parent star far outshines the light from the planet
itself, making the planet difficult to see. While taking a picture of a
small rocky planet like Earth is still not feasible, researchers have
made strides by snapping images of about 20 giant planet-like bodies.
These objects, known as planetary-mass companions, are more massive than
Jupiter, orbit far from the glare of their stars, and are young enough
to still glow with heat from their formation—all traits that make them
easier to photograph.
But one big question remains: Are these
planetary-mass companions actually planets, or are they instead small
"failed" stars called brown dwarfs? Brown dwarfs form like stars do—out
of collapsing clouds of gas—but they lack the mass to ignite and shine
with starlight. They can be found floating on the their own in space, or
they can be found orbiting with other brown dwarfs or stars. The
smallest brown dwarfs are similar in size to Jupiter and would look just
like a planet when orbiting a star.
Using the W. M. Keck
Observatory on Maunakea, Hawaii, researchers at Caltech have taken a new
approach to the mystery: they have measured the spin rates of three of
the photographed planetary-mass companions and compared them to spin
rates for small brown dwarfs. The results offer a new set of clues that
hint at how the companions may have formed.
"These companions with
their high masses and wide separations could have formed either as
planets or brown dwarfs," says graduate student Marta Bryan (MS '14),
lead author of a new study describing the findings in the journal Nature Astronomy. "In this study, we wanted to shed light on their origins."
"These
new spin measurements suggest that if these bodies are massive planets
located far away from their stars, they have properties that are very
similar to those of the smallest brown dwarfs,"
says Heather Knutson, professor of planetary science at Caltech and a co-author of the paper.
The
astronomers measured the spin rate, or the length of a day, of three
planetary-mass companions known as ROXs 42B b, GSC 6214-210 b, and VHS
1256-1257 b. They used an instrument at Keck Observatory called the Near
Infrared Spectrograph (NIRSpec) to dissect the light coming from the
companions. As the objects spin on their axes, light from the side that
is turning toward us shifts to shorter, bluer wavelengths, while light
from the receding side shifts to longer, redder wavelengths. The degree
of this shifting indicates the speed of a rotating body. The results
showed that the three companions' spin rates ranged between six to 14
kilometers per second, similar to rotation rates of our solar system's
gas giant planets Saturn and Jupiter.
Animation Credit: NASA/JPL-Caltech
For the study, the researchers also included the two
planetary-mass companions for which spin rates had already been measured. One,
β Pictoris b, has a rotation rate of 25 kilometers per
second—the fastest rotation rate of any planetary-mass body measured so far.
The researchers compared the spin rates for the five
companions to those measured previously for small free-floating brown dwarfs.
The ranges of rotation rates for the two populations were indistinguishable. In
other words, the companions are whirling about their own axes at about the same
speeds as their free-floating brown-dwarf counterparts.
The results suggest two possibilities. One is that the planetary-mass companions are actually
brown dwarfs. The second possibility is that the companions looked at in this
study are planets that formed, just as planets do, out of disks of material
swirling around their stars, but for reasons not yet understood, the objects ended
up with spin rates similar to those of brown dwarfs. Some researchers
think that both newly forming planets and brown dwarfs are encircled by
miniature gas disks that might be helping to slow their spin rates. In other
words, similar physical processes may leave planets and brown dwarfs with
similar spin rates.
"It's a question of nature versus nurture," says
Knutson. "Were the planetary companions born like brown dwarfs, or did
they just end up behaving like them with similar spins?"
The team also says that the companions are spinning more slowly
than expected. Growing planets tend to be spun up by the material they pull in
from a surrounding gas disk, in the same way that spinning ice skaters increase
their speed, or angular momentum, when they pull their arms in. The relatively
slow rotation rates observed for these objects indicate that they were able to
effectively put the brakes on this spin-up process, perhaps by transferring some
of this angular momentum back to encircling gas disks. The researchers are
planning future studies of spin rates to further investigate the matter.
"Spin rates of planetary-mass bodies outside our solar
system have not been fully explored," says Bryan. "We are just now
beginning to use this as a tool for understanding formation histories of
planetary-mass objects."
The study, titled, "Constraints on the Spin Evolution of Young Planetary-MassCompanions," was funded by NASA and the Sloan Research Fellowship
Program. Other authors include Caltech's Konstantin Batygin (MS '10, PhD '12),
assistant professor of planetary science and Van Nuys Page Scholar;
Björn Benneke, formerly of Caltech and now of the
University of Montreal; and
Brendan Bowler of the
University of Texas at Austin.
Media Contacts:
Mari-Ela Chock, Keck Observatory
(808) 554-0567
mchock@keck.hawaii.edu
Whitney Clavin, Caltech
(626) 395-1856
wclavin@caltech.edu
Source: W.M. Keck Observatory
Media Contacts:
Mari-Ela Chock, Keck Observatory
(808) 554-0567
mchock@keck.hawaii.edu
Whitney Clavin, Caltech
(626) 395-1856
wclavin@caltech.edu
About W. M. Keck
Observatory
The W. M. Keck
Observatory telescopes are among the most scientifically productive on
Earth. The two, 10-meter optical/infrared telescopes on the summit of
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 Keck Observatory, which is a private 501(c) 3 non-profit
organization 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 wish to recognize and acknowledge the very significant cultural
role and reverence that the summit of Maunakea has always had within
the indigenous Hawaiian community. We are most fortunate to have the
opportunity to conduct observations from this mountain.
Article Summary
Scientists are investigating the nature of planetary-mass
bodies that orbit stars, finding new clues to their origins.
