Two
direct images of the cometary dust and exoplanet surrounding the young
star HD 106906. The wider field in blue shows Hubble Space Telescope
data where the star’s blinding light is artificially eclipsed (gray
circular mask). The point to the upper right is an 11-Jupiter-mass
planet located more than 650 times the distance from the Earth to the
sun. A new discovery in these Hubble observations is an extremely
asymmetric nebulosity indicating a dynamically disturbed system of
comets. Surprisingly, the planet is located 21 degrees above the plane
of the nebulosity. The circular orange inset shows a region much closer
to the star that can only be detected using advanced adaptive optics
from the ground-based Gemini Observatory. Using the Gemini Planet Imager
(GPI), researchers found a narrow loop of nebulosity suggesting that a
planetary system formed close to the star, but somehow the architecture
of the outer regions is severely disrupted. The investigators also find
that the planet HD 106906 b may have a dusty ring system of its own,
motivating future work with Hubble and ground-based astronomical
observatories. Image by Paul Kalas/UC Berkeley. Download Full Image
ASU scientists zoom in to study the dynamics that drive planet formation.
A planet discovered last year sitting at an unusually large distance
from its star – 16 times farther than Pluto is from the sun – may have
been kicked out of its birthplace close to the star in a process similar
to what may have happened early in our own solar system’s history.
Images from the Gemini Planet Imager (GPI) in the Chilean Andes and
the Hubble Space Telescope show that the star has a lopsided comet belt
indicative of a very disturbed solar system, and hinting that the planet
interactions that roiled the comets closer to the star might have sent
the exoplanet into exile as well.
The planet may even have a ring of debris it dragged along with it when it was expelled.
“The measurements we made on the planet suggest it may be dustier
than comparison objects, and we are making follow-up observations to
check if the planet is really encircled by a disk – an exciting
possibility,” said Abhijith Rajan, a graduate student at Arizona State
University’s School of Earth and Space Exploration (SESE), who analyzed
the planet’s images.
“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud,” said Paul Kalas, an adjunct professor of astronomy at UC Berkeley and lead author of a paper about the results, published by the The Astrophysical Journal on November 20, 2015.
“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud,” said Paul Kalas, an adjunct professor of astronomy at UC Berkeley and lead author of a paper about the results, published by the The Astrophysical Journal on November 20, 2015.
Such planets are of interest because in its youth, our own solar
system may have had planets that were kicked out of the local
neighborhood and are no longer among of the eight planets we see today.
“Is this a picture of our solar system when it was 13 million years
old?” asks Kalas. “We know that our own belt of comets, the Kuiper belt,
lost a large fraction of its mass as it evolved, but we don’t have a
time machine to go back and see how it was decimated. One of the ways,
though, is to study these violent episodes of gravitational disturbance
around other young stars that kick out many objects, including planets.”
The new observations may have recorded this type of disk disruption
around a younger star. “Studying the planets and disks around young
stars is a way to investigate the processes that impacted the formation
and evolution of our Solar System” says Jennifer Patience, associate
professor at Arizona State University’s School of Earth and Space
Exploration, who co-lead the development of the target sample for the
survey.
The disturbance could have been caused by a passing star that
perturbed the inner planets, or a second massive planet in the system.
The GPI team also looked for another large planet closer to the star
that may have interacted with the exoplanet, but found nothing outside a
Uranus-sized orbit.
Kalas and Rajan will discuss the observations during a Google+
Hangout On Air at 7 a.m. Hawaii time (10AM Arizona time) on Dec. 1
during Extreme Solar Systems III, the third in a series of international
meetings on exoplanets that this year takes place on the 20 anniversary
of the discovery of the first exoplanet in 1995. Viewers without
Google+ accounts may participate via YouTube.
Young, 13-million-year-old star
The star, HD 106906, is located 300 light years away in the direction
of the constellation Crux and is similar to the sun, but much younger:
about 13 million years old, compared to our sun’s 4.5 billion years.
Planets are thought to form early in a stars history, however, and in
2014 a team led by Vanessa Bailey at the University of Arizona
discovered a planet HD 106906 b around the star weighing a hefty 11
times Jupiter’s mass and located in the star’s distant suburbs, an
astounding 650 AU from the star (one AU is the average distance between
Earth and the sun, or 93 million miles).
Planets were not thought to form so far from their star and its
surrounding protoplanetary disk, so some suggested that the planet
formed much like a star, by condensing from its own swirling cloud of
gas and dust. The GPI and Hubble discovery of a lopsided comet belt and
possible ring around the planet points instead to a normal formation
within the debris disk around the star, but a violent episode that
forced it into a more distant orbit.
Kalas and a multi-institutional team using GPI first targeted the
star in search of other planets in May 2015 and discovered that it was
surrounded by a ring of dusty material very close to the size of our own
solar system’s Kuiper Belt. The emptiness of the central region – an
area about 50 AU in radius, slightly larger than the region occupied by
planets in our solar system – indicates that a planetary system has
formed there, Kalas said.
He immediately reanalyzed existing images of the star taken earlier
by the Hubble Space Telescope and discovered that the ring of dusty
material extended much farther away and was extremely lopsided. On the
side facing the planet, the dusty material was vertically thin and
spanned nearly the huge distance to the known planet, but on the
opposite side the dusty material was vertically thick and truncated.
“These discoveries suggest that the entire planetary system has been
recently jostled by an unknown perturbation to its current asymmetric
state,” he said. The planet is also unusual in that its orbit is
possibly tilted 21 degrees away from the plane of the inner planetary
system, whereas most planets typically lie close to a common plane.
Kalas and collaborators hypothesized that the planet may have
originated from a position closer to the comet belt, and may have
captured dusty material that still orbits the planet. To test the
hypothesis, they carefully analyzed the GPI and Hubble observations,
revealing three properties about the planet consistent with a large
dusty ring or shroud surrounding it. However, for each of the three
properties, alternate explanations are possible.
The investigators will be pursuing more sensitive observations with
the Hubble Space Telescope to determine if HD 106906b is in fact one of
the first exoplanets that resembles Saturn and its ring system.
The inner belt of dust around the star has been confirmed by an
independent team using the planet-finding instrument SPHERE on the ESO’s
Very Large Telescope in Chile. The lopsided nature of the debris disk
was not evident, however, until Kalas called up archival images from
Hubble’s Advanced Camera for Surveys.
The GPI Exoplanet Survey, operated by a team of astronomers at UC
Berkeley and 23 other institutions, is targeting 600 young stars, all
less than approximately 100 million years old, to understand how
planetary systems evolve over time and what planetary dynamics could
shape the final arrangement of planets like we see in our solar system
today.
Among Kalas’s coauthors are associate professor Jennifer Patience and
graduate student Abhijith Rajan of Arizona State University’s School of
Earth and Space Exploration, a unit of ASU’s College of Liberal Arts
and Sciences. The research was supported by the NSF and NASA’s Nexus for
Exoplanet System Science (NExSS) research coordination network
sponsored by NASA’s Science Mission Directorate.
