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14 November 2012
from
ESO Website
Orphaned world may help
to explain
how planets and stars form
Astronomers using
ESO’s Very Large
Telescope and the
Canada-France-Hawaii Telescope have identified a
body that is very probably a planet wandering through space without
a parent star.
This is the most exciting free-floating
planet candidate so far and the closest such object to the Solar
System at a distance of about 100 light-years. Its comparative
proximity, and the absence of a bright star very close to it, has
allowed the team to study its atmosphere in great detail.
This object also gives astronomers a
preview of the exoplanets that future instruments aim to image
around stars other than the Sun.
Free-floating planets are planetary-mass
objects that roam through space without any ties to a star. Possible
examples of such objects have been found before [1], but
without knowing their ages, it was not possible for astronomers to
know whether they were really planets or brown dwarfs - “failed”
stars that lack the bulk to trigger the reactions that make stars
shine.
But astronomers have now discovered an
object, labeled
CFBDSIR2149 [2], that seems to be part
of a nearby stream of young stars known as the
AB Doradus Moving Group.
The researchers found the object in
observations from the Canada-France-Hawaii Telescope and harnessed
the power of ESO’s Very Large Telescope to examine its properties
[3].
The AB Doradus Moving Group is the
closest such group to the Solar System.
Its stars drift through
space together and are thought to have formed at the same time. If
the object is associated with this moving group - and hence it is a
young object - it is possible to deduce much more about it,
including its temperature, mass, and what its atmosphere is made of
[4]. There remains a small probability that the
association with the moving group is by chance.
The link between the new object and the
moving group is the vital clue that allows astronomers to find the
age of the newly discovered object [5].
This is the first isolated planetary
mass object ever identified in a moving group, and the association
with this group makes it the most interesting free-floating planet
candidate identified so far.
“Looking
for planets around their stars is akin to studying a firefly
sitting one centimeter away from a distant, powerful car
headlight,” says Philippe Delorme (Institut de planétologie
et d’astrophysique de Grenoble, CNRS/Université Joseph Fourier,
France), lead author of the new study.
“This
nearby free-floating object offered the opportunity to study the
firefly in detail without the dazzling lights of the car messing
everything up.”
Free-floating objects like CFBDSIR2149
are thought to form either as normal planets that have been booted
out of their home systems, or as lone objects like the smallest
stars or brown dwarfs.
In either case these objects are
intriguing - either as planets without stars, or as the tiniest
possible objects in a range spanning from the most massive stars to
the smallest brown dwarfs.
“These
objects are important, as they can either help us understand
more about how planets may be ejected from planetary systems, or
how very light objects can arise from the star formation
process,” says Philippe Delorme.
“If
this little object is a planet that has been ejected from its
native system, it conjures up the striking image of orphaned
worlds, drifting in the emptiness of space.”
These worlds could be common - perhaps
as numerous as normal stars [6].
If CFBDSIR2149 is not associated with
the AB Doradus Moving Group it is trickier to be sure of its nature
and properties, and it may instead be characterized as a small
brown
dwarf.
Both scenarios represent important
questions about how planets and stars form and behave.
“Further
work should confirm CFBDSIR2149 as a free-floating planet,”
concludes Philippe Delorme.
“This
object could be used as a benchmark for understanding the
physics of any similar exoplanets that are discovered by future
special high-contrast imaging systems, including the SPHERE
instrument that will be installed on the VLT.”
Notes
[1]
Numerous candidates for these kinds of planets have been found
before (with corresponding press releases and papers, e.g. from
Science Magazine,
Nature,
Royal Astronomical Society). These objects started to become
known in the 1990s, when astronomers found that the point at
which a brown dwarf crosses over into the planetary mass range
is difficult to determine. More
recent studies have suggested that there may be huge numbers
of these little bodies in our galaxy, a population numbering
almost twice as many as the main sequence stars present.
[2] The object was identified as
part of an infrared extension of the Canada-France Brown Dwarfs
Survey (CFBDS), a project hunting for cool brown dwarf stars. It
is also referred to as CFBDSIR J214947.2-040308.9.
[3] The team observed CFBDSIR2149
with both the WIRCam camera on the Canada France Hawaii
Telescope on Hawaii, and the SOFI camera on the ESO New
Technology Telescope in Chile. The images taken at different
times allowed the object’s proper motion across the sky to be
measured and compared to members of the AB Doradus Moving Group.
The detailed study of the object’s atmosphere was made using the
X-shooter spectrograph on ESO’s Very Large Telescope at the
Paranal Observatory.
[4] The association with the AB
Doradus Moving Group would pin down the mass of the planet to
approximately 4–7 times the mass of Jupiter, with an effective
temperature of approximately 430 degrees Celsius. The planet’s
age would be the same as the moving group itself - 50 to 120
million years.
[5] The team’s statistical analysis
of the object’s proper motion - its angular change in position
across the sky each year - shows an 87% probability that the
object is associated with the AB Doradus Moving Group, and more
than 95% probability that it is young enough to be of planetary
mass, making it much more likely to be a rogue planet rather
than a small “failed” star. More distant free-floating planet
candidates have been found before in very young star clusters,
but could not be studied in detail.
[6] These free-floating objects can
also reveal their presence when they pass in front of a star.
The light travelling towards us from the background star is bent
and distorted by the gravity of the object, causing the star to
suddenly and briefly brighten - a process known as gravitational
microlensing. Microlensing surveys of the Milky Way, such as
OGLE, may have detected free-floating planets in this way (for
example, a
Microlensing Experiment published in Nature in 2011).
More information
This research is presented in a paper, “CFBDSIR2149-0403
- A 4-7 Jupiter-Mass Free-Floating Planet in The Young Moving Group
AB Doradus?”
appeared in Astronomy &
Astrophysics on 14 November 2012.
The team is composed of,
-
P. Delorme (Institut de
planétologie et d’astrophysique de Grenoble, CNRS/Université
Joseph Fourier, France [IPAG])
-
J. Gagné (Université de
Montréal, Canada)
-
L. Malo (Université de Montréal,
Canada)
-
C. Reylé (Institut UTINAM, CNRS/OSU
THETA Franche-Comté-Bourgogne/Université de Franche Comté,
France)
-
E. Artigau (Université de
Montréal, Canada)
-
L. Albert (Université de
Montréal, Canada)
-
T. Forveille (Institut de
planétologie et d’astrophysique de Grenoble, CNRS/Université
Joseph Fourier, France [IPAG])
-
X. Delfosse (Institut de
planétologie et d’astrophysique de Grenoble, CNRS/Université
Joseph Fourier, France [IPAG])
-
F. Allard (Université Claude
Bernard Lyon 1, France), D. Homeier (Université Claude
Bernard Lyon 1, France)
The year 2012 marks the 50th anniversary
of the founding of the European Southern Observatory (ESO).
ESO is the foremost intergovernmental
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ground-based astronomical observatory by far. It is supported by 15
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ESO carries out an ambitious program
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ESO operates three unique world-class
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La Silla
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Paranal
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Chajnantor
At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
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VISTA works in the infrared and is
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ESO is the European partner of a
revolutionary astronomical telescope ALMA, the largest astronomical
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The Canada-France-Hawaii Telescope (CFHT)
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