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This artist's impression shows the
free-floating planet CFBDSIR J214947.2-040308.9. This is the closest
such object to the Solar System. It does not orbit a star and hence
does not shine by reflected light; the faint glow it emits can only
be detected in infrared light. Here we see an artist’s impression
of an infrared view of the object with an image of the central parts
of the Milky Way from the VISTA infrared survey telescope in the
background. The object appears blueish in this near-infrared view
because much of the light at longer infrared wavelengths is absorbed
by methane and other molecules in the planet's atmosphere. In visible
light the object is so cool that it would only shine dimly with a
deep red color when seen close-up.
Credit: ESO/L. Calçada/P.
Delorme/Nick Risinger (skysurvey.org)/R. Saito/VVV Consortium
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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, labelled CFBDSIR2149 [2], that seems to be part of a nearby
stream of young stars known as the [AB Doradus Moving Group][1]. 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 centimetre away from
a distant, powerful car headlight," says Philippe Delorme
(Institut de planetologie et d'astrophysique de Grenoble,
CNRS/Universite 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 characterised 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."
Contact: Richard Hook
Notes
[1] Numerous candidates for these kinds
of planets have been found before (with corresponding press releases
and papers, e.g. from [Science Magazine][1], [Nature][2], [Royal
Astronomical Society][3]). 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][2] 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][2] published in Nature in 2011).
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