Is This Speck of Light an Exoplanet? (Forwarded)
From: Andrew Yee (ayee_at_nova.astro.utoronto.ca)
Date: 09/10/04
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Date: Fri, 10 Sep 2004 10:44:14 -0400
ESO Education and Public Relations Dept.
--------------------------------------------------------------
Text with all links and the photos are available on the ESO
Website at URL:
http://www.eso.org/outreach/press-rel/pr-2004/pr-23-04.html
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Contacts:
Gael Chauvin
ESO-Santiago
Chile
Tel: +56-2-463-3063
Email: gchauvin@eso.org
Christophe Dumas
ESO-Paranal
Chile
Tel: +56-2-463-3060
Email: cdumas@eso.org
Ben Zuckerman
UCLA
Los Angeles, USA
Tel: +1-310-8259338
Email: ben@astro.ucla.edu
Anne-Marie Lagrange
LAOG
Grenoble, France
Tel: +33-1-44964377
Email: anne-marie.lagrange@obs.ujf-grenoble.fr
ESO Press Release 23/04
For immediate release: 10 September 2004
Is This Speck of Light an Exoplanet?
VLT Images and Spectra of Intriguing Object near Young Brown
Dwarf [1]
Summary
Is this newly discovered feeble point of light the long-
sought bona-fide image of an exoplanet?
A research paper by an international team of astronomers [2]
provides sound arguments in favour, but the definitive
answer is now awaiting further observations.
On several occasions during the past years, astronomical
images revealed faint objects, seen near much brighter
stars. Some of these have been thought to be those of
orbiting exoplanets, but after further study, none of them
could stand up to the real test. Some turned out to be
faint stellar companions, others were entirely unrelated
background stars. This one may well be different.
In April of this year, the team of European and American
astronomers detected a faint and very red point of light
very near (at 0.8 arcsec angular distance) a brown-dwarf
object, designated 2MASSWJ1207334-393254. Also known as
"2M1207", this is a "failed star", i.e. a body too small
for major nuclear fusion processes to have ignited in its
interior and now producing energy by contraction. It is
a member of the TW Hydrae stellar association located at
a distance of about 230 light-years. The discovery was
made with the adaptive-optics supported NACO facility [3]
at the 8.2-m VLT Yepun telescope at the ESO Paranal
Observatory (Chile).
The feeble object is more than 100 times fainter than
2M1207 and its near-infrared spectrum was obtained with
great efforts in June 2004 by NACO, at the technical
limit of the powerful facility. This spectrum shows the
signatures of water molecules and confirms that the
object must be comparatively small and light.
None of the available observations contradict that it may
be an exoplanet in orbit around 2M1207. Taking into account
the infrared colours and the spectral data, evolutionary
model calculations point to a 5 jupiter-mass planet in orbit
around 2M1207. Still, they do not yet allow a clear-cut
decision about the real nature of this intriguing object.
Thus, the astronomers refer to it as a "Giant Planet
Candidate Companion (GPCC)" [4].
Observations will now be made to ascertain whether the
motion in the sky of GPCC is compatible with that of a
planet orbiting 2M1207. This should become evident within
1-2 years at the most.
PR Photo 26a/04: NACO image of the brown dwarf object 2M1207
and GPCC
PR Photo 26b/04: Near-infrared spectrum of the brown dwarf
object 2M1207 and GPCC
PR Photo 26c/04: Comparison between the possible 2M1207
system and the solar system
Just a speck of light
ESO PR Photo 26a/04
The Brown Dwarf Object 2M1207 and GPCC
Caption: ESO PR Photo 26a/04 is a composite image of the
brown dwarf object 2M1207 (centre) and the fainter object
seen near it, at an angular distance of 778 milliarcsec.
Designated "Giant Planet Candidate Companion" by the
discoverers, it may represent the first image of an
exoplanet. Further observations, in particular of its
motion in the sky relative to 2M1207 are needed to
ascertain its true nature. The photo is based on three near-
infrared exposures (in the H, K and L' wavebands) with the
NACO adaptive-optics facility at the 8.2-m VLT Yepun
telescope at the ESO Paranal Observatory.
Since 1998, a team of European and American astronomers [2]
is studying the environment of young, nearby "stellar
associations", i.e., large conglomerates of mostly young
stars and the dust and gas clouds from which they were
recently formed.
The stars in these associations are ideal targets for the
direct imaging of sub-stellar companions (planets or brown
dwarf objects). The leader of the team, ESO astronomer Gael
Chauvin notes that "whatever their nature, sub-stellar objects
are much hotter and brighter when young -- tens of millions
of years -- and therefore can be more easily detected than
older objects of similar mass".
The team especially focused on the study of the TW Hydrae
Association. It is located in the direction of the
constellation Hydra (The Water-Snake) deep down in the
southern sky, at a distance of about 230 light-years. For
this, they used the NACO facility [3] at the 8.2-m VLT Yepun
telescope, one of the four giant telescopes at the ESO Paranal
Observatory in northern Chile. The instrument's adaptive
optics (AO) overcome the distortion induced by atmospheric
turbulence, producing extremely sharp near-infrared images.
The infrared wavefront sensor was an essential component of
the AO system for the success of these observations. This
unique instrument senses the deformation of the near-infrared
image, i.e. in a wavelength region where objects like 2M1207
(see below) are much brighter than in the visible range.
The TW Hydrae Association contains a star with an orbiting
brown dwarf companion, approximately 20 times the mass of
Jupiter, and four stars surrounded by dusty proto-planetary
disks. Brown dwarf objects are "failed stars", i.e. bodies
too small for nuclear processes to have ignited in their
interior and now producing energy by contraction. They emit
almost no visible light. Like the Sun and the giant planets
in the solar system, they are composed mainly of hydrogen
gas, perhaps with swirling cloud belts.
On a series of exposures made through different optical
filters, the astronomers discovered a tiny red speck of light,
only 0.8 arcsec from the TW Hydrae Association brown-dwarf
object 2MASSWJ1207334-393254, or just "2M1207", cf. PR Photo
26a/04. The feeble image is more than 100 times fainter than
that of 2M1207. "If these images had been obtained without
adaptive optics, that object would not have been seen," says
Gael Chauvin.
Christophe Dumas, another member of the team, is enthusiastic:
"The thrill of seeing this faint source of light in real-time
on the instrument display was unbelievable. Although it is
surely much bigger than a terrestrial-size object, it is a
strange feeling that it may indeed be the first planetary
system beyond our own ever imaged."
Exoplanet or Brown Dwarf?
ESO PR Photo 26b/04
The Brown Dwarf Object 2M1207 and GPCC
Caption: ESO PR Photo 26b/04 shows near-infrared H-band
spectra of the brown dwarf object 2M1207 and the fainter
"GPCC" object seen near it, obtained with the NACO facility
at the 8.2-m VLT Yepun telescope. In the upper part, the
spectrum of 2M1207 (fully drawn blue curve) is compared with
that of another substellar object (T513; dashed line); in
the lower, the (somewhat noisy) spectrum of GPCC (fully
drawn red curve) is compared with two substellar objects of
different types (2M0301 and SDSS0539). The spectrum of GPCC
is clearly very similar to these, confirming the substellar
nature of this body. The broad dips at the left and the
right are clear signatures of water in the (atmospheres of
the) objects.
What is the nature of this faint object [4]? Could it be an
exoplanet in orbit around that young brown dwarf object at a
projected distance of about 8,250 million km (about twice the
distance between the Sun and Neptune)?
"If the candidate companion of 2M1207 is really a planet, this
would be the first time that a gravitationally bound exoplanet
has been imaged around a star or a brown dwarf" says Benjamin
Zuckerman of UCLA, a member of the team and also of NASA's
Astrobiology Institute.
Using high-angular-resolution spectroscopy with the NACO
facility, the team has confirmed the substellar status of
this object -- now referred to as the "Giant Planet Candidate
Companion (GPCC)" -- by identifying broad water-band
absorptions in its atmosphere, cf. PR Photo 26b/04.
The spectrum of a young and hot planet -- as the GPCC may
well be -- will have strong similarities with an older and
more massive object such as a brown dwarf. However, when it
cools down after a few tens of millions of years, such an
object will show the spectral signatures of a giant gaseous
planet like those in our own solar system.
Although the spectrum of GPCC is quite "noisy" because of
its faintness, the team was able to assign to it a spectral
characterization that excludes a possible contamination by
extra-galactic objects or late-type cool stars with abnormal
infrared excess, located beyond the brown dwarf.
After a very careful study of all options, the team found
that, although this is statistically very improbable, the
possibility that this object could be an older and more
massive, foreground or background, cool brown dwarf cannot
be completely excluded. The related detailed analysis is
available in the resulting research paper that has been
accepted for publication in the European journal Astronomy &
Astrophysics (see below).
Implications
The brown dwarf 2M1207 has approximately 25 times the mass of
Jupiter and is thus about 42 times lighter than the Sun. As a
member of the TW Hydrae Association, it is about eight million
years old.
Because our solar system is 4,600 million years old, there is
no way to directly measure how the Earth and other planets
formed during the first tens of millions of years following
the formation of the Sun. But, if astronomers can study the
vicinity of young stars which are now only tens of millions of
years old, then by witnessing a variety of planetary systems
that are now forming, they will be able to understand much
more accurately our own distant origins.
Anne-Marie Lagrange, a member of the team from the Grenoble
Observatory (France), looks towards the future: "Our discovery
represents a first step towards opening a whole new field in
astrophysics: the imaging and spectroscopic study of planetary
systems. Such studies will enable astronomers to characterize
the physical structure and chemical composition of giant and,
eventually, terrestrial-like planets."
Follow-up observations
ESO PR Photo 26c/04
Comparison between the solar and 2M1207 systems
Caption: ESO PR Photo 26c/04 shows for illustration a
comparison between the solar system and the brown dwarf
object 2M1207 system with its possible planet at 55 AU
distance. The sizes of the objects are drawn to the same
scale, but the distances have been strongly compressed.
Taking into account the infrared colours and the spectral data
available for GPCC, evolutionary model calculations point to
a 5 jupiter-mass planet, about 55 times more distant from
2M1207 than the Earth is from the Sun (55 AU). The surface
temperature appears to be about 10 times hotter than Jupiter,
about 1000 C; this is easily explained by the amount of energy
that must be liberated during the current rate of contraction
of this young object (indeed, the much older giant planet
Jupiter is still producing energy in its interior).
The astronomers will now continue their research to confirm
or deny whether they have in fact discovered an exoplanet.
Over the next few years, they expect to establish beyond
doubt whether the object is indeed a planet in orbit around
the brown dwarf 2M1207 by watching how the two objects move
through space and to learn whether or not they move together.
They will also measure the brightness of the GPCC at multiple
wavelengths and more spectral observations may be attempted.
There is no doubt that future programmes to image exoplanets
around nearby stars, either from the ground with extremely
large telescopes equipped with specially designed adaptive
optics, or from space with special planet-finder telescopes,
will greatly profit from current technological achievements.
More information
The results presented in this ESO Press Release are based on a
research paper ("A Giant Planet Candidate near a Young Brown
Dwarf" by G. Chauvin et al.) that has been accepted for
publication and will soon appear in the leading research
journal "Astronomy and Astrophysics". A preprint is available
at
http://www.sc.eso.org/%7Egchauvin/index.html
Notes
[1]: This press release is issued simultaneously by ESO and
CNRS (in French) .
[2]: The team consists of Gael Chauvin and Christophe Dumas
(ESO-Chile), Anne-Marie Lagrange and Jean-Luc Beuzit (LAOG,
Grenoble, France), Benjamin Zuckerman and Inseok Song (UCLA,
Los Angeles, USA), David Mouillet (LAOMP, Tarbes, France) and
Patrick Lowrance (IPAC, Pasadena, USA). The American members
of the team acknowledge funding in part by NASA's Astrobiology
Institute.
[3]: The NACO facility (from NAOS/Nasmyth Adaptive Optics
System and CONICA/Near-Infrared Imager and Spectrograph) at
the 8.2-m VLT Yepun telescope on Paranal offers the capability
to produce diffraction-limited near-infrared images of
astronomical objects. It senses the radiation in this
wavelength region with the N90C10 dichroic; 90 percent of the
flux is transmitted to the wavefront sensor and 10 percent to
the near-infrared camera CONICA. This mode is particularly
useful for sharp imaging of red and very-low-mass stellar or
substellar objects. The adaptive optics corrector (NAOS) was
built, under an ESO contract, by Office National d'Etudes et
de Recherches Aérospatiales (ONERA), Laboratoire
d'Astrophysique de Grenoble (LAOG) and the LESIA and GEPI
laboratories of the Observatoire de Paris in France, in
collaboration with ESO. The CONICA camera was built, under an
ESO contract, by the Max-Planck-Institut für Astronomie (MPIA)
(Heidelberg) and the Max-Planck Institut für
extraterrestrische Physik (MPE) (Garching) in Germany, in
collaboration with ESO.
[4]: What is the difference between a small brown dwarf and
an exoplanet ? The border line between the two is still being
investigated but it appears that a brown dwarf object is
formed in the same way as stars, i.e. by contraction in an
interstellar cloud while planets are formed within stable
circumstellar disks via collision/accretion of planetesimals
or disk instabilities. This implies that brown dwarfs are
formed faster (less than 1 million years) than planets (~10
million years). Another way of separating the two kinds of
objects is by mass (as this is also done between brown dwarfs
and stars): (giant) planets are lighter than about 13 jupiter-
masses (the critical mass needed to ignite deuterium fusion),
brown dwarfs are heavier. Unfortunately, the first definition
cannot be used in practice, e.g., when detecting a faint
companion as in the present case, since the observations do
not provide information about the way the object was formed.
On the contrary, the above mass criterion is useful in the
sense that spectroscopy and astrometry of a faint object,
together with the appropriate evolutionary models, may
reveal the mass and hence the nature of the object.
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(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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