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VLT Takes Images and Spectra of Possible Exoplanet Near Young Brown Dwarf

Garching bei München, Germany – September 10, 2004:   A possible exoplanet five times more massive than Jupiter has been discovered near the brown dwarf 2M1207. Spectroscopic data provide strong evidence for its planetary status, but final proof must wait until proper motion studies have determined whether the putative exoplanet is a companion of the brown dwarf, or a background object.
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  On several occasions in recent years, astronomical images have revealed faint objects near much brighter stars. Some of these were initially thought to be orbiting exoplanets, but further study ruled that out. 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 just 0.8 arcseconds from a brown-dwarf object designated 2MASSW J1207334-393254. The brown dwarf, also known as "2M1207", is a "failed star". That is, it is too small to sustain nuclear fusion processes in its interior, and it produces energy by gravitational contraction. 2M1207 is a member of the TW Hydrae stellar association, which lies about 230 light-years from Earth. The discovery was made with the NACO adaptive optics facility on the 8.2-m Yepun Unit Telescope of the VLT at the ESO Paranal Observatory in Chile.
Science With Adaptive Optics, Brandner
The feeble companion object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was obtained in June 2004 by NACO. This difficult observation was 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.
The available observations are all consistent with the faint object being 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, but the data don't yet allow a clear-cut decision about the true nature of this intriguing object. Therefore, the astronomers refer to it as a "Giant Planet Candidate Companion (GPCC)".

Observations will now be made to ascertain whether the motion on the sky of GPCC is compatible with that of a planet orbiting 2M1207. This should become evident within one or two years at the most.

Since 1998, a team of European and American astronomers has been studying the environment of young, nearby "stellar associations", that is, 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, be they planets or brown dwarf objects. The leader of the team of scientists, ESO astronomer Gaël 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".
2M1207 A composite image of the brown dwarf object 2M1207 (centre) and the fainter object seen near it at an angular separation 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.
  Image:  © European Southern Observatory
The team focused on a study of the TW Hydrae Association. It is located in the direction of the constellation Hydra (The Water-Snake) deep in the southern sky, at a distance of about 230 light-years. For this, they used the NACO adaptive optics facility 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.

NACO's infrared wavefront sensor option was an essential component of the AO system for the success of these observations. Most wavefront sensors operate at visible wavelengths, but this unique instrument senses the wavefront distortion of the near-infrared image, in a wavelength region where objects like 2M1207 are much brighter than at visible wavelengths. Because of their low temperature, brown dwarfs emit almost no visible light.

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. 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 2MASSW J1207334-393254, ("2M1207"), as shown in the image above. 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 Gaël Chauvin.

Christophe Dumas, another member of the team, is excited about the discovery: "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."

What is the nature of this faint object? 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, 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, shown in the graph on the right.

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 signature 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
2M1207 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.
  Image:  © European Southern Observatory
possibility that this object could be an older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The detailed analysis is presented in a research paper that has been accepted for publication in the European journal Astronomy & Astrophysics.

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."

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. 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 determine whether or not 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.

Further proper motion studies of the brown dwarf 2M1207 and its possible planetary companion established that the two objects do move together. This confirmed that the companion is a planetary mass object, and that the companion, 2M1207 b, is indeed the first planet imaged outside our own Solar System:  
Astronomers Confirm the First Image of a Planet Outside of Our Solar System, Apr 2005.

G. Chauvin, A.-M. Lagrange, C. Dumas, B. Zuckerman, D. Mouillet, I. Song, J.-L. Beuzit and P. Lowrance,
"A giant planet candidate near a young brown dwarf," Astron. & Astrophys. 425, L29–L32 (2004)     (ArXiv e-print)
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