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VLT Captures Exoplanet’s Direct Spectrum

January 14, 2010

By studying a triple planetary system that resembles a scaled-up version of our own Sun’s family of planets, astronomers have been able to obtain the first direct spectrum “” the “chemical fingerprint” [1] “” of a planet orbiting a distant star [2], thus bringing new insights into the planet’s formation and composition. The result represents a milestone in the search for life elsewhere in the Universe.

“The spectrum of a planet is like a fingerprint. It provides key information about the chemical elements in the planet’s atmosphere,” says Markus Janson, lead author of a paper reporting the new findings. “With this information, we can better understand how the planet formed and, in the future, we might even be able to find tell-tale signs of the presence of life.”

The researchers obtained the spectrum of a giant exoplanet that orbits the bright, very young star HR 8799. The system is at about 130 light-years from Earth. The star has 1.5 times the mass of the Sun, and hosts a planetary system that resembles a scaled-up model of our own Solar System. Three giant companion planets were detected in 2008 by another team of researchers, with masses between 7 and 10 times that of Jupiter. They are between 20 and 70 times as far from their host star as the Earth is from the Sun; the system also features two belts of smaller objects, similar to our Solar System’s asteroid and Kuiper belts.

“Our target was the middle planet of the three, which is roughly ten times more massive than Jupiter and has a temperature of about 800 degrees Celsius,” says team member Carolina Bergfors. “After more than five hours of exposure time, we were able to tease out the planet’s spectrum from the host star’s much brighter light.”

This is the first time the spectrum of an exoplanet orbiting a normal, almost Sun-like star has been obtained directly. Previously, the only spectra to be obtained required a space telescope to watch an exoplanet pass directly behind its host star in an “exoplanetary eclipse”, and then the spectrum could be extracted by comparing the light of the star before and after. However, this method can only be applied if the orientation of the exoplanet’s orbit is exactly right, which is true for only a small fraction of all exoplanetary systems. The present spectrum, on the other hand, was obtained from the ground, using ESO’s Very Large Telescope (VLT), in direct observations that do not depend on the orbit’s orientation.

As the host star is several thousand times brighter than the planet, this is a remarkable achievement. “It’s like trying to see what a candle is made of, by observing it from a distance of two kilometers when it’s next to a blindingly bright 300 Watt lamp,” says Janson.

The discovery was made possible by the infrared instrument NACO, mounted on the VLT, and relied heavily on the extraordinary capabilities of the instrument’s adaptive optics system [3]. Even more precise images and spectra of giant exoplanets are expected both from the next generation instrument SPHERE, to be installed on the VLT in 2011, and from the European Extremely Large Telescope.

The newly collected data show that the atmosphere enclosing the planet is still poorly understood. “The features observed in the spectrum are not compatible with current theoretical models,” explains co-author Wolfgang Brandner. “We need to take into account a more detailed description of the atmospheric dust clouds, or accept that the atmosphere has a different chemical composition from that previously assumed.”

The astronomers hope to soon get their hands on the fingerprints of the other two giant planets so they can compare, for the first time, the spectra of three planets belonging to the same system. “This will surely shed new light on the processes that lead to the formation of planetary systems like our own,” concludes Janson.

Notes

[1] As every rainbow demonstrates, white light can be split up into different colors. Astronomers artificially split up the light they receive from distant objects into its different colors (or “wavelengths”). However, where we distinguish five or six rainbow colors, astronomers map hundreds of finely nuanced colors, producing a spectrum “” a record of the different amounts of light the object emits in each narrow color band. The details of the spectrum “” more light emitted at some colors, less light at others “” provide tell-tale signs about the chemical composition of the matter producing the light. This makes spectroscopy, the recording of spectra, an important investigative tool in astronomy.

[2] In 2004, astronomers used NACO on the VLT to obtain an image and a spectrum of a 5 Jupiter mass object around a brown dwarf “” a “failed star”. It is however thought that the pair probably formed together, like a petite stellar binary, instead of the companion forming in the disc around the brown dwarf, like a star”“planet system (see ESO 28/04, ESO 15/05 and ESO 19/06).

[3] Telescopes on the ground suffer from a blurring effect introduced by atmospheric turbulence. This turbulence causes the stars to twinkle in a way that delights poets but frustrates astronomers, since it smears out the fine details of the images. However, with adaptive optics techniques, this major drawback can be overcome so that the telescope produces images that are as sharp as theoretically possible, i.e. approaching conditions in space. Adaptive optics systems work by means of a computer-controlled deformable mirror that counteracts the image distortion introduced by atmospheric turbulence. It is based on real-time optical corrections computed at very high speed (several hundreds of times each second) from image data obtained by a wavefront sensor (a special camera) that monitors light from a reference star.

More information

This research was presented in a paper in press as a Letter to the Astrophysical Journal (“Spatially resolved spectroscopy of the exoplanet HR 8799 c”, by M. Janson et al.).

The team is composed of M. Janson (University of Toronto, Canada), C. Bergfors, M. Goto, W. Brandner (Max-Planck-Institute for Astronomy, Heidelberg, Germany) and D. Lafrenire (University of Montreal, Canada). Preparatory data were taken with the IRCS instrument at the Subaru telescope.

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organization in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory, and VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-meter European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Image 1: By studying a triple planetary system that resembles a scaled-up version of our own Sun’s family of planets, astronomers have been able to obtain the first direct spectrum of a planet around a star, thus bringing new insights into its formation and composition. The spectrum is that of a giant exoplanet, orbiting around the bright and very young star HR 8799, about 130 light-years away. This montage shows the image and the spectrum of the star and the planet as seen with the NACO adaptive optics instrument on ESO’s Very Large Telescope. As the host star is several thousand times brighter than the planet, this is a remarkable achievement at the border of what is technically possible. According to the scientists it is like trying to see what a candle is made of, by observing it from a distance of two kilometers when it’s next to a blindingly bright 300 Watt lamp. Despite the power of the VLT’s extraordinary adaptive optics system, the spectrum of the planet appears very faint, but still contains enough information for the astronomers to characterize the object. In the spectrum (right) several artifacts from the instrument are seen, such as internal reflections, or “ghosts”, and diffraction rings. Similarly, the image of the star (left) suffers from several diffraction artifacts, which should not be confused with the real signal of the exoplanet. Credit: ESO/M. Janson

Image 2: The bright and very young star HR 8799, about 130 light-years away from Earth, hosts a planetary system that looks like a scaled-up model of our own Solar System. Three giant planetary companions have been detected so far, with masses between 7 and 10 times that of Jupiter and being between 20 and 70 times as far from their host star than the Earth is from the Sun; the system also includes two belts of smaller objects, similar to our asteroid and Kuiper belts. This NACO image shows the star and the middle planet (marked), of which the astronomers were able, after more than five hours of exposure time, to tease out the planet’s spectrum from the host star’s much brighter light. This is the first time the spectrum of an exoplanet orbiting a normal, almost Sun-like star has been obtained directly. According to the scientists it is like trying to see what a candle is made of, by observing it from a distance of two kilometers when it’s next to a blindingly bright 300 Watt lamp. Despite the power of the VLT’s extraordinary adaptive optics system, the spectrum of the planet appears very faint, but still contains enough information for the astronomers to characterize the object. The image of the star suffers from several diffraction artifacts, which should not be confused with the real signal of the exoplanet. Credit: ESO/M. Janson

Image 3: By studying a triple planetary system that resembles a scaled-up version of our own Sun’s family of planets, astronomers have been able to obtain the first direct spectrum of a planet around a star, thus bringing new insights into its formation and composition. The spectrum is that of a giant exoplanet, orbiting around the bright and very young star HR 8799, about 130 light-years away. This spectrum of the star and the planet was obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. As the host star is several thousand times brighter than the planet, this is a remarkable achievement, at the border of what is technically possible. According to the scientists it is like trying to see what a candle is made of, by observing it from a distance of two kilometers when it’s next to a blindingly bright 300 Watt lamp. Despite the power of the VLT’s extraordinary adaptive optics system, the spectrum of the planet appears very faint, but still contains enough information for the astronomers to characterize the object. In the spectrum several artifacts from the instrument are seen, such as internal reflections, or “ghosts”, and diffraction rings. ï»Â¿Credit: ESO/M. Janson

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