First observations with the world's newest planet-hunter instrument, HiCIAO, have revealed a faint companion to the star GJ 758, resulting in what could be the first image of a cool extrasolar planet orbiting a Sun-like star. With an estimated mass of 10 - 40 times Jupiter's mass, GJ 758 B is either a giant planet or a lightweight brown dwarf. Its orbit is somewhat larger than Neptune's, and its temperature of 600 K makes it the coldest companion to a Sun-like star ever resolved in an image.
The number of known exoplanets - planets orbiting stars other than our Sun - has recently surpassed 400. Most were discovered indirectly, by detecting a planet's influence on its host star's motion or brightness. Producing a direct image of an exoplanet is much more difficult - as difficult as producing an image of a firefly sitting next to a 300 Watt floodlight from a distance of several kilometers, given that stars are significantly brighter than planets. But whenever imaging is successful, the rewards are considerable, as images can provide invaluable information about the planet's orbit, and about the temperature and composition of its atmosphere.
Now a new planet candidate, designated GJ 758 B, has been discovered and imaged with the 8-meter Subaru telescope on the summit of Mauna Kea in Hawai`i, using state-of-the-art adaptive optics to correct the blurring effect of Earth's turbulent atmosphere. The planet orbits the star GJ 758 in the constellation Lyra. While, on each of the images taken, the tiny spot representing the planet is lost in the host star's residual glare, it was possible to obtain an image of the planet by combining time-series of individual images, using a technique known as angular differential imaging (ADI) . In this way, the astronomers were able to remove the star's halo from the image sufficiently well to reveal GJ 758 B's feeble glow.
Prior to this discovery, only ten possible exoplanets had been imaged directly. In each of these cases, conditions are markedly different from those in our own Solar System: Either the planetary orbits are extremely wide (hundreds of times the distance between the Earth and the Sun), the companion's temperature is closer to that of a star than that of a planet (greater than 1000 Kelvin), or the host stars are quite different from the Sun (such as A-stars, which are more massive, or late M-stars, whose mass is much smaller). Compared with these other candidates, GJ 758 B is much more similar to the planets of our own Solar System: It orbits a Sun-like star, at a distance comparable to that of our own Solar System's outer planets - as viewed from Earth, it currently appears to be as far from its host star as Neptune is from the Sun ; the overall size of GJ 758 B's orbit can only be estimated, with the most likely estimate putting the planet at a mean distance of around 59 astronomical units from its Sun (compared to 39 astronomical units for Pluto). Most interestingly, the planet's temperature amounts to a balmy 550 - 640 K (280 - 370 degrees Celsius, or 530-700 Fahrenheit). "This corresponds to the heat of a baking oven at full power, or to the day-side temperature of the planet Mercury", says Dr. Christian Thalmann of the Max Planck Institute for Astronomy (MPIA), lead author of the discovery paper. "It makes GJ 758 B the coldest companion to a Sun-like star ever resolved in imaging."
The outermost planet in our own Solar System, Neptune, receives only about 1/900 of the sunlight that reaches Earth, and has a surface temperature of a mere 70 K (-200 degrees Centigrade, or -330 Fahrenheit). GJ 758 is at least as far away from its host star. Its considerably higher temperature suggests that this object is still contracting - an intermediary stage in the life of giant gas planets, in which gravitational energy is converted to heat. For such a contracting object, temperature, age and mass are related: The larger a giant planet, the longer it takes to radiate the excess heat out into space and settle down into thermal equilibrium. "This is also why the mass of the companion is not well known: The measured infrared brightness could come from a 700 million year old planet of 10 Jupiter masses just as well as from a 8700 million year old companion of 40 Jupiter masses," explains Dr. Markus Janson from the University of Toronto, a former MPIA scientist. As stars and their planets form at the same time, an accurate age estimate for the host star would eliminate the uncertainties; so far, however, the available data is insufficient to allow an estimate with the required precision.
GJ 758 B was detected in two independent observations in May and August 2009. The images provide clear evidence that GJ 758 B and the star GJ 758 are not merely unrelated objects that happen to occupy the same patch of sky: As many other nearby stars, GJ 758 exhibits what astronomers call "proper motion", that is, it changes its position in the night sky (albeit very slowly). The images show that GJ 758 B moves exactly as expected for an object that is bound by gravity to GJ 758: It's motion in the night sky is a combination of GJ 758's proper motion and the companion's orbital motion around its host star.
The August image, which is of higher quality, reveals one additional object, at a closer separation from the star. This could represent a second companion, GJ 758 C. Yet another observation is needed to confirm common proper motion of this object with the GJ 758 system, and thus to prove that it is no mere background star. "This looks very promising," says Dr. Christian Thalmann, and adds: "If this is indeed a second companion, it is most likely that GJ 758 B and C are young planets rather than old brown dwarfs. A system with two brown dwarfs on such close orbits is unlikely to remain stable in the long term." Dr. Motohide Tamura (National Astronomical Observatory of Japan), principal investigator of HiCIAO, elaborates: "It is unlikely that the companions formed in their current orbits. They might have ended up there as a result of a scattering event."
"The discovery of GJ 758 B, an exoplanet or brown dwarf orbiting a star that is similar to our own sun, gives us an insight into the diversity of substellar objects that may form around Solar-type stars," says MPIA's Dr. Joseph Carson, the second author of the publication. "This in turn helps show how our own Solar system, and the environments that are conducive to life, are just one of many scenarios that may be the outcome of planet or brown dwarf formation around Sun-like stars."
The HiCIAO instrument will now be used in a five-year strategic survey mission "SEEDS" to detect extrasolar planets and circumstellar disks on a large scale. As Prof. Dr. Thomas Henning, Managing Director of the MPIA, says: "The spectacular discovery of GJ 758 B during the commissioning run bodes well for this survey, proving that the instrument is in excellent shape to face this challenging task."
 More information on this imaging technique can be found here: http://www.mpia.de/homes/thalmann/adi.htm
 Just as objects can appear foreshortened when viewed at a certain angle, a planet's apparent distance from its host star, as observed from Earth, will in generally be smaller than the true distance. Observations show that GJ 758 B must be at least as far away from its host star as Neptune. The most likely reconstruction of its orbit - which could change as new data becomes available, puts GJ 758 on an eccentric orbit, with a mean distance of 59 AU from its host star.
The results reported here will be published in the journal Astrophysical Journal Letters. They are based on data obtained by the commissioning team of the HiCIAO high-contrast imaging instrument at the Subaru telescope on Mauna Kea, Hawai`i, operated by the National Astronomical Observatory of Japan (NAOJ). The team includes members from NAOJ, the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Princeton University, the University of Toronto and the University of Hawai`i . It is led by principal investigator Dr. Motohide Tamura (NAOJ). The lead authors of the discovery paper are MPIA scientists Dr. Christian Thalmann and Dr. Joseph Carson. The data reduction of the ADI images was performed using an optimized algorithm ("LOCI") generously provided to the MPIA team by its inventor, Dr. David Lafrenière from the University of Montréal.
Dr. Christian Thalmann (first author of the discovery paper)
Max Planck Institute for Astronomy, Heidelberg
Phone: (0|+49) 6221 - 528 403
Dr. Jakob Staude (Public relations)
Max Planck Institute for Astronomy, Heidelberg
Phone: (0|+49) 6221 - 528 229
http://www.mpia.de/Public/menu_q2e.php?Aktuelles/PR/2009/PR091203/PR_091203_en.h... - Press release on the MPIA website, with additional images (including hi-res versions)
http://arxiv.org/abs/0911.1127 Science article
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