Wednesday, December 23, 2009


The first observations with the world's newest planet-hunter instrument on the Subaru Telescope, HiCIAO (High Contrast Instrument for the Subaru next generation Adaptive Optics), have revealed a companion to the Sun-like star GJ 758. 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 comparable in size to Neptune's, and its temperature of 600 K makes it the coldest companion to a Sun-like star ever resolved in an image. A second companion with a similar mass at the Uranus's orbit is also suggested. The presence of such massive planets at these large distances challenges standard assumptions about planetary system formation based on the Solar System. Since a strategic search for exoplanets and their formation sites has just started, further observations will eventually answer questions about whether the Solar System is ubiquitous or not.

The number of planets known outside of our Solar System has recently surpassed 400. Most of those were discovered with indirect methods, such as Doppler spectroscopy (determining the "wobble" of the parent star due to the orbiting planet's gravitational tug) or transit photometry (measuring the ever-so-slight dimming of the parent star when the planet passes through the line of sight, obscuring part of the star). Actually resolving the planet's faint speck of light against the host star's overwhelming glare is much more challenging, yet ultimately more rewarding. It grants access to invaluable information about the planet's orbit and the temperature and composition of its atmosphere, and after all, "seeing is believing".

Direct imaging of exoplanets has been a hot topic in recent years. In 2005, a 4 Jupiter-mass companion around a young brown dwarf and several 10-a few 10s Jupiter-mass companions at a large distance of ~100 AU or more were discovered, but the results have been controversial. In 2008, 3 A-type stars (stars with about twice of the Sun's mass) were reported to be circled by planets. Among them, 3 planets around HR8799 have been confirmed by others including the Subaru Telescope. They are 7-10 Jupiter-mass at 24-68AU. However, there have been no reports on planet candidates orbiting close enough around the Sun-like stars (G-type stars).

The new planet candidate, GJ 758 B, was discovered around a G star JG758A at a distance of 50 light-years from the Sun by an international team composed of members from Germany, Japan, and the USA. Interestingly, its temperature is only a balmy 600 K; it is the coldest companion around G stars imaged so far.

Two epoch observations in May and August, 2009 established that the candidate is not a background star. It is found at a projected separation of 29 AU (Neptune's semi-major axis). The companion mass is estimated from its brightness and age. Although there is some uncertainty about the age of the host star, the mass of the companion is about 10 Jupiter-masses with an age of 700 million years old. Therefore, the companion can certainly be called a giant planet candidate.

In other August 2009 observations, another candidate with a similar mass at only 18 AU (Uranus's semi-major axis) was also detected, suggesting that it may be part of a planetary system with at least two giant planets at Neptune and Uranus orbits. However, this hypothesis should be confirmed with other observations.

All of these observations were conducted with the 8.2-meter Subaru Telescope at the summit of Mauna Kea in Hawaii, using the new instrument HiCIAO with its state-of-the-art adaptive optics to correct for the blurring effect of Earth's turbulent atmosphere in real time. The data and findings reported here were obtained during the commissioning phase of HiCIAO.

Although the Subaru Telescope has been equipped with its previous coronagraph CIAO (Coronagraphic Imager with Adaptive Optics) since 2000, the new planet/disk-hunting instrument HiCIAO employs not only the standard coronagraph but also various differential techniques (wavelength, polarization, angular differential imaging) to suppress the noise due to the residual stray-light halo of the host star. After five years of development funded by the MEXT grants-in-aid for Priority Area, its performance has demonstrated that it is more than 10 times better than CIAO.

The current discovery obtained by direct imaging provides information that cannot be acquired with the Doppler method, and it gives a much deeper understanding of planets around the Sun-like stars. In order to directly detect and observe exoplanets and disks, we need special instruments such as coronagraphs to block the light from a nearby bright host star, so that nearby faint objects can be viewed. A total eclipse, like the one seen in some regions of Japan in July 2009, is a "natural coronagraph" in which the Moon suppresses the bright light of the Sun. By using such direct imaging, we can answer the question, "Is the Solar System unique in the Universe?"

The Subaru Telescope recently began its first strategic observations for direct exoplanet hunting in October 2009. Over the next five years, its project SEEDS (Subaru Strategic Exploration of Exoplanets and Disks with HiCIAO/AO188) will search for exoplanets and circumstellar disks around about 500 stars. SEEDS is an international project composed of about 100 members from Japan, the USA, Germany, and the UK. By detecting more planet candidates similar to the ones reported here, we are able to not only answer questions about whether our Solar System is unique but also to understand how planets form from protoplanetary disks.

(Photo: Subaru Telescope)

National Astronomical Observatory of Japan

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