An international team of astronomers, led by Joseph Carson (College of Charleston and Max Planck Institute for Astronomy), has discovered a "super-Jupiter" orbiting the massive star Kappa Andromedae. Using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) and the Infrared Camera and Spectrograph (IRCS) mounted on the Subaru Telescope, the team was able to directly image the new exoplanet, a gas giant with a mass about 13 times that of Jupiter and an orbit somewhat larger than Neptune's. The host star has a mass 2.5 times that of the Sun, making it the highest mass star to ever host a directly imaged planet or very low-mass brown dwarf.
Successful direct imaging of an exoplanet is difficult at best, because the brightness of the central star obscures the fainter light emitted from a planet orbiting it. A major goal of the SEEDS Project (Note), of which the current team is a part, is to explore hundreds of nearby stars in an effort to directly image extrasolar planets and protoplanetary/debris disks. The team used Subaru Telescope's high-contrast imaging instrument, HiCIAO, with the AO 188 Adaptive Optics System to hunt for exoplanets. They targeted many young high-mass stars and then concentrated follow-up observations on the relatively young star Kappa Andromedae. Located 170 light years from our own Solar System, Kappa Andromedae is a member of the Columba stellar moving group, with a youthful estimated age of 30 million years (compared with the Sun's older age of about 5 billion years). Young star systems are attractive targets for directly imaging planets because young planets retain significant heat from their formation, thus enhancing their brightness at infrared wavelengths.
Kappa And b, a so-called "super-Jupiter" (a gas giant significantly more massive than Jupiter), was detected in independent observations in January and July 2012 at four different wavelengths. Comparison of its relative positions between the two time periods revealed that Kappa And b exhibits "common proper motion" with the host star, proving that the two objects are gravitationally bound. A comparison of Kappa And b's brightness between the four different wavelengths revealed infrared colors similar to those of a handful of other gas giant planets successfully imaged around stars.
Such direct imaging of an extrasolar planet is exceptionally rare, especially for objects with orbital separations akin to the planets in our own Solar System. In a single infrared snapshot, the glare of the host star completely overwhelms the tiny point of light that is Kappa And b. The SEEDS observing team distinguished its distinct light only after using a technique known as angular differential imaging, which combines a time-series of individual images in a manner that allows for removal of the otherwise overwhelming glare of the host star from the final, combined image.
The large masses of the host star and its gas giant planet sharply contrast to objects in our own Solar System. In recent years some observers and theoreticians have argued that large stars like Kappa Andromedae are likely to have large planets, perhaps conforming to a simple scaled-up model of our own Solar System. Other experts suggest that there are limits to extrapolating from our own Solar System; if a star is too massive, its powerful radiation may disrupt the "normal" planet formation process that would otherwise occur in the disk surrounding a star, its circumstellar disk. The discovery of the super-Jupiter around Kappa Andromedae demonstrates that stars as large as 2.5 solar masses are still fully capable of producing planets within their circumstellar disks.
The SEEDS research team is continuing to study the light emitted from Kappa And b across a broad wavelength range in order to better understand the atmospheric chemistry of the gas giant and define its the orbital characteristics. The team also continues to explore the system for possible secondary planets, which may have influenced the formation of Kappa And b and its orbital evolution. These follow-up studies will yield further clues not only about the formation of the Super-Jupiter but also about principles of planet formation around massive stars.
References:The paper describing the research leading to this discovery, "Direct Imaging Discovery of a 'Super-Jupiter' Around the Late B-Type Star κ And", has been accepted for publication in the Astrophysical Journal Letters.
Core members of the research team are:
・J. Carson, College of Charleston, USA and Max Planck Institute for Astronomy, Germany
・C. Thalmann, University of Amsterdam, The Netherlands and Max Planck Institute for Astronomy, Germany
・M. Janson, Princeton University, USA
・T. Kozakis, College of Charleston, USA
・M. Bonnefoy, Max Planck Institute for Astronomy, Germany
・B. Biller, Max Planck Institute for Astronomy, Germany
・J. Schlieder, Max Planck Institute for Astronomy, Germany
・T. Currie, University of Toronto, Canada
・M. McElwain, Goddard Space Flight Center, USA
・M. Goto, Ludwig Maximilians University, Germany
・T. Henning, Max Planck Institute for Astronomy, Germany
・W. Brandner, Max Planck Institute for Astronomy, Germany
・M. Feldt, Max Planck Institute for Astronomy, Germany
・R. Kandori, National Astronomical Observatory of Japan, Japan
・M. Kuzuhara, National Astronomical Observatory of Japan and University of Tokyo, Japan
・H. Tamura, National Astronomical Observatory of Japan, Japan