Astronomers in the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) Project have recently discovered and captured an image of the least massive planet ever imaged so far -- a so-called "second Jupiter" . This discovery marks an important step toward the direct imaging of much fainter Earth-like planets in the future and may lead to new models of planet formation.
Observations of Exoplanets
Exoplanets are planets orbiting stars other than our Sun, outside of our Solar System. As of July 2013, most of the 890 exoplanets reported thus far have been discovered by indirect observation techniques, e.g. monitoring the host star for radial velocity variation or planetary transits (Note 1). Such techniques require observations over at least one orbital period and are impractical for detecting planets that are widely separated from their host stars and have long orbital periods. In contrast, direct imaging may be the most important way to observe exoplanets, because it yields information about the planet's luminosity, temperature, atmosphere, and orbit. However, this method is technically challenging, because these distant planets are not only faint but also close to their bright central stars, the glare of which easily obscures an indication of a planet. It's like trying to see a firefly around a distant lighthouse. Only a dozen planets with a location similar in scale to that of the Solar System have been discovered. In spite of these technical challenges, 8 m class telescopes with adaptive optics and coronagraphs that block the light of the central star have reached the high performance necessary to image massive planets at large orbital separations.
Discovery of a "Second Jupiter"
Since 2009, SEEDS, an international project approved by the National Astronomical Observatory of Japan (NAOJ) and led by Motohide Tamura (University of Tokyo and NAOJ), has been probing this new frontier with its five-year direct imaging survey of exoplanets and disks around a targeted total of 500 stars. The team is composed of more than 120 members (two-thirds from Japan and one-third in the United States and Europe) and over 25 institutes.
During the SEEDS survey, a team led by members from the Tokyo Institute of Technology (TiTech), the University of Tokyo, and NAOJ used the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) (Note 2) mounted on the Subaru Telescope to discover and image planet candidate GJ 504 b that orbits a Sun-like star GJ 504, which is in the constellation Virgo about 60 light years from Earth. They used a mature (160 Myr) host star as a natural guide star for the AO 188 (adaptive optics with 188 elements). GJ 504 is bright and visible to the naked-eye (~5 magnitudes), but the planet is very dim (17-20 magnitudes at infrared wavelengths).
Since a single direct imaging cannot distinguish planet candidates from background stars that happen to be within the small field of view of the camera, the team conducted seven observations, including the use of IRCS (Infrared Camera and Spectrograph) with AO 188, to confirm that GJ 504 b orbits around GJ 504 and is NOT a background star.
What is the mass of this planet? Planet mass is estimated from age and luminosity. However, mass estimates for planets suffer from the uncertainties of cooling models for giant planets. Planetary mass estimates usually assume a hot start, in which the planet is initially in a hot-temperature and luminous state. However, recent theoretical models suggest that giant planets could be much colder initially. The difference between the relation of mass to luminosity decreases with an exoplanet's age and converges after 100 Myr for a five-Jupiter-mass planet. Regardless of which model is used to calculate the mass of this cold, dim object, GJ 504 b, the central star GJ 504 is mature enough (160 Myr) to predict a planetary rather than brown dwarf mass.
Based on the relation of its observed luminosity and estimated age in comparison with the theoretical model, scientists can infer that GJ 504 b has a mass as small as three Jovian masses. If so, it is the lightest-mass planet ever imaged. The apparent distance between the central star and planet is 44 AU (astronomical unit), which is larger than Neptune's orbit and comparable to Pluto's, and the team concludes that they have finally imaged a "second Jupiter." This is an important step toward the direct imaging of much fainter Earth-like planets in future.
The question remains: How did such a giant planet form around a Sun-like star? The standard core accretion model based on our Solar System does not provide an adequate explanation for the formation of these outer planets situated so far from their central stars. This model maintains that the core reaches a critical mass from the accretion of bodies of rock or ice (planetesimals) and subsequently increases with the rapid and direct accumulation of gas from its protoplanetary disk. However, it falls short of explaining how planets beyond 30 AU have developed.
Perhaps another model like gravitational instability of formation sites could supply the answer. This model maintains that a massive protoplanetary disk becomes gravitationally unstable in its outer regions. Consequently, the outer disk fragments, collapsing directly into one or more giant planets. However, gravitational instability has its own challenge. The theory requires that Sun-like stars have very massive disks around them, and it is unlikely that they could obtain enough mass in such outer regions.
At this point scientists cannot definitely say what led to the formation of GJ 504 b. Its features provide the basis for continuing investigation. Subsequent observations reveal that the planet has a unique blue color, which indicates that its atmosphere is less cloudy than that of other imaged planets.
Its relatively cold temperature, less cloudiness, and light mass place it in a physically distinct category from previously imaged planets, which are hotter and have cloudier atmospheres. Additional interesting features include its age (oldest among all directly imaged planets) as well as its wide separation and orbit around a main sequence star with a Sun-like mass.
Other Fruits of the SEEDS Project
In line with the project's aim of using the Subaru Telescope to directly image young planetary-mass objects and circumstellar structures located at relatively large distances from central stars, observers have used HiCIAO (Notes 2 & 3) with the Subaru Telescope to directly image two exoplanets and provide detailed images of more than ten protoplanetary and two debris disks. Observations have yielded reports of the following;
They have also detected interesting fine structures of the disks around many young stars, which show gaps, spiral arms, and rings; these structures may be "signposts of planets." In addition, the image of a companion around HAT-P-7 may explain the retrograde motion of its planet HAT-P-7 b ("The Origin and Maintenance of a Retrograde Exoplanet").
The Promise of Future Research
The SEEDS Project has finished more than three-quarters of its allotted 120 observation nights. About 500 stars will eventually be observed, and the statistical results on the frequency of outer planets will provide clues to planet formation. Scientists can address such questions as: "How numerous are outer planets?"; "Where and how did they form?"; "What do their atmospheres look like ?"
The team has detected many gaps and spiral-arm structures in disks at the same radial distance where outer planets were discovered. Planets within the disks could explain the formation of these structures and allow scientists to hypothesize that outer planets have formed or are close to their birth. At the very least, the SEEDS results demonstrate that it is now time to revisit current planet formation theory and include the findings about outer planets and disk structures.
SEEDS also will explore the inner regions near the central star with a very advanced adaptive optics system called the Subaru Telescope Coronagraphic Extreme Adaptive Optics (SCExAO, Note 4), which will become available soon. Observations with this technology promise to contribute to better understandings of exoplanetary systems.
Since observations from SEEDS include data on very young stars as well as relatively old ones, these data are and will be used to explore the links between planets and disks as well as the evolution of protoplanetary systems and debris disks. The SEEDS data set will become a dominant source of information in this important field of research for many years to come.
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