Press Release

Astronomers successfully obtained images of an extra-solar planet

May 21, 2009

Are there any planetary systems like our own in the Universe? Is there life out there? Astronomers now try to answer these long-standing philosophical questions scientifically. The extra-solar planets have been increasingly found, and about 350 planets are known so far. However, most of them were discovered in indirect ways such as the radial velocity method finding the wobble of the parent star due to planets (Doppler shifts in the star’s spectra). Direct detection, which separates between the light from a planet and the stellar light, has obviously more impact as it directly demonstrates the existence of a planet there. Direct imaging is technically difficult, however, since planets are much fainter than the star and located too close to it.

The breakthrough came in November of 2008: astronomers achieved direct detections of extra-solar planets orbiting HR 8799 and Fomalhaut, obtained with Keck, Gemini, and Hubble Space Telescopes. There still are uncertainties in the planet mass estimates, but they are recognized as more promising planet candidates than the several cases reported until then. HR 8799, in particular, hosts three planets, and is the first planetary system ever imaged. Following this epoch-making discovery, researchers in Osaka University, Kobe University, and National Astronomical Observatory of Japan took a detailed look at the Subaru data of HR 8799 obtained in 2002, and indeed found an outmost planet, HR 8799b.

HR 8799 has been known as having a debris disk which is generally thought to be maintained by planet formation activity, i.e., mutual collision between planetismals. The Subaru observations were originally aimed to image the dusty disk, thus the combination of adaptive optics (AO) and coronagraph was used to suppress the bright stellar light and detect the faint structure in the vicinity of the star. This setup is fortunately suited for planet detection, too. The team analyzed the data in an optimized way for planets, not for the disk this time, and found the planet HR 8799b at around the expected location. The measurement of the relative position to the star is useful to determine the accurate orbit of HR 8799b in the future. The positions from 1998 to 2008 that they have now are not inconsistent with a planet having a circular orbit, but measurements should be continued to cover the large fraction of the orbital phase.

HR 8799 planetary system is quite difference from the Solar system: three planets with about 10 Jupiter masses are located at 24, 36, and 69 AU from the star (recall that the distance from the Sun to Neptune is about 30 AU). How is such a system formed? Is it possible for these planets to orbit without drastically disturbing each other? The discovery of HR 8799 system raises questions that keep astronomers busy.

Now, Subaru has an upgraded AO and new coronagraph to get more images of planets and study protoplanetary/debris disks. There is no doubt that Subaru will yield fruitful results in the field of extra-solar planets.

This research was published in The Astrophysical Journal Letters vol.696, pp. L1-L5 (issue published on 1 May 2009).

Subaru image of HR 8799b at 1.6 micron. A yellow arrow shows HR 8799b. A star sign denotes the position of the central star.

Face-on view assuming that three planets found so far have completely circular orbits. HR 8799 is thought to be surrounded by a debris disk which would exist outside of HR 8799b. Two out of three planets are farther than Neptune (red dotted circle).



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