Subaru Telescope
National Astronomical Observatory of Japan
Hilo, Hawaii

A Solar System research team from the National Astronomical Observatory of Japan (NAOJ), the Graduate University for Advanced Studies, and the Science University of Tokyo have discovered small objects extended within the outer solar system using Subaru Telescope. Although more than 350 such objects are now known, these are the first discovered by Japanese astronomers.

Around 1950, Edgeworth and Kuiper independently proposed that there should be many small objects in the outer solar system that never became planets. We call them "Edgeworth-Kuiper Belt Objects (EKBOs)." Since we believe that they are composed of the materials of the early Solar System in their original state, EKBOs should be very useful objects for teaching us about how the Solar System formed. EKBOs may also be the source of the short- period comets, according to computer simulations.

The first EKBO was discovered with the University of Hawaii's 24" telescope at the summit of Mauna Kea in August 1992. In the nine years since the discovery of this EKBO (named 1992 QB1), more than 350 EKBOs have now been observed.

On UT February 21 and 24, 2001, the Japanese team of astronomers [, 69KB] discovered nine candidates for new EKBOs using the Subaru prime-focus camera (Suprime-Cam). Since two of the nine objects have observations on both days and their initial orbits have been determined, the International Astronomical Union has classified them as EKBOs and given them provisional designations "2001 DR106" and "2001 DS106." The distance between the objects and the earth is approximately 6.3 billion kilometers and their brightness is approximately 25th magnitude, a value about forty million times fainter than what can be seen with the unaided human eye (sixth magnitude). The objects are estimated to be approximately 100 kilometers in diameter, about 10% the size of the largest asteroid Ceres (910 kilometers). These observations show that the projected surface density of objects the size of these two EKBOs is approximately 10 per square degree along the plane of the ecliptic, consistent with previous results.

Compared to the asteroids which exist between the orbits of Mars and Jupiter, the motion of EKBOs against the stellar background is very slow because they are much further from the Sun. Mr. Daisuke Kinoshita (Grad. Univ. for Advanced Studies) and Mr. Naotaka Yamamoto (Science Univ. of Tokyo) have developed an auto-detection program for slow moving EKBOs. They comment, "I tremendously realized the great performance of Subaru and Suprime-Cam"; "it's impossible to describe the impression when I look at the results of the program." According to Dr. Jun-ichi Watanabe (NAOJ), the team leader, "Subaru's wide-field and big mirror offers the highest performance in the world for this type of work, and I fully expect more discoveries to be made." This is just the starting point for Subaru's EKBO studies.

The results are reported in Minor Planets Electric Circular MPEC 2001-J33 ( issued on May 15, 2001.

[, 56KB] The images of 2001 DR106: the left is at 07:35:36UT on 2001/02/24 and the right is 08:31:03UT on the same date.

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