It was detected because of its extremely slow motion, only 3 arcsec/hour. Six accurate positions were measured and allowed to determine an approximate distance of about 42 AU, that is 6300 million kilometres from the Sun. This is far outside the orbit of the outermost, large planet, Neptune (4500 million kilometres); hence the classification as a "transneptunian" object. These six images show the motion of 1994 TG2 in front of the background images.
It has in the meantime been given the designation "1994 TG2" by the Minor Planet Center of the International Astronomical Union. The observed magnitude is about 24, i.e., it is about 16 million times fainter than the faintest objects that can be perceived with the unaided eye. Its diameter is probably 100 - 200 kilometres.
1994 TG2 is the 17th transneptunian object found during the past two years. Nine of these have distances between 31 and 36 AU, the other eight between 40 and 45 AU. None have so far been found in the gap in between; this may be an effect of Neptune's gravitational attraction. Colour measurements of some of the brightest have shown that they are unusually red.
The transneptunian objects represent an entirely new class of objects in the solar system. It is not yet clear how they may be related to other minor bodies like comets and minor planets, and whether Pluto and its moon Charon, as well as the Neptunian moon Triton (which was observed from close quarters by Voyager 2 in 1989), also belong to this class.
Technical information: 16 minutes gunn-R exposure with EMMI. North is up and East is to the left. 1 pixel = 0.265 arcsec; the field measures 51 x 38 arcsec.
This is ESO Press Photo 02/94 [with some additions]. It may be reproduced, if credit is given to the European Southern Observatory. Copyright: ESO Information and Photographic Service, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany.