Contact:
Claudine Laurent, 01 40 51 20 32, claudine.laur...@obspm.fr

1 May 2001

Discovery of a satellite around the transneptunian object 1998 WW31

Alain Doressoundiram (Observatoire de Paris) and Christian Veillet (CFH
Institute) have just discovered that the transneptunian object 1998 WW31
is in fact a double object. It is during their multi-color photometry and
recovery of transneptunian objects program that they made this discovery.
This program is carried out on the Canada-France-Hawaii Telescope of 3.6 m
located at 4200m on Mauna Kea, Hawaii. In December 2000, they pointed the
telescope at 1998 WW31, a transneptunian object which required additional
observation, without which they would have lost its position. Its double
nature or its elongated shape did not appear to them immediately at the
time of the observations, but only later during the meticulous analysis of
the data.

The transneptunian objects are small bodies of the Solar system located
beyond the orbit of Neptune, at more than 30 astronomical units from the
Sun (1 astronomical unit = distance Sun-Earth). They are icy bodies, very
primitive as fossil remnants of the formation of the Solar system, 4.6
billion years ago. The study of these objects, discovered for the first
time less than ten years ago, is currently in full development. It can
provide invaluable indices on the composition of the primitive nebula and
on the processes prevailing at the beginning of the Solar system. Moreover,
the knowledge of the physical properties of the transneptunian objects
could be a significant step in the study of circumstellar discs and planet
formation around other stars that the Sun.

1998 WW31 like all the objects of the Solar system moves relative to fixed
stars of the field. It is a weak object of 23.6 in magnitude and distant
by 45.6 astronomical units from the Earth. 1998 WW31 appears undoubtedly
double. The exceptional quality of the Mauna Kea site allowed the
resolution of the pair (1.2 seconds of arc). With only the images, two
alternative interpretations were possible:

* They are two transneptunian objects on distinct orbits but having by pure
  coincidence a common apparent motion over two nights!
* 1998 WW31 is undoubtedly a twin (like Pluto-Charon)

It is the second alternative, much more probable than the first, which proved
to be right. Indeed, the CFHT archive images of January 7 2000, made it
possible to find 1998 WW31. But the orientation of the pair, as well as the
separation between the two components had changed: 1998 WW31 is actually a
twin!

1998 WW31 is thus the second transneptunian object (after Pluto) to have a
satellite. The combined diameter of the pair is estimated at 150 km. The
angular separation of 1.2" observed in December corresponds to an effective
distance between the two components of approximately 40,000 km (Pluto and
Charon are distant by 19,636 km). The difference in magnitude between
1998 WW31 and its satellite is estimated at 0.4, which thus gives a mass
ratio of approximately 1.7 (ratio of ~8 for Pluto-Charon) the system
1998 WW31 is more a double "planet" than a planet-satellite system.

With all the archive images available and new observations expected in the
future, it will be possible to get a full orbit determination. This will
make it possible to determine for the first time the mass and the density
of a transneptunian object, capital information on the intimate nature of
these primitive objects, playing the role of reservoir for short period
comets.

NOTES FOR EDITORS

Further information from:

Alain Doressoundiram
Département DESPA, Observatoire de Paris
01 45 07 71 11, Alain.Doressoundi...@obspm.fr

Reference:

Veillet C, Doressoundiram A., Shapiro J, 2001, IAU Circular num. 7610

[NOTE: Images supporting this release are available at
http://www.obspm.fr/actual/nouvelle/tno_en.html]

--
Andrew Yee
a...@nova.astro.utoronto.ca