Binary minor planets are a relatively recent discovery, although they had been hypothesized to exist since shortly after the discovery of Ceres, more than 200 years ago. It is not unusual that TNOs have binary companions, and more are being discovered all the time.
"A binary planet" is a pair of worlds that are similar in mass. Each orbits the other around a gravitational balance point that is between the two - a location called the center of mass, or the barycenter. The pair may have born like twins, or may be produced by collisions, where a single body is split in two. The two objects usually have similar size, so it's not always clear which is the TNO and which is the satellite. There are even contact binaries - the objects are literally touching each other!
The first transneptunian binary (TNB) was identified in 1978 with the discovery of Charon. However, since at that time the existence of the Kuiper Belt had yet to be established, Pluto was considered a major planet.
The first discovery of a binary KBO (1998 WW31) was announced on April 16, 2001. Within a year after this discovery six other KBOs were discovered to be binaries. "It's amazing that something that seems so hard to do and takes many years to accomplish can then trigger an avalanche of discoveries," said Veillet, the discoverer of the satellite.
Photo: Copyright © Susanna Kauppinen.
Altjira and S/2007 (148780) 1
Borasisi and Pabu
Ceto and Phorcys
Logos and Zoe
Orcus and Vanth
Pluto and Kharon
Teharonhiawako and Sawiskera
Typhon and Echidna
1998 SM165 and S/2001 (1998 SM165) 1
1998 WW31 and S/2000 (1998 WW31) 1
1999 OJ4 and S/2002 (1999 OJ4) 1
1999 TC36 and S/2001 (47171) 1
2000 CF105 and S/2002 (2000 CF105) 1
2000 CM105 and S/2002 (2000 CM105) 1
2000 CQ114 and S/2003 (2000 CQ114) 1
2000 OJ67 and S/2003 (2000 OJ67) 1
2000 YW134 and S/2002 (2000 YW134) 1
2001 QW322 and S/2001 (2001 QW322) 1
Kuiper Belt Binaries by Kirsti Melto
EKOs From the Distant Past
New Horizons site
NASA/HST news release
David Jewitt's Kuiper Belt site
SkyTonight.com article about contact binaries