P2X-555 is the planet where SG-1 was scheduled to conduct a standard reconnaissance mission. The planet's location with respect to Earth's meant the wormhole would be passing near Earth's sun, which was experiencing a great deal of solar flare activity.
A solar flare affected the wormhole and SG-1 instead travelled back in time and returned to Earth. Equipped with information about the timing of solar flares in 1969, SG-1 dialed the Stargate to P2X-555 and arrived at a future Earth.
The SGC determined that SG-1 never arrived on P2X-555. SG-5 conducted the reconnaissance mission on the P2X-555 in SG-1's stead. No information about the planet or the results of that mission is known.
--Aurora 17:50, 12 May 2006 (PDT)