Contact Report 251 explains the main points of interest regarding the Destroyer planet.
The threatening wandering planet in the past, over a long period, had caused a great deal of destruction and wreaked great havoc in the SOL system. According to details formerly presented by Sfath, this celestial body named the "Destroyer" had previously caused the most horrifying destruction on their homeworlds in days gone by, before it streaked through space on its unpredictable course. The "Destroyer" ensconced itself in the SOL system where it also wreaked much havoc. Over time it decreased its orbiting period to an average of 575.5 years; however, the Plejaren have since increased this interval again. As a result, the "Destroyer" will again appear in the SOL system in about 1180 years. The Plejaren were unable to completely knock the planet off course, as they had planned; nonetheless, they did change the Destroyer's orbiting period and rotation time in such a way that it presents no longer any danger - at least for the time being. Normally, the planet would have returned in the year 2255 with devastating, destructive results. The High Council failed to give the Plejaren permission to offset the wandering "Destroyer's" course by any other means or to destroy it, and it requested the Plejaren refrain from doing so as the Destroyer's obliteration, or a change of its course, would have generated an even greater destruction and possibly even affected the Earth.
In Contact Report 238 Billy states the age of The Destroyer.
645. Then the Wanderer, or now the Destroyer, as you have named it, must actually be about 5,000,000,000 years old, I mean as a solid body.