last book aliens

In the third book, the prequel The Prophet of Yonwood, a government scientist is attempting to use a form of space folding technology to communicate with alien life. It's not really all that significant since in about two generations a thermonuclear war will decimate humanity but it's just meant to show the limitless of human ingenuity.

In the fourth book, The Diamond of Darkhold, there is the minor mention that an alien civilization picked up some of those human signals during the experiments of that scientist and they sent a probe to investigate. However, during the several centuries that the probe was enroute to Earth, some bad things happened and when it reached orbit, it found that the powerful civilization was gone and only a tiny, primitive society existed that appeared to be rebuilding itself. The probe was to remain and monitor its progress.

Doon and Lina, nor anyone else is actually mentioned interacting with the probe. They just think it's a falling star. It's not really integral to the plot. I think it's just a mechanism to show that humanity will persevere no matter what.

I hope that helps.