Final answer:
Nerva, who was childless, followed Domitian as emperor and later adopted Trajan as his successor, thus ensuring the stability of the Roman Empire.
Step-by-step explanation:
The childless senator who succeeded Domitian and was later threatened by his soldiers was Nerva. Emperor Domitian was assassinated in 96 CE, and Nerva was then proclaimed emperor by the Senate. Although Nerva lacked a biological heir, he adopted Trajan as his successor, bypassing any living relatives he had. This was a decision prioritising the stability and safety of Rome over family ties. Nerva passed away in 98 CE after a short reign, but not before ensuring the continuation of strong leadership for the Roman Empire through his choice of Trajan. It is important to note that Hadrian and Antoninus Pius, who came after Trajan, did not achieve similar military successes but maintained Roman stability through other means, including major building projects and defending the empire's borders.