Solar System in fiction close to the Sun and its surface was exposed to extremes of temperature. The asteroids were too tiny and airless. By the early 20th century, prospects for life in the Solar System focused on Venus, the larger moons of Jupiter and Saturn, and especially Mars. With the onset of the Space Age, planetary probes cast increasing doubt on the likelihood of extraterrestrial life in the Solar System, at least life of any magnitude greater than organisms such as bacteria. By the mid-1960s, it was firmly established that life could have no foothold on the hostile surfaces of Mercury or Venus,
Five-planet Nice model distant massive ninth planet is responsible for the alignment of the perihelia of several trans-Neptunian objects with semi-major axes greater than 250 AU. Although the mechanism for the ejection of the fifth giant planet in the five-planet Nice model is reminiscent of the origin of Planet Nine, with a gravitational instability including an encounter with Jupiter, it is unlikely to be the same planet. The estimated timing of the capture of Planet Nine onto its distant orbit, three to ten million years after the formation of the Solar System, when the Sun was still in its birth cluster, is inconsistent