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,
Planet Nine some combinations of semi-major axis and mass) and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected. Brown speculates that the predicted planet is most probably an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas. In fact, if it once orbited the region of the gas/ice giants, the planet probably