Discovery and exploration of the Solar System Earth to be orbited by an artificial satellite ("Luna 10"), followed by Mars in 1971 ("Mariner 9"), Venus in 1975 ("Venera 9"), Jupiter in 1995 ("Galileo"), the asteroid 433 Eros in 2000 ("NEAR Shoemaker"), Saturn in 2004 ("Cassini–Huygens"), and Mercury and Vesta in 2011 ("MESSENGER" and "Dawn" respectively). "Dawn" is orbiting the asteroid–dwarf planet Ceres as of 2015. The first probe to land on another Solar System body was the Soviet "Luna 2" probe, which impacted the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966
Planet Nine perihelia also reached values where the objects would not be observed, requiring a population of Sednas significantly larger than current estimates. They then, after declining, fell to values low enough for the objects to enter planet-crossing orbits leading to their ejection from the Solar System, requiring an initial population inconsistent with current models of the early Solar System to explain current observations. Based on these challenges Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing TNOs is a temporary phenomenon that will disappear as more objects are detected.