least one planet orbiting within one. In 2013, astronomers using the Kepler space telescope's data estimated that about one-fifth of G-type and K-type stars (sun-like stars and orange dwarfs) are expected to have an Earth-sized or super-Earth-sized planet ( Earths wide) close to an Earth-like orbit (), yielding about 8.8 billion of them for the entire Milky Way Galaxy. Uncertainty over Jupiter's role The requirement for a system to have a Jovian planet as protector (Rare Earth equation factor ) has been challenged, affecting the number of proposed extinction events (Rare Earth equation factor ). Kasting's 2001 review of Rare Earth questions whether a Jupiter protector has any bearing on the incidence of complex life. Computer modelling including the 2005 Nice model and 2007 Nice 2 model yield inconclusive results in relation to Jupiter's gravitational influence and impacts on the inner planets. A study by Horner and Jones (2008) using computer simulation found that while the total effect on all orbital bodies within the Solar System is unclear, Jupiter has caused more impacts on Earth than it has prevented. Lexell's Comet, a 1770 near miss that passed closer to Earth than any other comet in recorded history, was known to be caused by the gravitational influence of Jupiter. Grazier (2017) claims that the idea of Jupiter as a shield is a misinterpretation of a 1996 study by George Wetherill, and using computer models Grazier was able to demonstrate that Saturn protects Earth from more asteroids and comets than does Jupiter. Plate tectonics may not be unique to Earth or a requirement for complex life Ward and Brownlee argue that for complex life to evolve (Rare Earth equation factor ), tectonics must be present to generate biogeochemical cycles, and predicted that such geological features would not be found outside of Earth, pointing to a lack of observable mountain ranges and subduction. There is, however, no scientific consensus on the evolution of plate tectonics on Earth. Though it is believed that tectonic motion first began around three billion years ago, by this time photosynthesis and oxygenation had already begun. Furthermore, recent studies point to plate tectonics as an episodic planetary phenomenon, and that life may evolve during periods of "stagnant-lid" rather than plate tectonic states. Recent evidence also points to similar activity either having occurred or continuing to occur elsewhere. The geology of Pluto, for example, described by Ward and Brownlee as