Still others see extraterrestrial life as a possible threat to humankind; although alien life has never been found, scientists such as Carl Sagan have postulated that the existence of extraterrestrial life is very likely.
In April 2008, it was announced that two simulations of long-term planetary movement, one at Paris Observatory and the other at University of California, Santa Cruz indicate a 1% chance that Mercury's orbit could be made unstable by Jupiter's gravitational pull sometime during the lifespan of the sun. Were this to happen, the simulations suggest a collision with Earth could be one of four possible outcomes (the others being colliding with the Sun, colliding with Venus, or being ejected from the solar system altogether). If this were to happen, all life on Earth would be obliterated and the impact may displace enough matter into orbit to form another moon. Note that an asteroid just 15 km wide is said to have destroyed the dinosaurs; Mercury is some 5,000 km in diameter.