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The average distance of the earth to the sun is used as a standard for measuring distances in the solar system and is called an astronomical unit (AU). One AU is about 93 million miles or 150 million kilometers. Mercury, the planet closest to the sun, is at about 0.387 AU. Pluto is the outermost planet, and is 39.44 AU from the sun. The boundary between the solar system and interstellar space is called the heliopause, and is about 100 AU from the sun. Some comets have highly eccentric orbits, and can travel out as far as 50,000 AU from the sun.
In 1992 radio astronomers reported evidence that strongly suggested the existence of two to three planets orbiting a pulsar in the Milky Way about 1,500 light-years from earth. Since then, astronomers have discovered many more planets outside our solar system - theextrasolar planets. These are planets orbiting around a star other than the Sun. Extrasolar planets were first discovered in the 1990s as a result of improved telescope technology, CCD and computer-based image processing which allowed far more accurate measurements of stellar motions.
These discoveries, and recent advances in astrobiology biochemistry applied to astronomy), are leading more and more scientists to speculate that life on other planets is not only possible, but even, perhaps, rather likely - stimulating enthusiasm for SETI.
