Passage FourQuestions 16to20are based on the following passage.Comets orbiting a distant star have been found to contain water, the first time the ingredient considered key to life has been discovered in objects circling a star other than our sun.The discovery adds weight to the theory that other planetary systems may be similar to our own and may contain chemicals that are essential to the formation of life as we know it.Scientists have no way of knowing whether life ever existed on any comets or planets orbiting this particular star, known as CW Leonis and located some 3,000 trillion miles from Earth in the constellation (星座) Leo.But they are certain that no life could exist there now. That is because as it burns out the last of its nuclear fuel, the star has swelled and increased in luminosity (亮度), engulfing nearby objects and melting comets in orbits as distant as Neptune is from the sun — releasing the water that astronomers spotted using a radio telescope in other space."Ironically, the existence of these bodies is measured through their destruction." Said Gary Melnick, chief scientist on the project and an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.Mr. Melnick said the discovery was of particular importance because if there was a reservoir of water farther from the star, there could have been water closer to the star, supporting the idea that life may have existed outside our solar system. Comets are believed to have helped build life on Earth, Mr. Melnick and other scientists said. Besides bringing water to the oceans, they are also thought to have transported important chemicals as they hurtled through Earth's atmosphere in past eons (极漫长的时期).