For the first time, scientists have found a planet beyond the solar system that not only is the same size as Earth, but has the same proportions of iron and rock, a key step in an ongoing quest to find potentially habitable sister worlds.     The planet, known as Kepler-78b, circles a star that is slightly smaller than the sun located in the constellation Cygnus , about 400 light years away.     Kepler-78b was discovered last year with NASA's now-idled Kepler space telescope, which detected potential planets as they circled in front of their parent stars, blocking a bit of light. That measurement not only revealed that Kepler-78b was  ly small, with a diameter just 20 percent larger than Earth's, but that it was practically orbiting on the surface of its host star. Scientists do not know how the planets ended up so close to their host stars.  While the planet's presumably molten surface and hot temperatures make it ill-suited for life, two independent teams of astronomers jumped at the opportunity to follow up the discovery with ground-based measurements to try to determine the density of Kepler-78b. In two papers in this week's journal Nature, the teams report that not only were they successful, but that they came to the same conclusion: Kepler-78b has roughly the same density as Earth, suggesting that it also is made primarily of rock and iron. Kepler-78b is among a dozen or so recently discovered small planets that orbit very close to their parent stars. Kepler-78b, for example, completes an orbit in just 8.5 hours. We can infer from the passage that   _
Scientists have never before found a planet beyond the solar system Kepler-78b is the only one of the planets found till now that has been orbiting so close to its parent star Kepler-78b is much alike the earth in size, proportions of iron and rock, and density Among recently discovered small planets, it takes Kepler-78b the shortest time to complete an orbit