Rocky planet discovery stokes ET life hopes The planet Kepler-78B is so close to its host star that its surface is molten lava. Art by Karen Teramura, University of Hawaii Institute for Astronomy. ASTRONOMERS have found earth's "virtual twin" after high-precision telescopes allowed them to "weigh" an earth-sized planet in another solar system. The first-time feat revealed that a planet known as "Kepler-78B" is not only a similar size to earth, but also roughly the same mass, suggesting it is composed of rock rather than gas. While the planet has earth-like dimensions, its "hellish" surface - an ocean of lava roasted by a sun so close that it takes up half the sky - has no chance of supporting life. But scientists say the discovery, reported in a series of papers in the journal Nature, marks a big step in the search for extra-terrestrial life. "The fact that we have found another rocky earth-sized planet in a different solar system suggests that nature is capable of making these things," said Andrew Howard, an astronomer with the University of Hawaii at Manoa and lead author of one of the papers. Astronomy magazines have been abuzz with reports of earth "twins" since NASA put its 1-metre Kepler Telescope in orbit around the sun in 2009. The telescope identified over 3600 possible planet in a section of the northern sky. So far over 150 of them have been confirmed as planets, but many are gas giants like Jupiter and others have "fluffy" atmospheres not considered conducive to life. The holy grail of interstellar life - a planet with earth-like size, composition and surface temperatures - remains elusive. Kepler-78B ticks the first two boxes but definitely not the third. The planet, identified earlier this year by a graduate student at Massachusetts Institute of Technology, takes just 8.5 hours to orbit a star some 400 light years from our own solar system. Surface temperatures are estimated at close to 3000C. The researchers determined its mass by measuring "wobbles" in the star's light as the planet passed in front of it. Dr Howard said it was currently impossible to perform similar measurements on a planet orbiting further out in the "habitable zone" where water can exist in liquid form. "The wobble it produces is something like 20 times smaller," he said. But University of Maryland astronomer Drake Deming said new telescopes and instruments would improve the odds by focusing on stars cooler than the sun. "It will allow future scientific teams to probe habitable planets orbiting small stars," Professor Deming wrote in a Nature analysis.