Planets with sizes between that of Earth (with radius R⊕) and Neptune (about 4 R⊕) are now known to be common around Sun-like stars. Most such planets have been discovered through the transit technique, by which the planet’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet’s mass—and hence its density, which is a clue to its composition—is more difficult. Planets of size 2–4 R⊕ have proved to have a wide range of densities, implying a diversity of compositions, but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6). Given a radius of 1.20 ± 0.09 R⊕ and a mass of 1.69 ± 0.41 M⊕, the planet’s mean density of 5.3 ± 1.8 g cm^(−3) is similar to Earth’s, suggesting a composition of rock and iron.
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机译：现在已知大小在地球（半径为R⊕）和海王星（约4R⊕）之间的行星在类似太阳的恒星中很常见。大多数此类行星都是通过过境技术发现的，通过这种技术，可以根据行星在恒星通过时阻挡的星光比例确定行星的大小。测量行星的质量及其密度是其组成的线索，这一点更加困难。大小为2 R-4的行星已被证明具有广泛的密度，这意味着成分的多样性，但是这些测量结果并未扩展到像地球一样小的行星。在这里，我们报告了对地球大小的开普勒78b行星质量的多普勒光谱测量，开普勒78b每8.5小时将其恒星绕轨道旋转一次（参考文献6）。假设半径为1.20±0.09R⊕，质量为1.69±0.41M⊕，则行星的平均密度为5.3±1.8 g cm ^（-3）与地球相似，表明岩石和铁的成分。
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