摘要：We present spectroscopic measurements of the Rossiter–McLaughlin effect for the planet b of the Kepler-9 multitransiting planetary system. The resulting sky-projected spin–orbit angle is λ=−13°±16°, which favors an aligned system and strongly disfavors highly misaligned, polar, and retrograde orbits. Including Kepler-9, there are now a total of four Rossiter–McLaughlin effect measurements for multiplanet systems, all of which are consistent with spin–orbit alignment.
