Complex sentence: On 29 September 2010, astronomers at Keck Observatory detected two additional planets, Gliese 581 f and Gliese 581 g, both in nearly circular orbitsbased on analysis of a combination of data sets from the HIRES and HARPS instruments.
Split into simpler sentences: On 29 September 2010, astronomers at Keck Observatory announced the discovery of Gliese 581 f and Gliese 581 g, both in nearly circular orbits. The detection was based on analysis of a combination of data sets from the HIRES and HARPS instruments.