RADIAL VELOCITY PLANETS DE-ALIASED: A NEW, SHORT PERIOD FOR SUPER-EARTH 55 Cnc e

Abstract

Radial velocity measurements of stellar reflex motion have revealed many\nextrasolar planets, but gaps in the observations produce aliases, spurious\nfrequencies that are frequently confused with the planets' orbital frequencies.\nIn the case of Gl 581 d, the distinction between an alias and the true\nfrequency was the distinction between a frozen, dead planet and a planet\npossibly hospitable to life (Udry et al. 2007; Mayor et al. 2009). To improve\nthe characterization of planetary systems, we describe how aliases originate\nand present a new approach for distinguishing between orbital frequencies and\ntheir aliases. Our approach harnesses features in the spectral window function\nto compare the amplitude and phase of predicted aliases with peaks present in\nthe data. We apply it to confirm prior alias distinctions for the planets GJ\n876 d and HD 75898 b. We find that the true periods of Gl 581 d and HD 73526\nb/c remain ambiguous. We revise the periods of HD 156668 b and 55 Cnc e, which\nwere afflicted by daily aliases. For HD 156668 b, the correct period is 1.2699\ndays and minimum mass is (3.1 +/- 0.4) Earth masses. For 55 Cnc e, the correct\nperiod is 0.7365 days -- the shortest of any known planet -- and minimum mass\nis (8.3 +/- 0.3) Earth masses. This revision produces a significantly improved\n5-planet Keplerian fit for 55 Cnc, and a self-consistent dynamical fit\ndescribes the data just as well. As radial velocity techniques push to\never-smaller planets, often found in systems of multiple planets,\ndistinguishing true periods from aliases will become increasingly important.\n