relativity is expected to have strong effects on planet b, because it orbits at a distance of just ~0.05 AU from the parent star. The apsides of planet c and d, instead, oscillates with time; the orbit of Upsilon Andromedae c thus returns to a nearly circular state every 9,000 years. The eccentricity of those planets may have arisen from a close encounter between the outer planet and a fourth planet, with the result that the third planet was ejected from the system or destroyed. Such a mechanism could have been triggered by perturbations on the orbit of the companion star, which arise from close encounters with other stars and from the tidal field of the Milky Way. The orbits of the two inner planets seems to be shaped by tidal interactions, while the evolution of c and d orbits is secular. The existence of further planets too small or distant to detect has not been ruled out, though the presence of Jupiter-mass planets as close as 5 AU from Upsilon Andromedae A would make the system unstable. However, a potential fourth planet (Upsilon Andromedae e) was discovered in 2010. This planet seems to be in a 3:1 resonance with Upsilon Andromedae d. Subsequent studies in 2011 and 2014, while finding some evidence for a fourth planet, found large inconsistencies in the estimated orbital period of Upsilon Andromedae e depending on what dataset was used, suggesting that the apparent planetary signal is more likely to be an instrumental artifact. Upsilon Andromedae does not appear to have a circumstellar dust disk similar to the Kuiper belt in the Solar System. This may be the result of perturbations from the companion star removing material from the outer regions of the Upsilon Andromedae A system. See also 51 Pegasi Kepler-56 Lists of exoplanets PSR 1257+12 Notes References External links HR 0458 CCDM 01367+4125 F-type main-sequence stars 2 Andromedae, 50 Andromedae, Upsilon 007513 0458 009826 M-type main-sequence stars Durchmusterung objects Binary stars 0061 J01364784+4124200 Planetary systems with three confirmed planets