in an orbit that is close to edge-on. Between them, no measurement of c's nor f's inclinations have been made. It had been thought that with five planets, the system cannot deviate far from coplanar in order to maintain stability. An attempt to measure the spin-orbit misalignment of the innermost planet reported that it was in a nearly polar orbit, but this interpretation of the data has since been challenged by a subsequent study, with noted inconsistencies between the implied and measured stellar rotation. The approximate ratios of periods of adjacent orbits are (proceeding outward): 1:20, 1:3, 1:6, 1:20. The nearly 1:3 ratio between 55 Cancri b and c is apparently a near resonance, rather than a genuine mean motion resonance. Between planets f and d, there appears to be a huge gap of distance where no planets are known to orbit. A 2008 paper found that as many as 3 additional planets of up to 50 times the mass of Earth could orbit at a distance of 0.9 to 3.8 AU from the star, and stable resonances of a hypothetical planet g with the known planets were found to be 3f:2g, 2g:1d, and 3g:2d. A study released in 2019 showed that undiscovered terrestrial planets may be able to orbit safely in this region at 1 to 2 AU; this space includes the outer limits of 55 Cancri's habitable Zone. In 2021, it was found that terrestrial planets with comparable water content to Earth may have indeed been able to form and survive between the planets f and d. As for the space outside d's orbit, its stability zone begins beyond 10 AU, though there is a stability zone between 8.6 and 9 AU due to a 2:1 resonance. Communication A METI message was sent to 55 Cancri. It was transmitted from Eurasia's largest radar—the Evpatoria Planetary Radar. The message was named Cosmic Call 2; it was sent on July 6, 2003, and it will arrive at 55 Cancri in May 2044. See also Sudarsky's gas giant classification Cancer in Chinese astronomy Kepler-186 Lists of exoplanets Gliese 1132 b – rocky exoplanet with a confirmed atmosphere Mu Arae c – At constellation Ara Planetary system PSR B1257+12 References Further reading External links When the Gods Fall 55 Cancri in fiction. Extrasolar Planet Interactions by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona The First and the