Announced in 1996, 55 Cancri b was the fourth extrasolar planet discovered around a Sun-like star. It was also the second "Hot Jupiter" discovered, after 51 Pegasi b. Although other Hot Jupiters have orbital distances less than 0.05 AU and estimated temperatures over 1000 Kelvin, 55 Cancri b is less extreme. Its mean distance of 0.115 AU and its cooler star give planet b an estimated temperature of only about 700 Kelvin. Assuming a system inclination of 53°, the planet has a mass 0.982 times that of Jupiter, and estimates of the level of stellar heating suggest that its atmosphere is puffed up to give the planet a radius of about 1.2 times that of Jupiter. Recent observations have indicated that 55 Cancri b might be in or near a 3:1 resonance with Planet c. More interesting is the effect Planet b may have had on its inward sibling, the Neptune mass 55 Cancri e. Planet b is the most massive of the three inner planets, and would have been the first to open a gap in 55 Cancri's protoplanetary disk and begin migrating in towards the star. If this happened before Planet e could reach the mass required for rapid growth into a gas giant, it could have been carried along by Planet b's migration, placing it in a star hugging orbit before it had the chance to grow past the rocky core phase.
55 Cancri b is likely to appear as a nearly featureless deep blue ball of haze. The planet orbits in a temperature range too hot for clouds of water ice to form near the surface, yet too cool for silicate clouds. Without clouds to obstruct our view, we would be able to look deep into the clear atmosphere. But even a cloudless atmosphere is not completely transparent. Rayleigh scattering gives a thick enough region of atmosphere a blue hue. This is the case when we look up into the the daytime sky on Earth, and this would also be the case when we look down into the cloudless atmosphere of 55 Cancri b. Given the close distance to 55 Cancri, Planet b may be tidally locked, or else have a slow rotation. If this is the case, the planet would have little or no magnetic field. Close in planets have been associated, theoretically, with superflares on their host stars. But for this to work, the planet in question would need a powerful Jupiter-like magnetic field that could entangle and reconnect with that of the star. If planet b's rotation is locked, its weak magnetic field and its comparative distance (far for a Hot Jupiter) would make it an unlikely candidate for inducing superflares. If 55 Cancri b had moons after its formation, tidal locking or slow rotation make it unlikely that those moons have survived. However, it is possible that the planet may have a collection of captured asteroids in irregular orbits, similar to the outer moons of Jupiter.
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