Orbit and Mass
When 55 Cancri d was discovered, it was thought to be on a fairly low eccentricity orbit similar to Jupiter in our solar system, though the orbital elements were not well determined. As more data were collected, the best-fit solution for this planet turned out to be highly eccentric, more so than any of the planets in our solar system (including Pluto). In 2008, after a complete orbit of this planet had been observed, the true orbit was revealed, indicating that as had been originally suspected, the planet's 14 year orbit was in fact near-circular, located about 5.77 AU from the star.
A limitation of the radial velocity method used to discover 55 Cancri d is that only a lower limit on the planet's mass can be obtained. In the case of 55 Cancri d, this lower limit was around 3.835 times the mass of Jupiter. In 2004, astrometric measurements with the Fine Guidance Sensors on the Hubble Space Telescope suggest that the planet's orbit is inclined by around 53° with respect to the plane of the sky. If this measurement is confirmed, it implies that the planet's true mass is 25% greater than the lower limit, at around 4.8 Jupiter masses; and it will not be coplanar with the innermost planets e and b (both 85°).
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