Apsidal alignment in v Andromedae

One of the parameters fitted by Doppler radial velocity measurements of ext rasolar planetary systems is omega, the argument of pericenter of a given p lanet's orbit referenced to the plane of the sky. Curiously, the omega 's o f the outer two planets orbiting upsilon Andromedae are presently nearly id entical : Delta omega equivalent to omega (D) - omega (C)= 4.degrees8 +/-4. degrees8 (1 sigma). This observation is least surprising if planets C and D occupy orbits that are seen close to edge-on (sin i(C), sin i(D) greater t han or similar to 0.5) and whose mutual inclination Theta does not exceed 2 0 degrees. In this case, planets C and D inhabit a secular resonance in whi ch omega librates about 0 degrees with an amplitude of similar to 30 degree s and a period of similar to 4x10(3) yr. The resonant configuration spends about one-third of its time with \ Delta omega \ less than or equal to 10 d egrees. If Theta greater than or similar to 40 degrees, either Delta omega circulates or the system is unstable. This instability is driven by the Koz ai mechanism, which couples the eccentricity of planet C to Theta to drive the former quantity to values approaching unity. Our expectation that Theta less than or similar to 20 degrees suggests that planets C and D formed in a flattened, circumstellar disk, and this may be tested by upcoming astrom etric measurements with the FAME satellite.