A Triple-Planet System Orbiting Upsilon Andromedae

Geoff Marcy's Research Group
S.F. State University and the University of California at Berkeley
December 1, 1999

A diagram of the orbits of the 3 planets around Upsilon Andromedae. The red dots mark the orbits of planets b,c and d. The dashed circles show the orbits of Mercury, Venus, Earth and Mars to give the scale of the orbits.

The Doppler Shift of the Host Star, Upsilon Andromedae: Evidence for Three Planets (Click to enlarge)

Orbital Parameters as of 21 Aug 2000:
Planet Period (d) T_peri (JD-2440000) ecc omega (deg) Vel Amp, K (m/s)  Msini (M_jup) a (AU)
b 4.6171 0.6383 0.02 316.4 70.2 0.69 0.059
c 241.3 153.87 0.24 240.3 58.1 2.05 0.828
d 1308.5 12.9 0.31 241.8 70.4 4.29 2.556

The measured radial velocities for Upsilon Andreomdae are fit with a model containing three companions. The inner planet is subtracted (4.617-day sine wave) to render more visible the wobble caused by the outer two planets.

The inner planet was announced by Butler et al. in Jan 1997. A velocity plot containing a few months of data looks clean ( Fig 1 ), during a few months. However, a larger duration of velocities reveals additional velocity variations. Recognizing that the velocity variations could be due to a second companion, Butler and Marcy alerted the astronomical community to this possibility.

The second companion appeared to have an orbital period of about 3.5 years from the long-term excursions of the velocities. These velocity variations were also found by the AFOE team of Drs. R.Noyes, T.Brown, S.Korzennik, S.Horner, A.Contos, P Nisenson, et al.

A Keplerian orbital model was constructed to fit the data. However, the best solution still left velocity points scattering above and below the 2-planet theoretical curve. After subtracting the theoretical velocities that would be produced by a 2-planet system from the observed velocities, a periodic pattern was observed in the "residual" velocities, suggesting the possibility of one more companion. A 3-planet model was found to produce a dramtically superior fit to the data, reducing the residual velocities to the expected variations, with no periodicity in the residuals.

With the orbital solution for 3 planets that fit our observed velocities, we can subtract the predicted velocities for any one planet, to reveal the velocities caused by the remaining two planets:




    Our collaborators on this work are the AFOE team.

    Images of the Star Field around Upsilon Andromedae.

    Simulated view of Planets around Upsilon Andromedae by Sylvain Korzennik.

    The Published Paper: "Evidence for Multiple Companions to Upsilon Andromedae", ,

    Appeared in the Astrophysical Journal, 1999, v.526, p916, Butler, R. Paul, Marcy, Geoffrey W., Fischer, Debra A., Brown, Timothy M., Contos, Adam R., Korzennik, Sylvain G., Nisenson, Peter, Noyes, Robert W.