A Triple-Planet System Orbiting 55 Cancri

Marcy et al. 2002, ApJ 581, 1375

Above: Artist's conception by Lynette Cook, showing the star 55 Cnc in the distance with the innermost two planets visible as tiny points. In the foreground is a hypothetical moon orbiting the large outer planet. From Earth, this planetary system is 70,000 times more distant than Pluto.

Revised 25 June 2002

Orbital Parameters and Planet Mass
Lick Observatory

Data provided by California & Carnegie Planet Search Team

Latest Orbital Parameters
Planet Period Tperi (JD-2450000) ecc omega (deg) Vel Amp, K (m/s) M sin i (Mjup) a (AU)
b 14.653 d 1.479 .02 99 72.2 0.84 0.115
c 44.28 d 31.4 .34 61 13.0 0.21 0.24
d 5360 d 2785 .16 201 49.3 4.05 5.9

The inner planet was announced by Butler et al. in January 1997. Analysis of extended velocity measurements reveals additional velocity variations.


Two-Planet Fit:

A Two-Keplerian model (solid line) is fit to the measured velocities. Here the velocity wobble caused by the outer planet has been removed for clarity, leaving the wobble caused by the inner planet. These residual velocities (dots) are phased at the 14.65-d period of the inner planet, showing the significant reduction in the residuals compared with the one-planet model. By including a second planet in the model, the RMS of the residuals dropped from 39.5 m/s to 11.6 m/s.

One-Planet Fit:

Velocities for 55 Cancri, phased at the best-fit period of 14.65 d. The dashed line represents the best-fit (single-planet) keplerian. The RMS of 38 m/s is well above the typical velocity errors of 3-10 m/s, showing the inadequacy of a single-planet model.
Note: Assumes Mstar=0.95. The outer planet is 0.5 arcsec from the star, making it the best extrasolar planet for direct imaging by a coronograph.
Stellar Characteristics
Spectral Type Mass (MSun) Apparent magnitude Distance (pc) Prot(d) [Fe/H]
G8V 0.95 5.95 12.53 35-42 0.27

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