Doppler Planets
51 Peg
- nearby star very similar to Sun
- from Spectral Analysis
- same T, Luminosity, and mass
- starlight shows clear Doppler shifts
- periodic, with period 4.2 days
- nearly perfect “sine” wave means circular orbit
- From Kepler’s III law, the planet is 0.05 AU from star
- much closer than even Mercury to the sun
- amplitude 50 m/s
- That’s the star’s velocity. The planet must be travelling around a circle of radius 0.05 AU every 4.2 d, i.e. about 120 km/s.
- planetary mass > 0.4 MJupiter
- from the ratio of velocities and the mass of the star
- teeter-totter implies the ratio of masses = (ratio of velocities)-1
- if orbit is pole-on, the implied mass is higher