51 Pegasi B Star51 Pegasi ConstellationPegasus DistanceDistance50.9 ± 0.3 ly (15.61 ± 0.09 pc)lypc Spectral typeSpectral typeG2.5IVa or G4- 5Va MassMass(m)1.06 x Mass of Sun RadiusRadius(r)1.237 ± 0.047 Rsun TemperatureTemperature(T)5571 ± 102 KK MetallicityMetallicity[Fe/H]0.20 ± 0.0 AgeAge6.1-8.1 GyrGyr Semimajor axis(a)(a) 0.0527 ± 0.0030 AU (7.89 Gm)AUGm Periastron(q)(q) 0.0520 AU (7.79 Gm)AUGm Apastron(Q)(Q) 0.0534 AU (7.99 Gm)AUGm Eccentricity(e)(e)0.013 ± 0.012 Orbital period(P)(P)4.230785 ± 0.000036 dd (101.5388 h)h Argument of periastron (ω)(ω)58° Time of periastronperiastron(T0)(T0)2,450,001.51 ± 0.61 JDJD Semi-amplitudeamplitude(K)(K)55.94 ± 0.69 m/sm/s Physical characteristics Minimum mass(m sin i) 0.472 ± 0.039 M J (150 M )M JM The planet The star
How do we know this Radial velocity of Star It move toward and away from us with period of 4.2 days.
How do we know that? Doppler Effect! Change in wavelength depends on speed!
Problem: not all systems are edge on! We dont always know the tilt! Thus the mass measured is a minimum mass Here I = 90..cant determine m p Here m p = 0, we can can know exact mass. In General, mass listed is really: M sin i where i is inclination of orbit
Then a bit of algebra….and Astronomers know star masses from their spectra and lots of work from predecessors over the years! V star and p are obtained from the radial velocity graph!
Limits of Radial velocity measurements Star surfaces move up and down about 1 m/s, so this is smallest practical speed for star. Big Planet close to small star creates the biggest wobble, so we can see these most easily. To see a complete wobble, we need to watch for one periodhard to do for planets more distant than a few AUs. Earth Makes sun move about 1 cm/s, so this would be lost in the Noise of the sun if someone was trying to detect us! So guess what we found around sun like stars ?
Hot Jupiters! Here are the first nine planets discovered (as of 1997)
So where are we know (2010) Note 20 multiple planet systems!
So how are they finding smaller or more distant planets? Use Astrometry (motion of stars in photographs) Watch for longer time periods! (its been 12 years now!) Improve precision of methods (techology continues to improve) Gets lots more people doing it! Search around smaller stars! Use Transit Photometry for edge on systems (finds smaller planets?) Lets see how these methods are working!