Extra Solar Planets. The first—51 Pegasi b ( Bellerophon ). The star. The planet. 51 Pegasi B. Star 51 Pegasi Constellation Pegasus Distance 50.9 ± 0.3 ly (15.61 ± 0.09 pc ) Spectral type G2.5IVa or G4-5Va Mass ( m )1.06 x Mass of Sun Radius ( r )1.237 ± 0.047 Rsun
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Extra Solar Planets
The first—51 Pegasi b (Bellerophon )
51 Pegasi B
Distance50.9 ± 0.3 ly(15.61 ± 0.09 pc)
Spectral typeG2.5IVa or G4-5Va
Mass(m)1.06 x Mass of Sun
Radius(r)1.237 ± 0.047 Rsun
Temperature(T)5571 ± 102 K
Metallicity[Fe/H]0.20 ± 0.0
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?
Change in wavelength depends on speed!
In practice…its done with spectra
Problem: not all systems are edge on! We don’t always know the tilt!Thus the mass measured is a minimum mass
Here mp = 0, we can can know exact mass.
Here I = 90..can’t determine mp
In General, mass listed is really: M sin i
where i is inclination of orbit
Astronomers know star masses from their spectra and lots of work from predecessors over the years!
Vstar 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 period—hard to do for planets more distant than a few AU’s.
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 ?
Here are the first nine planets discovered (as of 1997)
Note 20 multiple planet systems!
Use Astrometry (motion of stars in photographs)
Watch for longer time periods! (its been 12 years now!)
Transit of Mercury in 2006
Transit of Venus in 2004
Check out: http://www.esnips.com/doc/868644b5-3d2d-46f7-8497-90255b80e3d7