The first—51 Pegasi b
(Bellerophon )
Extra Solar Planets
51 Pegasi B
The star
How do we know this
The planet
Star51 Pegasi
ConstellationPegasus
Distance50.9 ± 0.3 ly
(15.61 ± 0.09 pc)
Spectral typeG2.5IVa or G45Va
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
Age6.1-8.1 Gyr
Semimajor axis
(a)
0.0527 ± 0.0030 AU
(7.89 Gm)
Periastron
(q)
0.0520 AU
(7.79 Gm)
Apastron
(Q)
0.0534 AU
(7.99 Gm)
Eccentricity
(e)
0.013 ± 0.012
Orbital period
(P)
Radial velocity of Star
It move toward and away from us with
period of 4.2 days.
4.230785 ± 0.000036 d
(101.5388 h)
Argument of
periastron
(ω)
58°
Time of periastron
(T0)
2,450,001.51 ± 0.61 JD
Semi-amplitude
(K)
55.94 ± 0.69 m/s
Physical characteristics
Minimum mass
(m sin i)
0.472 ± 0.039 MJ
(150 M )
How do we know that?
In practice…its done with spectra
Doppler Effect!
Change in wavelength depends on speed!
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Problem: not all systems are edge on!
We don’t always know the tilt!
Thus the mass measured is a minimum mass
Then there is a bit more physics:
I.
rp + rstar = d
Here mp =
0, we can
can know
exact
mass.
M starVstar = m planet v planet
p2 = (
(conservation of momentum)
4π 2
+m
star
planet
)
)d 3
p = orbital period
d = semi - major axis
G = constant of universal gravitation
II .
Here I = 90..can’t
determine mp
p2 = (
In General, mass listed is
really: M sin i
G(M
4π 2
+m
star
planet
)
)d 3
p = orbital period
d = semi - major axis
p2 = (
G = constant of universal gravitation
p2 = (
M star p 2G
4π 2
so you only need M star , Vstar , and p!
d =3
G( M
4π 2
)d 3
+m
)
star
planet
p = orbital period
d = semi - major axis
G = constant of universal gravitation
Astronomers know star masses
from their spectra and lots of
work from predecessors over
the years!
planet
)
)d 3
IV. vstar =
2πrplanet
2πrstar
, v planet =
p
p
Limits of Radial velocity
measurements
Then a bit of algebra….and
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M star
p
2πG
4π 2
+m
star
G = constant of universal gravitation
III. m p + M star ≈ M star
the result , is :
G( M
p = orbital period
d = semi - major axis
where i is inclination of orbit
m p = Vstar 3
G( M
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 ?
Vstar and p are obtained from the
radial velocity graph!
Hot Jupiters!
Why search around sun like stars?
Here are the first nine planets discovered (as of 1997)
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A more complete list (2000)
A Growth Industry?
So where are we going?
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!
Note 20 multiple planet
systems!
• Use Transit Photometry for edge on systems
(finds smaller planets?)
• Lets see how these methods are working!
Astrometry
Transit Photmetry
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Provides lots of info!
Not as easy as it looks!
Transit of Mercury in 2006
Transit of Venus in 2004
Zoom in…
Check out: http://www.esnips.com/doc/868644b5-3d2d-46f7-849790255b80e3d7
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