extra solar planets. the first51 pegasi b (bellerophon )

Extra Solar Planets

The first—51 Pegasi b (Bellerophon )

51 Pegasi B

Star51 PegasiConstellationPegasusDistance50.9 ± 0.3 ly(15.61 ± 0.09 pc)Spectral typeG2.5IVa or G4-5VaMass(m)1.06 x Mass of SunRadius(r)1.237 ± 0.047 RsunTemperature(T)5571 ± 102 KMetallicity[Fe/H]0.20 ± 0.0Age6.1-8.1 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) 4.230785 ± 0.000036 d

    (101.5388 h)

Argument ofperiastron

(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⊕)

The planetThe 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!

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 I = 90..can’t determine mp

Here mp = 0, we can can know exact mass.

In General, mass listed is really: M sin i

where i is inclination of orbit

Then there is a bit more physics:

momentum) ofion (conservat

.

planetplanetstarstar

starp

vmVM

drr

I

ngravitatio universal ofconstant G

axismajor -semi d

period orbitalp

3))(

24(2

d

planetm

starMG

p

ngravitatio universal ofconstant G

axismajor -semi d

period orbitalp

3))(

24(2

.

d

planetm

starMG

p

II

ngravitatio universal ofconstant G

axismajor -semi d

period orbitalp

3))(

24(2

d

planetm

starMG

p

p

rv

p

rv planet

planetstar

star

2 ,

2 IV.

starstarp MMm III.

Then a bit of algebra….and

ngravitatio universal ofconstant G

axismajor -semi d

period orbitalp

3))(

24(2

d

planetm

starMG

p

p! and ,V ,M needonly you so4

2

:is,result

starstar

32

2

3

2

GpMd

G

pMVm

the

star

starstarp

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 ?

Hot Jupiters!

Here are the first nine planets discovered (as of 1997)

Why search around sun like stars?

A more complete list (2000)

A Growth Industry?

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!

Astrometry

Transit Photmetry

Provides lots of info!

Not as easy as it looks!

Transit of Venus in 2004Transit of Mercury in 2006

Zoom in…

Check out: http://www.esnips.com/doc/868644b5-3d2d-46f7-8497-90255b80e3d7