exoplanets detection-radial velocity

[Pierebean]

Is it possible to have some informations about this method of detection?

something is not clear:
how the varation of radial velocity of a star around the barycenter of its system can allow us to have informations about the planet orbiting around this barycenter as well?
I am aware than we can deduce from the star orbit the planet orbit, perhaps even the planet mass. However i think that itn be done only with a lot of other informations about the star.

In the case of exoplanet detection, we barely know the star mass. We also do not know the angle "i" between the orbital plan of the star system and the axis of observation.

so after this idiomatic nonsense for which i'm sorry. I would like to ask you simple question:

how can we have accurate informations about an exoplanet with only the radial velocity of its "mother-star"?

[Ynjevi]

how the varation of radial velocity of a star around the barycenter of its system can allow us to have informations about the planet orbiting around this barycenter as well?
I am aware than we can deduce from the star orbit the planet orbit, perhaps even the planet mass. However i think that itn be done only with a lot of other informations about the star. In the case of exoplanet detection, we barely know the star mass.

You're correct, the mass of the star must be known. That is probably one reason why mainly main sequence FGKM stars are targeted, because it is rather easy to deduce the mass of a main sequence star from its brightness, age and metallicity. There are few candidate planets orbiting around evolved stars, and in those cases parameters for the planets are less well constrained.

We also do not know the angle "i" between the orbital plan of the star system and the axis of observation.

That's correct, and it is probably the biggest shortcoming in the radial velocity method.

so after this idiomatic nonsense for which i'm sorry. I would like to ask you simple question:

I am not aware of any nonsense here. One reason why this forum exist is that people can ask question concerning astronomy. And your questions are good.

how can we have accurate informations about an exoplanet with only the radial velocity of its "mother-star"?

Because it is enough -- no other process than stellar pulsations, starspots and such (which can be ruled out) can create similar radial velocity pattern as a planet orbiting star. Only a few observations are not enough, of course, and therefore the star must be studied long enough; the planet must complete at least one full orbit. When the mass of the star and the planet's orbit are known, it is easy to measure the minimum mass of the planet using Kepler's laws. There can't be much ambivalence here.

Because the inclination is not known, there remains possibility that some of the planets are actually too massive to be planets, but brown dwarfs or even stars instead. Again, it is statistically extremely unlikely. In addition, for some reason there are very few brown dwarfs around Sun-like stars in short orbits.

Of course, as the radial velocity method is indirect, one might think that the planets were not real after all for some reason. However, since the first transiting planet was discovered and thus proved to exist, any remaining doubt has vanished.

[Pierebean]

thx for your quick and precise answers.

to be continued...

[Pierebean]

When the mass of the star and the planet's orbit are known, it is easy to measure the minimum mass of the planet using Kepler's laws

How is it possible to have that planet's orbit? does the knowlegde of {radial velocity}=f(t) (f a sinus-like fonction I assume) allows that?

[maxim]

The orbital period is also known, isn't it?

maxim

[Pierebean]

The orbital period is also known, isn't it?

if it is so, then how? does the spectrometric detection of radial velocy method give that orbital period?

[selden]

They've been watching the changes in radial velocity of many stars for several years, now. The periods of the exoplanets' orbits are observed as the radial velocities change while the planets move around their suns. New planets are announced when they've accumulated enough data samples to be able to specify the orbital parameters fairly accurately. As the data accumulates over the years, they're able to measure the periods of planets with larger and larger orbits.

[Pierebean]

I heard that when you had that kind of data:

you were able to determine from P (also called T) the distance,r , between the exoplanet and its star (we assume it's an circular orbit). We have to use the keplerian law:

rp= ( (T^2*G*M)/(4*Pi^2) )^(1/3)

with G=6,67E-11 IU
T=P*24*3600 s
M=the mass of the star (it is supposed to be an sun-like star)
Pi=well, it's Pi


Then to have the mass of the planet we proceed like that:

after a long calculation i've got:


mp=(T*K*M) / ( ( T^2*G*M)^(1/3) -T*K )

(you will have to trust me on that formula)


with:
T= still the period in second
G=the same
M= the same
K= the amplitude of the velocity in m.s-1


On internet from sure sources i've got the distance between 51 pegasi b and its star (0,05 AU) , I also have its mass (0,47*Mj )(Mj is the mass of jupiter).

My probleme is, with the data up there, I can't find out 0,05 AU and 0,47*Mj

let me explain:

if I take 2E29 kg for the mass of the star, i've got :
0,053421655 AU (good)
and 0,178244783*Mj (too low)

however if I take 8,55E29 kg for the mass of the star i've got:
0,086702053 AU (too much)
0,469200978*Mj (good)

In conclusion, I can't refind the data i've got. Did I forget something?