# Plenty of wet, rocky planets

by marcus
Tags: planets, plenty, rocky
 Astronomy Sci Advisor PF Gold P: 23,233 http://arxiv.org/abs/astro-ph/0209383 "Much indirect evidence suggests that wet, rocky planets are common." fairly recent article by Lineweaver (September 2002)
 P: 333 Hmm, interesting. Just two questions for now. What does he mean by the i in M*sin(i)? And what's the signifigance of M*sin(i). Afraid I've not come across this before.
Astronomy
PF Gold
P: 23,233
 Originally posted by Lonewolf Hmm, interesting. Just two questions for now. What does he mean by the i in M*sin(i)? And what's the signifigance of M*sin(i). Afraid I've not come across this before.
this is a standard term in the exoplanet search (exosolar planet search) where i is the inclination of the orbit relative to the observer

in most cases the planet is detected by wobble, changes in the radial speed of the star, by doppler effect

If the observer is in the plane of the star and planet two-body motion (inclination = 0) then one measures the whole wobble

if the inclination is 90 degrees one wouild not see any wobble

in general the inclination is unknown and one sees a fraction of the wobble which is sine(inclination).

therefore one systematically underestimates the mass of the planet, in fact ones estimate of the mass is only sin(i) times the mass, where the inclination is an unknown number between zero and one

and so the tables show sin(i)M because one cannot actually say anything about M itself

as parttime astronomer you will see that the mass of the star itself is told by the color etc. -----so knowing the period will tell the distance from the star and knowing the wobblespeed and the distance will tell the mass of the planet----but one only knows sine(inclination) X wobblespeed

EXCEPT in a few cases where the planet actually passes in front of the disk of the star and notches the lightcurve----there are a few of these inclination=0 cases where one has two handles on the problem

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