Is the net angular momentum vector of our solar system pointing in roughly the same direction as the Milky Way galaxy's net angular momentum vector? If yes or no, is that common for most stars in the galaxy?
[Physics] Do solar systems typically spin in the same direction as their galaxy
angular momentumastrophysicsgalaxiessolar systemstars
Best Answer
There is no alignment between the Sun or the Solar System's net angular momentum and the "spin axis" of the Galaxy. Think for a moment about whether the line of the ecliptic (which marks the "equatorial line" of the Solar System) and the Milky Way (which roughly marks the plane of the Galaxy) are lined up? If this were so, then you would always see the planets (Jupiter, Mars, etc.) projected against the Milky Way. In fact, the spin axes of the Solar System and Galaxy are inclined at an angle of 63 degrees with respect to each other (see cartoon below - note, the Solar System is not drawn to scale compared with the Galaxy!).
We do not know much about the alignments of other solar systems. Both the Doppler shift discovery method and the transit discovery method have a rotational ambiguity about the plane of the exoplanets' orbits. In other words, if we were to observe a transiting planet, we know that the inclination is close to 90 degrees to the line of sight, but we could rotate the system around our line of sight by any angle, and would see the same observational signatures.
The general assumption is that there is no relationship between the angular momentum directions of stars (and their planetary systems) and the Galaxy. Turbulence in molecular clouds on relatively small scales compared with the dimensions of the Milky way randomises the angular momentum vectors of collapsing prestellar cores. A possible alignment mechanism could come about through the threading of giant molecular clouds by the Galactic magnetic field.
If we knew what fraction of stars had close-in, potentially transiting planets, we could use the numbers of detected transiting exoplanets in the Kepler field to say whether that number was consistent with random orientations or not. Alternatively, if we had another Kepler field pointing in a different Galactic direction, but with similar sensitivity to the original Kepler field, then the relative numbers of detected transiting planets in the two fields might tell us of any non-random orientations. For example, if orbital planes were all aligned with the Galactic plane, then no transits would be seen for any star viewed out of the Galactic plane. (I think this extreme possibility can already be ruled out.)