Extrasolar planets and the rotation and axisymmetric mass‐loss of evolved stars

I examine the implications of the recently found extrasolar planets on the planet‐induced axisymmetric mass‐loss model for the formation of elliptical planetary nebulae (PNe). This model attributes the low departure from spherical mass‐loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of orbital angular momentum of the planets. Since about half of all PNe are elliptical, i.e., have low equatorial to polar density contrast, it was predicted that about 50 per cent of all Sun‐like stars have Jupiter‐like planets around them, i.e., a mass about equal to that of Jupiter, M J , or more massive. In the light of the new findings that only 5 per cent of Sun‐like stars have such planets, and a newly proposed mechanism for axisymmetric mass‐loss, the cool magnetic spots model, I revise this prediction. I predict that indeed ∼50 per cent of PN progenitors do have close planets around them, but the planets can have much lower masses, as low as ∼0.01 M J , in order to spin‐up the envelopes of AGB stars efficiently. To support this claim, I follow the angular momentum evolution of single stars with main‐sequence mass in the range of 1.3–2.4 M ⊙ , as they evolve to the post‐AGB phase. I find that single stars rotate much too slowly to possess any significant non‐spherical mass‐loss as they reach the upper AGB. It seems, therefore, that planets, in some cases even Earth‐like planets, are sufficient to spin‐up the envelope of these AGB stars for them to form elliptical PNe. The prediction that on average several such planets orbit each star, as in the Solar system, still holds.