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Are there any situations in which the existence of a planet measurably affects its star, aside from orbital wobble? I am curious about possible ongoing affects, not stuff that happened during stellar system formation. For example, I know there was a controversy a few years ago about whether planets cause their host stars to be lithium depleted, but don't know if that was resolved.

This question is inspired by some tangential comments on a physics question "Do solar panels act as an electrical load on the sun?". I very much doubt there are any direct measurable affects of the Earth on the Sun, but I then started to wonder about systems with small stars (e.g. red or brown dwarfs) and very large planets. If their orbit is close enough, I assume the star might slowly strip the planet which could affect the star. Or as was suggested during the lithium controversy, planets might gravitationally affect the internal evolution of their host star. Or maybe there is some process I haven't thought of.

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    $\begingroup$ I remember reading a recent study showing our Sun's magnetic pole flip was apparently caused by alignments of the inner planets. Can't seem to locate it now. $\endgroup$ Commented Nov 14, 2022 at 0:36
  • $\begingroup$ They can cause interference in spectrograpic data in the starlight. And yes they can strip a planet at proximity. $\endgroup$ Commented Nov 15, 2022 at 1:04

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A star and a planet are always subject to mutual tidal interaction.

Usually, it is assumed that close planets are synchronised. This assumption, in fact, is not always justified. A close-in Jupiter or a semimolten terrestrial planet may end up in a pseudosynchronous rotation state, see this work. If the planet is terrestrial and its orbit is noticeably eccentric (say, due to the pull of the external planets), then the planet may share the fate of our Mercury and get trapped in a higher rotation state, permanently or temporarily; see this paper.

Likewise, a close and sufficiently heavy planet may, in principle, synchronise (or, more likely, pseudosynchronise) its host star's rotation -- just like Charon has synchronised Pluto. I emphasise the words ``in principle'' because, to understand how probable such an episode can be, one has to compare the timescales. That is, say, to make sure that the timescale of the stellar spin (pseudo)synchronisation is shorter that the timescale of the planet's orbit shrinking. I am not aware of such (pseudo)synchronised stars ever obsered -- but in principle they may exist.

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