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I'm working on a fantasy story set in a slightly modified version of the Trappist-1 system (all orbits are perfectly resonant rather than only nearly so), and I am trying to get as good of an understanding of the various phenomena that might occur in that system which would affect an alien civilization on the potentially habitable worlds of said system. One of the things that has crossed my mind is the potential tidal affects the nearest planets might have on their parent star, Trappist-1, and how that might affect stellar brightness. My basic idea is that the tidal forces, particularly of the innermost planet, might cause the hotter, lighter, more luminous material to rise into the photosphere leading to those planets experiencing slightly more sunlight then other regions 90 degrees or so from their current position. All of the planets will probably do this to some extent, but only the 2 or 3 innermost to any notable extent. This would also mean that the outer planets would experience unusually bright days when they transit with one of the inner planets (or are opposite to them?), which s very good for worldbuilding if these bright days are notably warmer or brighter as they could have cultural significance on the outer worlds.

My basic question is whether or not this might actually occur in the given real system or a slightly modified form of it. I am thinking it might be possible due to the significantly high mass of each of the seven worlds, the fact that the star is relatively low mass, and how close some of the planets orbit. Is this actually possible?

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  • $\begingroup$ It's worth remembering that the sun contains 99.8% of the mass of the solar system, and this is not atypical based on the exoplanets we've found. Unless something unusually massive is right next to its star, resultant tides are going to be things determined by careful solar observation, not personal day-to-day observations. $\endgroup$
    – jdunlop
    May 25 at 17:39

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We do expect tidal interactions to increase a star's luminosity in certain cases, but not quite through the mechanism you propose. Particularly in the case of massive exoplanets in tight orbits, like hot Jupiters, we expect tidal bulges to form on the star, just like those created on Earth by the Sun and Moon (Saar & Kuntz 2001, Vidotto 2019). The expansion and contraction of these bulges dissipates energy, leading to what you could refer to as non-radiative energy generation.

The same setup might produce heating through other mechanisms - for instance, expanding the effects of waves in the outer photosphere, or increasing magnetic field strength leading to magnetic heating. All of these effects depend on the motion of the planet, with tidal effects repeating twice per orbital period and magnetic effects repeating once per orbital period.

Could this happen in the TRAPPIST-1 system? I'd bet against it. There is the issue that the planets are quite low-mass, much less than 1% or so the mass of the hot Jupiters we expect to produce significant heating. I don't think we'd see something as dramatic as the effect you're looking for. It would likely be detectable by careful observations, but probably not by someone stepping outside and noticing a temperature change.

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I would imagine that if a planet is close enough to a star to cause tides on the star then it's probably going to disintegrate from the heat. You might consider other options - rotating star, solar flares, solar wind being blocked by other bodies, binary stars, ...

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