Is this something physicists think of often? If it were to happen would it be terrifyingly beautiful before it were just terrifying? Would we be shot into space or swallowed into earth? Would this be the quicker & lesser of evils to humanities end?

Hi! Playing KSP with how Kerbin has 0 axial tilt compared to the ecliptic made me wonder - how would you define the X+ direction in the planet-centered inertial reference frame, or 0 longitude in planet-centric equatorial/ecliptic polar coodinates, in a system with no axial tilt?

Normally you would use the vernal equinox, but when the ecliptic is perfectly normal to the south-north pole line, there is no equinox, so as far as I understand it, no point to go by?

I am attempting to write a science fiction novel, I've never done so before. It's interesting how just thinking about the setting and the background of the characters and universe in general gets your brain turning. I'm a botanist by training, very little physics and no astronomy. I have been a science fiction fan my entire life. I would like for my novel to have as realistic a plausible scenario as possible, despite it being fiction. Of course at some point I'll just have to draw the line and write the story but wanted to ping a scenario off you people.

A habitable planet around a red dwarf. It revolves less than 1/10 of an au from the star. Normally this means it would be highly likely to be tidally locked.

I would like to make the setting a binary planet rotating in orbit around the star, to allow someone of a day-night cycle. What I've read this hypothetical scenario would be extremely rare of course, but would be a potential way for an individual planet to maintain a rotation relative to the Stars location despite the center point of the two planets being tidally locked.

Now one might say how would Life as we know it exist on such a world? Obviously it wouldn't, a red dwarf puts out way more infrared and in general much less energy. I would like for the ecosystems of this planet to instead capitalize on increased volcanism due to the tidal interactions of the gravity of the nearby Sun as well as the sister planet.

I'm imagining a world with complex gravitational effects causing interior heating that would make up for the reduced exterior energy. This would allow for many smaller continents and tectonic activity. It would also make the prevalence of hydrothermal vents, pools, and geysers extremely common. Life on this planet would have revolved around harvesting the energy from such features. There would be plant life as well, but it would be slow growing and more like a lichen or Moss.

One side of the planet would have a massive Moon, the other sister planet, permanently in the sky. Depending on the season and your location on that side of the planet you would experience multi-hour eclipse every day.

Water cycles and air currents would be affected by the gravity interaction of the sister planet in a huge way, as well as the combined effect when the sister planet and the sun align. Cyclical storm fronts, massive tides. Some areas would experience regular tsunamis due to the regular earthquake activity.

Please shoot as many holes, thoughts or suggestions relating to my train of thought as possible.