# What does and doesn't work with this planetary system around a red dwarf?

So I have the following scenario for a story, and I want to know if it runs into any ridiculous no-nos from a scientific point of view:

There is an earth-like world orbiting a red dwarf (similar to AD Leonis, though it is not tidally locked. It doesn't have to be hospitable but does have to be habitable (there will be other factors drawing humans to this world). It's much colder than earth, maybe only habitable in a narrow band around the equator, and with a lower gravity (say 0.8 g, somewhere in that neighbourhood).

Smaller worlds are more likely to be in tight orbits around a red dwarf from what I understand so it'll be the third planet in the system with an orbital period of ~190 (earth) days (close enough to half an Earth year to make it easy to count). Days would be a bit longer, maybe 29 hours or so.

The most important parts are that it is possible to survive, if not exactly comfortably, outdoors, and that reaching orbit is at worst as much work as it is here on Earth. A smaller planet with lower gravity (assuming close to as dense an atmosphere as Earth) should make it a little easier to get things into space.

Here is the rest of the system (briefly), from star and going outwards:

• 1 hot molten (lava) world
• 1 cold rocky world
• 1 Earth-like world (described above)
• 1 gas giant
• 1 gas giant
• 1 cold gas world (Neptune-like)
• 1 tiny rock-world (Pluto analog)
• Cometary halo and asteroid belt
• Hi, LeftLiner, welcome to Worldbuilding Stack Exchange! Would you mind adding in some more details about the planets - perhaps masses and orbital radii, as well as some properties of the star (mass, luminosity, etc.)? That would make it much easier to answer your question. Thanks. Oct 15, 2019 at 14:22
• Thank you! Well, I'll be honest I don't have that kind of detail thought out - though I can give you the data of the star since I'm using AD Leonis for reference: en.wikipedia.org/wiki/AD_Leonis Oct 15, 2019 at 14:29
• I am not certain that a habitable planet (thus in the habitable zone of its star) could orbit a red dwarf star with a year 190 Earth days long. It is possible that even the farthest orbit in the habitable zone of even the brightest red dwarf might be less than 190 Earth days long. And you need to think about possible reasons for the planet to not be tidally locked to its star. Oct 15, 2019 at 14:45
• That was exactly what I needed to hear. I can fix some of those, but not all, so I think I need to go back to the drawing board on this. Thank you very much. Oct 15, 2019 at 19:45

• A lack of tidal locking. Planets around red dwarfs are likely to be tidally locked, for two reasons. The timescale for tidal locking scales as $$\tau\propto a^6/M^2$$, where $$a$$ is the planet's semi-major axis and $$M$$ is the mass of the star. If you have a planet with a tight orbit around a low-mass star, as we do here, the planet will be tidally locked quite quickly, possibly long before life has a chance to arrive.
To mitigate this, you might consider placing the giant planets in orbits far from the star, away from the other planets, to prevent instabilities. Similarly, the Earth-like planet could also have a large semi-major axis, to increase the tidal locking timescale (even doubling $$a$$ increases the timescale by a factor of 64); unfortunately, this would then make the planet colder and uninhabitable. I don't have a great solution to the issue of gas giant formation, but perhaps some external source of matter could be responsible.