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I am currently working on a world orbiting around a red dwarf star (M0V to M2V, I haven't decided yet) and I need some help with designing the solar system. My original plan was having 4 planets orbiting the star:

  • 1st planet: a small rocky planet the size of Mars, very close to the star, a lot of volcanic activity, thick atmosphere (basically a smaller Venus)

  • 2nd planet: an Earth-like planet with a surface gravity of 1.5 g. Habitable, orbits in the star's goldilocks zone, has a composition of elements similar to Earth (rich in metals and minerals, Earth-like atmosphere, liquid water) tidally locked

  • 3rd planet: a gas giant about the size of Saturn with 5 named moons (I say named because only 5 of them are big enough to be spherical, there are several other moons that resemble meteorites)

  • 4th planet: an ice giant similar in size and chemical composition as Uranus with 3 named moons

  • Asteroid belt: a collection of asteroids and other dwarf planets at the outskirts of the solar system

From what I understand planets form from the debris disk that surrounds the star at the start of their life cycle. But red dwarfs are considered to be the smallest and dimmest of stars. Would their debris disk have the amount of elements required for the formation of these planets?

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  • $\begingroup$ Multiple questions. Please ask one question per question. $\endgroup$
    – Zeiss Ikon
    Aug 3, 2022 at 12:01
  • $\begingroup$ Perhaps you should edit your question, specifically reducing that last paragraph to contain a single focus. It seems to me that you are really getting at: can a red dwarf star form this solar system? $\endgroup$
    – PipperChip
    Aug 3, 2022 at 15:19
  • $\begingroup$ They didn't necessarily have to form that way. Could be there was another star originally in there and it got "kicked out", perhaps by another passing star or mutually with another gas giant or etc. It's speculated that our own solar system started with a gas giant in near orbit and it got kicked out, which is why we ended up with only enough in the inner system for a few small, rocky planets. Solar systems can have a lot of chaos before they settle down. I don't even think an explanation of "how" is necessary. Unless there's something gravitationally wrong with your model, it's plausible. $\endgroup$
    – JamieB
    Sep 22, 2022 at 18:24

2 Answers 2

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Believable

Red dwarf stars have mass of about 0.08-0.45 solar masses. But one solar mass is about 333,000X the mass of Earth. Even at their smallest average, a red dwarf is 26,600X the mass of Earth. it seems beyond believable that there could be enough mass in the star's debris field to create planets.

Even Jupiter is just 0.001 solar masses or 1/80th the mass of the smallest red dwarf.

Wait... you didn't use the word "plausible."

I'm not a fan of "plausible" or "realistic." The universe is constantly showing us wonders that "just last year" we didn't think were possible. Besides, who's the audience for your efforts? PhD Astronomers? That's why I prefer the word "believable." Even if our current knowledge base about red dwarf stars suggests none have planets, the basic statistics say there could be planets. And all that means is we need more time to find the so-called exception that breaks the rule.

So, the only real issue here is just how big is you red dwarf? The larger it is, the more believable your solar system becomes.

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  • $\begingroup$ You mention there is not enough mass, maybe a combination of removing the first planet and downgrading the gas giant to a smaller ice giant could make the planet more believable without changing the star? $\endgroup$
    – PeriodicParticle
    Aug 5, 2022 at 20:27
  • $\begingroup$ @PeriodicParticle On the contrary, my answer says, "it seems beyond believable that there could be enough mass in the star's debris field to create planets." However, you're correct that the smaller the planets are the more believable the situation becomes just as increasing the star size also increases believability. $\endgroup$
    – JBH
    Aug 5, 2022 at 23:52
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The answer is: your setup is definitely plausible.

I have a personal story about this question. Back in 2007 I wrote a scientific paper (I'm an astrophysicist) arguing that very low-mass stars might not have enough material to form Earth-mass planets, as you speculate. (Link to paper here).

Well, turns out that I was wrong: there are lots and lots of Earth-sized or larger planets around even the smallest stars. There's actually more (by number and total mass) than around Sun-like stars! The most extreme example is the Trappist-1 system, with seven roughly Earth-sized planets around a super-puny star. (The main reason for this, we think, is that a lot of stuff migrates inward close to the star, providing plenty of material from which to build planets, even around small stars -- see here for a description of our model for how they form).

This image shows some of the exoplanet candidates in the habitable zone, and you can see that there are plenty around small red stars.

The only part of your story that is less likely is the asteroid belt. Given how close they orbit to their host stars, collisions are generally more energetic around small stars and belts of asteroids grind down and become pretty puny (like our own) really fast. So your system could have an asteroid belt, but it should be pretty depleted.

Does this all make sense?

enter image description here

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