# I've determined how many planets my solar system could plausibly have. How do I figure out what kind of planets they are?

The 12 planets in my solar system orbit the G-type star of Nemo. It's about nine-tenths of the size of the sun, and has a stellar luminosity of 0.67 L☉, a diameter of 1.28 million kilometres, a surface temperature of 5,980 degrees Kelvin and (Starting from its birth) will live for 13 billion years.

I mentioned that there are twelve planets in this system. The 4th one from Nemo is habitable and the 8th one is the system's largest gas giant. The respective distances of all these planets from Nemo are shown below.

Starting from Nemo's solar mass, I was able to work my way up and build the solar system to that point, maintaining plausibility. Now that I know how many planets there are and where they are, how do I know what kind of planets they are?

Which ones are gas giants, which ones are rocky, how big are they? Are some clouded over like Venus?

These are all questions I need to answer to complete the construction of the solar system and move on to the habitable world. Here are the distances of the planets from the star:

Inner limit: 0.09 AU,

First planet: 0.17 AU,

Second planet: 0.32 AU,

Third planet: 0.45 AU,

Habitable zone inner limit: 0.72 AU, Fourth planet: 0.86 AU,

Habitable zone outer limit: 1.03 AU,

Fifth planet: 1.63 AU,

Sixth planet: 2.30 AU,

Seventh planet: 3.30 AU,

Frost line: 3.94 AU,

Eighth planet (Gas giant): 4.95 AU,

Ninth planet: 6.97 AU,

Tenth planet: 9.98 AU,

Eleventh planet: 15.96 AU,

Twelfth planet: 28.1 AU,

Outer limit: 36 AU

Sorry, that was probably a slog to read through, but I thought I'd include those statistics should they be important for answering this question.

Any other figures you require about the star Nemo are shown in the first paragraph.

Now, the question in full: How do I figure out what kind of planets these bodies are, or is it even possible? What factors influence this; distance, temperature, or something else? Are there any other important things to consider? You don't have to build the whole system for me, I'd be fine with just equations to do it myself.

I am not an astrophysicist, nor am I a wizard of mathematics. If you're including very complex terminology in an answer which I can't find the definition of without trawling through a Wikipedia page, then it'd be appreciated if you could explain them. Also, I'm just as capable of performing (But not always forming) equations myself, but please present equations (If they are relevant) in a way that would be comprehensible to the layman (Who can do maths and knows a lot about biology).

If you need more details about the star and its system, please ask me and I will add that in and notify you.

If you have objections of implausibility, please put them in the comments - and as soon as possible. While the project is young is the best time to point out weaknesses.

If you think this question is broad, unclear, or off-topic, please say so, and I will amend the question accordingly and promptly.

If you feel the need to downvote the question, by all means do, but I beg that you say why. Downvoting without critiquing isn't very helpful.

• @FoxElemental They'd only be important to answerers, and I'm not even sure if they are, in addition to being not the easiest thing to read. If you think those aren't good reasons, though, I'll readily change it back to normal. – SealBoi Jun 23 '18 at 15:35
• It is unbelieveably broad! We still don't know enough about the dynamics of a planet's formation to determine its conformation. In a nutshell you're asking for opinions. To give you an example, without the event that created the Moon, Earth would be extremely different from today. The gas giants actually moved from their orbits*. The quantity of metal in each planet depends on how much dirty was the star nursery...so, no. There are no fixed parameters to refer to. And 12 planet is an effing lot! – Valerio Pastore Jun 23 '18 at 15:41
• I think the key issue with the question is: it does not specify if you are asking about how the planets formed originally or how they ended up where they are 'currently', two very different things. Consider: Mercury (probably) had most of it's mantel blown off in a collision with a planet, Theia (probably) collided with Earth, forming the moon, Jupiter (probably) migrated in towards the sun, and back out again, ejecting at least one planet in the process, Uranus' axial tilt is (probably) from 2 (not just 1) planet size collisions. That's 8 planets, of 12 originals, not where/how they started – Dalila Oct 10 '18 at 20:10

# Massive objects beyond the frost line are likely giant planets.

Let's talk about the frost line, which you helpfully specified (3.94 AU). For anyone not familiar with the term, the frost line is the distance beyond which - in the protoplanetary nebula, while the system was still forming - heavy elements we call volatiles could condense into solid grains. These small bits of solid matter could then clump together into larger conglomerates, which in turn made it possible for larger planetesimals to form. This in turn encouraged the formation of giant planets.

The same thing doesn't happen inside the frost line, meaning that smaller protoplanets formed, and their growth was limited. They would eventually become the smaller, less-massive terrestrial planets. There are some exceptions to the rule; Hot Jupiters are giant exoplanets found orbiting extremely close to their parent stars. However, these giant planets likely moved to their present locations via planetary migration, which could have happened in a number of ways:

• Through interactions with the protoplanetary disk
• Through interactions (scattering) with other planets and protoplanets
• Through interactions with gas nearby

The system here has 12 planets - more than any known planetary system. I find it unlikely that any giant planets could have stably migrated inwards without disrupting the system. Assuming giant planets can only form beyond the frost line, then, we conclude that the inner planets (1-7) are terrestrial.

# Calculating the frost line

The radius of the frost line changes with time, increasing as the star becomes more luminous. For instance, I believe the Sun's current frost line is around 5 AU, whereas when the Solar System formed, it was around 3 AU. The flux from a star drops off as $$r^{-2}$$, so I would argue that the relationship between the radius of the frost line and the luminosity of the star should go as $$r_f=r_{f,\odot}\left(\frac{L_*}{L_{\odot}}\right)^{1/2}$$ where $$r_{f,\odot}$$ is the Solar System's frost line radius, and $$L_*$$ is the luminosity of the star. I thought I had found a formula of that sort in Hayashi (1981), but looking back, I can't find it.

Now, you've said that $$L_*\approx0.67L_{\odot}$$, so we find that, at the present time, $$r_f\approx2.2\text{ AU}$$ - much less than your value of 3.94 AU. Plus, the young star would have been even less luminous than the Sun. A value of perhaps 1.5-1.75 AU is more realistic, meaning that only planets 1-4 would be terrestrial.

# Stellar parameters

I also want to do a quick reality check of sorts on your star. Stars are pretty good black bodies, which is a thermodynamic term describing how they absorb and radiate radiation. We can use something called the Stefan-Boltzmann law to calculate the luminosity of a star if we know its radius ($$R_*$$) and temperature ($$T_*$$) using the Stefan-Boltzmann constant, $$\sigma$$: $$L_*=4\pi\sigma R^2T^4$$ Plugging in your parameters for radius and temperature, I find that for your star, $$L_*\approx3.88L_{\odot}$$ - much larger than you calculated. I would recommend reducing the surface temperature to about 4,000 K, and similarly reducing the radius to achieve the desired luminosity. This would make it a K-type main sequence star.

# Orbital resonances

The largest exoplanetary system we know of is Kepler-90, with 8 planets. It's pretty compact, with all 8 planets being within 1.01 AU of the parent star. I'd like to note that some of the planets have fallen into stable orbital resonances. Essentially some of their orbits are in integer ratios of one another. We see the same thing in some of the moons in our Solar System. In the large and complex system you're describing, I'd also expect some of the planets to be in orbital resonances. If you care enough to calculate some stable ones, you can do that (note that not all resonances are stable).

Also, on the subject of Kepler-90, I'd like to mention that the outer two planets are in fact giant planets, and they're likely inside the frost line. That's an interesting twist; perhaps giant planet migration is indeed possible. That said, it's also possible that Kepler-90 was significantly dimmer in its past, or that the migration of these two planets resulted in the scattering of other former planets, now gone. We don't know for sure.

• On luminosity; I first selected the mass of my star, at 0.9 solar masses. I then used three equations to determine luminosity, temperature and diameter respectively. To get luminosity, I used the equation M^4 = L. The other two were M^0.505 = T and M^0.74 = D. So, where have I gone wrong in this? Possible solutions would also be appreciated; the only necessities in the outcome is that Nemo will be a G-type star and have a habitable zone, but the closest I could get to what I had originally would be ideal. – SealBoi Jun 23 '18 at 17:48
• @SealBoi Can I ask where you got the equations? It's possible that they're only approximations, valid in certain regimes, or that certain assumptions were made. – HDE 226868 Jun 23 '18 at 17:52
• Not sure, but I think even beyond the frost line, gas giants may fail to form if they run out of accretion material or if gravitational influences from bigger neighbours prevent them from doing so (in the latter case, I think it's more likely to find asteroid belts) – Rafael Jun 23 '18 at 17:58
• – HDE 226868 Jun 23 '18 at 18:23
• @αCVn I've essentially given up on finding it; searching hasn't given me any results. I've kept a brief argument for why the equation should work, but I don't have a reference. – HDE 226868 Nov 7 '18 at 15:14