# Habitable planet in a multistar system

I'm trying to design a world in the Alcyone A system of the Pleiades.

Alcyone A consists of three stars. The main star is 3.4-3.8 solar masses. There is a very low mass star < 15 million miles away which orbits every 4 days. There is another star ~ 480 million miles away which orbits every 830 days. It is about 1.7-1.9 solar masses.

I want to have a habitable planet/moon in this system. The main star has ~2400x solar luminosity, so I think by that measure the planet needs to be about 50 AU from the main star. But it also needs to be far enough from the two main stars to be in a stable orbit. Is there a range that is close enough to be habitable but far enough away to be stable? Could a moon of a gas giant be warm enough for tidal heating?

I understand that a blue giant star like this doesn't have a long enough lifespan for life to evolve, but that's OK. I just need a habitable planet suitable for terraforming/colonization.

• Sorry, but that seems unlikely. The page you linked to says it is an open cluster that is only 440 million years old. It seems fairly impossible for conditions to be stable enough to allow formation of habitable planets with all the close encounters with massive stars and radiation from hot young stars. A protoplanet would need to be massive enough to resist all of that and that probably means a gas giant at a minimum. Jan 31, 2018 at 22:38
• @Ville Niemi Wiki says it's even younger, 100 million years old, but that should be enough for both rocky and gaseous planets to form. The surface could be very unstable, but that can be handwaved away. Jan 31, 2018 at 23:27
• @Alexander The system is 100M, the cluster 440M years, IIRC. I was focussed on the cluster since my issue is that the environment (ie. the cluster) is too unstable for small planets to form. Jan 31, 2018 at 23:53
• @Ville Niemi not that much is known about planet formation in a cluster. It could be that our Sun had started in a cluster just like Pleiades. I'm totally willing to give the benefit of the doubt here. Feb 1, 2018 at 0:02
• This doesn't address some of the specifics that you mentioned, but this video might help with some brainstorming. youtu.be/8_RRZcqBEAc Feb 1, 2018 at 0:10

# Stable

I ran this simulation using Rebound, and orbital particle simulator. You can find the code I used for your simulation at my github, under the file orbit_ryanrussel_180131.py. These are the initial conditions that I used:

m_alc_a = 3.6      # As a fraction of the mass of the sun
m_alc_b =  0.1
m_alc_c =  1.8
m_planet = 3e-6

a_alc_b =  0.16  # AU
a_alc_c =  5
a_planet = 50

e_alc_b = 0.01
e_alc_c =  0.01
e_planet = 0.01


Those are the mass, semi-major axis, and eccentricity of each of your objects, which I labeled alc_a, alc_b, and alc_c. The planet's info is in planet. The planet is the same size as Earth, and the stars are as you specified. Masses are in multiples of the Sun's mass, distances are in AU. For the 'low mass star' I used a red dwarf of 0.1 solar masses, as the results will show, I don't think the mass there really mattered.

This ended up being by far the most stable simulation I've run. Usually I like to post graphics of orbits or cool orbital resonances, but there were none. Just three concentric circles. The three orbiting objects (two stars and a planet) have not changed their semi-major axis or eccentricities by even 0.01% in the simulation, so I think we can assume that the planet is sufficiently far away not to be bothered by any of the stars.

So far the simulation has run for 47 million years as I'm typing this. I don't think I'll let it go any farther, since your planets are obviously stable.

• for 47 million years wow! Wish I had that kind of time, I been meaning to learn a bit of Python. PHP guy here. Don't frown on PHP, I'm like a next level PHP programmer. Feb 1, 2018 at 5:08
• @ArtisticPhoenix Python my man! You can do anything with it! Feb 1, 2018 at 16:55

Going with the Kasting et. al. estimate for the Solar System's habitable zone and your estimate of 2400x Solar luminosity, the inner edge of Alcyone A's habitable zone is 46.5 AU from the center of the star, while the outer edge is 67 AU out, well outside the entire three-star system. A stable circumbinary orbit is one that's more than 2-4 times the star-to-star distance. In your situation, the star-to-star distance is about 5 AU for a worst-case minimum stable radius of 20 AU. The entire habitable zone is stable.

Actually inhabiting such a planet would be a bit tricky, though. As a blue giant, Alcyone A emits far more ultraviolet than the Sun. It also has an exceptionally strong stellar wind. Between these two problems, you probably can't terraform a planet even if it has a reasonable size and temperature.