# Super-Earths around binary dwarf stars

1. Feb 8, 2017

### RyderP

I may be writing a story, set on a rocky planet, 15 times the size of Earth, around a binary dwarf star (one red, one orange). What kind of flora and fauna can I expect?

2. Feb 9, 2017

### shihab-kol

Well, the flora and fauna will vary depending on the type of vegetation you choose i.e. if you want the time-honoured traditional slimy one-eyed species or want to create something fresh, new and unconventional.

3. Feb 9, 2017

### RyderP

Actually, I want something a little more true to life.

4. Feb 9, 2017

### RyderP

On the other hand, how about combining Star Wars, Space: 1889, and D&D?

5. Feb 9, 2017

### RyderP

I also know thar the sky will, most likely, be orange.

6. Feb 9, 2017

Human

Elf

Half Elf

Halfling

Ogre

Gnome

Dwarf

Goblin

7. Feb 9, 2017

### RyderP

15 florins = 1 ducat

30 ducats = 1 kreutzer

Electrum = 1

Copper = 3

Silver = 5

Gold = 10

8. Feb 10, 2017

### RyderP

Sorry I got carried away.

9. Feb 20, 2017

### RyderP

Add the Todi (insectoids) and the Morrix (delphinoids).

10. Jun 26, 2017

### RyderP

How much gravity can I expect from a planet orbiting a red and an orange dwarf star?

11. Jun 27, 2017

### Bandersnatch

Gravity of a planet does not depend on the type of the parent star(s).
All that matters is its size and density. So...
If you mean 15 times the radius, and assume the same or very similar composition, then it'd by 15 times Earth's gravity. At that size, the planet would also be unlikely to be a terrestial planet any more. It would likely be a dwarf star itself, actually.

However, if you mean 15 times the surface area, then the planet would be approx 4 times larger than Earth in terms of radius, and with 4 times the gravity. You could fudge the density a bit to bring the gravity to about 3g, but anything lower than that would be improbable.

As a rule of thumb, you get as many times higher/lower gravity as many times the planet (its radius) is larger/smaller than Earth's.

12. Jun 27, 2017

### Algr

How much could you reduce gravity at the equator by having the planet rotate very fast? Perhaps due to a collision 10 million years ago. You'd then have an obviously non round planet, and lots of heavier elements in the crust.

13. Jun 27, 2017

### RyderP

So, 3G's okay?

14. Jun 27, 2017

### RyderP

15. Jun 27, 2017

### stefan r

You can use the surface gravity equation
g = 4π/3 × G ×ρ × r
Four thirds pi times gravitational constant times density times radius. So a planet with earth's density, 5514 kg/m3, and 15x radius you get:
g = 4.19 × 6.674 10-11× 5514 × (15 ×6371000) = 147.
Which is also 15x earth's gravity.

However, that rule of thumb does not work. Saturn has around 1g and radius 10x earth's radius because Saturn is has a lot of hydrogen and earth has a lot of iron. Jupiter has a lot of hydrogen too but 15x radius and 2.5x gravity. Mars has 0.38g and 0.5 radius and is similar to earth in composition. Mercury comes close to your thumb with .38g and .38 radius but only because Mercury has more iron and earth has a lot of oxygen and silicon.

A 15x radius iron planet will compress the electrons in the core. A planet made out of the same materials as earth does has a much higher density.

16. Jun 27, 2017

### Bandersnatch

3g is acceptable. Magnetosphere should not be a problem.

It works for terrestial planets - i.e. under the somewhat stretched assumption of Earth's density. If you want to add density to the mix, then it's another simple linear relationship.

17. Jun 27, 2017

### RyderP

I was thinking of making the magnetosphere of the planet more like what Earth's would have been, if Tesla had his way.

18. Jun 27, 2017

### RyderP

So that only longwave, shortwave, and hyperspace radio work. And local comm systems are provided by direct use of the magnetosphere.

19. Jun 27, 2017

### stefan r

make it strong enough to allow superconductor speeder bikes.

20. Jun 27, 2017

### RyderP

And make glass glow, while we're at it. Why not?