How Would a Red Dwarf Companion Affect Our Sunset Views?

[Vanadium 50]

Just for fun, I calculated what Proxima Centauri would look like from a planet orbiting Alpha Centauri A. It would be somewhere between 3rd and 4th magnitude. Maybe noticed by the natives (if any), maybe not.

 

[Algr]

I still have doubts that a planet will not tide-lock.

My understanding of gravity is that once you are a certain distance away, it won't matter if the mass you are orbiting is all in the center or in that of two stars. Perhaps if it was a 4:1 or 10:1 ratio, but I was describing a 93:1 ratio.

Just for fun, I calculated what Proxima Centauri would look like from a planet orbiting Alpha Centauri A. It would be somewhere between 3rd and 4th magnitude. Maybe noticed by the natives (if any), maybe not.

Interesting. Not many planets anywhere will have Red Dwarfs naked-eye-visible at all.
So I'm looking this up on Wikipedia and the skies from any planet in that system could never look like Tatooine. My drawing up top might be possible at some times of the year, if a planet was in the habitable zone of A or B. But then the distant star would appear far from the large one most of the year, and could be on the opposite side of the sky.

 

[Vanadium 50]

The degree of torque on the planet depends on how aspherical the body they are orbiting. The sun is spherical (mostly). Two suns are not.

Yes, Proxima will appear as a star in the sky, and as such the primary sun will move through its equivalent of the zodiac.

 

[snorkack]

The degree of torque on the planet depends on how aspherical the body they are orbiting. The sun is spherical (mostly). Two suns are not.

No. The torque depends on how aspherical the planet is.

 

[sophiecentaur]

I still have doubts that a planet will not tide-lock.

Stars are pretty fluid objects so I'd imagine that they would form a fast rotating spheroid. Would the presence of a nearby mass disturb this, bearing in mind that the orbit period would have to be many times greater than the rotation period (years vs hours)? That implies no locking(?).

 

[Vanadium 50]

No.

Wrong, And I'm out of here - I have decided not to participate in the arns race of correcting misinformation.

 

[snorkack]

How do we figure out the correct information then? By analyzing the first principles reasoning? By referring to an authoritative text (which?)

In the short term, when you have tidal interactions between two bodies, first of which is nonspherical and with its axis not aligned along or across the direction to the second body, but the second of which is a point mass or perfectly spherical, the first body experiences a torque around its own axis. The second body does not. It does experience a torque around first body´s axis - due to action and counteraction - but for the second body it is merely a force along its orbit. It affects second body´s revolution, not its rotation.

Is it correct so far, or where does this reasoning err?

 

[snorkack]

Interesting. Not many planets anywhere will have Red Dwarfs naked-eye-visible at all.

Most planets in solar neighbourhood are planets of red dwarfs.
Within 20 lightyears:
https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs
I seem to find non-red-dwarfs with confirmed planets:

1. Sun 8
2. e Eridani 3
3. τ Ceti 4
4. o² Eridani 1
5. ε Eridani 1
6. ε Indi 1

total 18
red dwarfs with confirmed planets:

1. Lacaille 9352 2
2. Gliese 832 1
3. Gliese 229 2
4. GX Andromedae 2
5. Lalande 21185 2
6. Gliese 752 1
7. Gliese 251 1
8. Wolf 1061 3
9. Gliese 687 2
10. Gliese 674 1
11. Gliese 876 4
12. Luyten Star 2
13. Gliese 3323 2
14. FI Virginis 1
15. YZ Ceti 3
16. TZ Arietis 1
17. Gliese 1061 3
18. Gliese 1002 2
19. Proxima Centauri 2
20. Teegarden star 2

total 39

So I'm looking this up on Wikipedia and the skies from any planet in that system could never look like Tatooine. My drawing up top might be possible at some times of the year, if a planet was in the habitable zone of A or B. But then the distant star would appear far from the large one most of the year, and could be on the opposite side of the sky.

The known circumbinary planets:
https://en.wikipedia.org/wiki/Circumbinary_planet
Start of the list, in order of discovery...

1. PSR B1620−26. The stars are a pulsar and a white dwarf, orbital period 191 days, planet orbital period about 100 years
2. HD 202206. A Sun-like star, with a red dwarf satellite at a 256 day orbit (quite eccentric) and a brown dwarf at 1260 day orbit (also eccentric)
3. DP Leonis. A tight white dwarf/red dwarf binary at 1,5 hour orbit. A planet out at 28 year orbit.
4. Kepler-16. A binary of a K dwarf and red dwarf. "Luminosity" ratio at about 26/1 - apparently the bolometric not visual luminosity. Stars orbit at 41 days, planet at 229 days
5. Kepler-34. A binary of two Sun-like stars, orbiting at semiaxis 0,22 AU. The first that could look like Tatooine.

The article
https://en.wikipedia.org/wiki/Kepler-34
does not actually spell out things like luminosities. So need to calculate them:
34A radius is 1,16 solar. Temperature 5913 K, which is 141 K warmer than Sun, so about 2,5 % warmer. This means the luminosity (bolometric) of A is about 150% solar.
34B radius is 1,09 solar. Temperature 5867 K which is 95 K warmer than Sun, so about 1,6 % warmer. So bolometric luminosity of B is about 125% solar.
Since the combined luminosity of two stars is about 275% solar, Earth-like radiation would be received at about 1,66 AU.
If both were at their average distance from 34"c" (the imaginary Earth-like planet, 34b is the real, Saturn-like one) then they would be respectively 70% and 65% of apparent size of solar disc.
Since the semimajor axis of the orbit of the stars is 0,23 AU and eccentricity 0,52 then when "c" is at the apside line, the distance between the stars is 0,34 AU, so one of them is at 1,49 and the other 1,83 AU. That 22% difference in distance will overwhelm the smaller differences in the size and brightness.
(Small back-of-the-envelope errors may exist; redo calculations if important. Note that the insolation of Earth varies 6% by season, yet we do not notice).

 

[Algr]

Most planets in solar neighbourhood are planets of red dwarfs.

I meant seeing them as stars at night, not orbiting them as a sun. A developing civilization might face a small
hurdle with this, as the stars they see in their night sky would all be fundamentally different than their sun.

Kepler-34. A binary of two Sun-like stars, orbiting at semiaxis 0,22 AU. The first that could look like Tatooine.

Now this is interesting. It occurs to me we have no idea what a tidally locked gas giant would be like. Would it be round? Shaped like a pear or comet? The same forces that cause tidal locking would also make moons unstable and short lived, right?

"Look like Tatooine" actually turns out to have some rather stringent requirements:
- Stable Habitable planet. (This may or may not preclude tidal locking.)
- Two stars of near equal size.
- Distance from one star to the other is no more that 6x the radius of the stars.

 

[snorkack]

I meant seeing them as stars at night, not orbiting them as a sun.

What do you classify as "seeing them as stars at night"?
As "distant and unrelated field star", red dwarfs are rare. None on Earth. Note that the definition of "red dwarf", that is the K7/M0 classification border (sic! K8 and K9 are usually not defined) has nothing really interesting physically changing at that point. The brightest red dwarf in Earth sky, AX Microscopii, at 13 ly away, at +6,67 visual is not a naked eye star, but at +8,69 absolute it is not so much dimmer than 61 Cygni B which is +8,31 absolute, +6,03 visual. Exchange AX Microscopii with Lalande 21185 (8,3 ly away, +10,44 absolute and +7,47 visual, in northern sky at that), and we would have the naked eye red dwarf.

A developing civilization might face a small
hurdle with this, as the stars they see in their night sky would all be fundamentally different than their sun.

They also would not see night sky.

Now this is interesting. It occurs to me we have no idea what a tidally locked gas giant would be like. Would it be round? Shaped like a pear or comet?

Moon is tidally locked but not so conspicuously out of round.

The same forces that cause tidal locking would also make moons unstable and short lived, right?

Agreed.

"Look like Tatooine" actually turns out to have some rather stringent requirements:
- Stable Habitable planet. (This may or may not preclude tidal locking.)
- Two stars of near equal size.
- Distance from one star to the other is no more that 6x the radius of the stars.

How stringent is the last condition?
Note that even when the stars have a maximum distance rather more than that, they will nevertheless have conjunctions. There are good stability reasons why planet´s orbital plane is likely to be close to the stars´ orbital plane.

 

[Algr]

What do you classify as "seeing them as stars at night"?

https://en.wikipedia.org/wiki/Night
Astronomical night would do.

They also would not see night sky.

Unless they have feet and can walk to where the night is.

Moon is tidally locked but not so conspicuously out of round.

Moon is not a jovian. It does not have a density significantly higher on one side than the other. It's mass isn't mobile via air currents.

How stringent is the last condition?
Note that even when the stars have a maximum distance rather more than that, they will nevertheless have conjunctions. There are good stability reasons why planet´s orbital plane is likely to be close to the stars´ orbital plane.

I'm thinking that the planet's orbit would destabilize if the two stars were too far apart. So the second star has to be either very close to the first, or so far as to have no effect on the planet. (not circumbinary).

 

[snorkack]

Unless they have feet and can walk to where the night is.

Yes, travel thousands of kilometres into a hostile environment.

I'm thinking that the planet's orbit would destabilize if the two stars were too far apart. So the second star has to be either very close to the first, or so far as to have no effect on the planet. (not circumbinary).

Well, we have Kepler-34. The second star is close enough. What it means: B semimajor axis 0,228 AU, eccentricity 0,52, gives maximum distance 0,346 AU. About 52 million km, which is about 65 radii of A.
b is at 1,09 AU as semimajor axis, which makes AB maximum distance 0,317 b semimajor axis.
Compare Mercury - semimajor axis 0,387 AU, aphelion 0,467, perihelion 0,307.
What it suggests is that a satellite as far out as Venus´ orbit has a potential of perturbing the orbit of a planet outside and should not be too massive... but a satellite inside Mercury´s orbit can be as massive as the primary and still leave the planetary perturbations practical to handle.
A more thorough celestial mechanic handling is needed, though.