If a planet had two suns...

snorkack

The list of binary star planets is obsolete.

Consider Toliman B planets.

Bb is generally believed to exist. Since planets are lettered in order of discovery, not location, it shall stay Bb.

Since the bolometric luminosity of Toliman B is estimated at 0,500 solar, the habitable zone lies at 0,707 AU. Let´s call a planet there as Bc.

The smallest distance between B and A is 11,2 AU at periastron. Then the distance between Bc and A goes down to 10,5 AU.

The luminosity of Toliman A is 1,52 solar. Thus, at the nearest approach, the light cast by Toliman A on BC is almost 1,4 % of sunlight.

Regarding comparisons: full sunlight is about 129 000 lux.
Clear blue sky with Sun at zenith but in shade is about 20 000 lux, in Earth atmosphere at sea level.

The illumination of a horizontal surface right at dawn or dusk by clear sky illuminated by sun right below horizon is 400 lux.

With Toliman A at closest approach and at zenith - not only would it cast shadows (so does Moon) but the sky would be blue, and the direct light would be enough not only for general illumination but for demanding tasks. The blue sky would be only slightly dimmer than at setting or rising Toliman B.

mfb

α Centauri B has one known exoplanet, and that is not in the habitable zone.

Do you have any reason to expect such a planet? In addition, how would its orbit look like?

Depends on the 3-dimensional orientation of the orbits.

snorkack

No specific reason to expect exactly this to exist.

On the other hand, γ Cephei B approaches about as close to A as does α Centauri A to B, and γ Cephei Ab manages to orbit unperturbed as far away as 2 AU. Thus, there is no specific reason for α Centauri B habitable zone to be empty.

Relative ordinary low eccentricity orbit around α Centauri B with minor perturbations from A (presumably precession of the nodes and apsides).
Probably at a low inclination to the AB orbital plane. High inclination would be more vulnerable to perturbations.

Whovian

:D That would be interesting.

Unfortunately, Sirius B doesn't seem to be too bright, so it wouldn't have much of an effect.

Now, even more interesting would be a trinary star system. (Or even quaternary.) That would allow for some pretty bizarre life. From an earthling's point of view, I mean.

mfb

There is a factor of 2 between the semi-major axes, and the closest approach corresponds to a higher velocity -> short time there.

ε=0.115
Roughly circular, but more eccentric than any planet in our solar system apart from mercury.

In addition, it is the only known system with those properties, indicating that those planets could be quite rare. Possible, but rare - even more if you want to decrease the semi-major axis by a factor of 2.

snorkack

Errai AB semimajor axis - 20,2 AU
eccentricity 0,41
period 67,5 years

Toliman AB semimajor axis - 23,4 AU
eccentricity 0,51
period 79,9 years.

mfb

Oh, I misinterpreted your previous post:
Ok, the stellar orbits are similar.

Gamma Cephei A (with planet) has 1.4 solar masses, Gamma Cephei B has 0.4 solar masses. The companion has less mass and less gravitational influence.

Alpha Centauri B (with planet) has 0.9 solar masses, Alpha Centauri A has 1.1 solar masses.
The companion has more mass.
The relative ratios differ by a factor of 4,6.

dodo

If you allow me the license, I thought I would add a marginally off-topic post; perhaps still useful if comparisons to Earth are intended. It involves changes to our own solar system, rather than planets formed from scratch in a binary system, so admittedly not the same.

I was rewatching the movie "2010" recently, which ends (I assume this is no spoiler by now) with Jupiter becoming a second sun. I am no physicist myself, but perhaps many of the other posters may confirm my Wikipedia speculation that this would certainly be no paradise for us.

First, if we are worrying about global warming, now imagine adding a second sun sourcing heat to the planet. But this is not really the worst.

For Jupiter to emit visible light, it would have to increase its mass at least 70-80 times (fusion of elements other than hydrogen may start to occur at a smaller mass, but these would only make it glow in the infrared, like what we know as "brown dwarf" stars; for a shiny second sun you really need to fuse hydrogen). But, as the mass of Jupiter increases that much, it will also start falling towards the sun. Guess who is in the middle. At the very least, a large asteroid belt is. So now you have sort of a kitchen blender splashing rocks at the outskirts of your planetary system. But this is not still the worst.

As Jupiter normally orbits the sun, its present mass makes the sun "wobble" - but not by much: just about as much as the sun's radius, but not really much more. A Jupiter that is 70-80 more massive would make the wobbling extremely close to the orbit of Mercury, at least at Jupiter's current distance (granted, less so as Jupiter starts falling in). Now Mercury gets into a violent eccentric orbit; it may or may not fall into the sun, but it may eventually get fractured in pieces by the violence of its new orbit. So now you have another splashing blender in the middle. But this is not yet the worst.

The rest of the inner planets, Venus, Earth and Mars, will likely leave their almost-circular orbits acquired after billions of years of relative quietness. So now, instead of some dozen degrees of temperature difference per year, your planet (assuming it doesn't smash into something, which it eventually will) may likely have a couple hundred degrees of difference between summer and winter. During the summer, lava will run the surface and stones will melt; in the winter, you will be breathing liquid nitrogen instead of air. Bad for your health.

As said, not the same as the OP's request. But the post was intended to illustrate how relatively peaceful, boringly circular our neighborhood is. And how very different life may have to be (perhaps even subterranean, away from the surface; perhaps without an atmosphere at all) in order to survive planets in eccentric orbits, subject to violent tidal forces and temperature extremes.

mfb

I think the movie did not mention any increase in the Jupiter mass - the object somehow catalyzed fusion (?).
A significant larger Jupiter mass would scatter objects in the asteroid belt all over the solar system - even if our orbit stays reasonable, this would be a serious threat.

snorkack

On the limit of extremely massive perturbing body, Moon is perfectly stable long term and has a modest eccentricity although it completes only 13 7/19 orbits in one orbit of te perturbing body. And around Jupiter, S/2003 J 2 has a period almost one quarter that of the perturbing body.

aerrowknows

As was said earlier, there are many possible permutations and outcomes depending on the exact orbital size and mass of the planets. One possible permutation that was not brought up is that the planet will orbit around both stars as they orbit each other.

To understand this, imagine the figure 8 or infinity sign if you will. The planet will orbit in this shape with 2 suns at the focii of the figure ( means the two suns would be in 2 spaces in the figure ).

The implications would be that the planet experences 9 distinct seasons: 4 around one star, 4 around the other star, and one in between the two. The one in between will have the condiition of constant sunlight for that season since the sun shines from both directions. The 4 seasons per star are governed by the distance,size and temperature of that particular star. For example , a winter around 1 and a very cold winter around the other.

ImaLooser

Not so. Masses and densities have nothing to do with orbits. What really matters is angular velocity. If the extra mass changed angular velocity then the orbit would become elliptical.

It is true that randomly adding 80 Jupiter masses to the Solar System would have a major effect on its stability.

I bet Clarke added a Baim Capital Fu-Zo-Matic to catalyze fusion. I don't know whether that is possible.

ImaLooser

Hah! Beat you to it. I think that that is the most interesting scenario. There would be temperature extremes so human life seems impractical, you can have fun inventing some other forms.