It’s a staple of Star Trek to be sure. Planets, supporting life no less, around binary stars. (For those of you not scientifically-minded, that’s a solar system with two suns;).)
But is that scientifically possible? Can life exist on a planet, revolving around a binary system? Cause to me, in my relatively non-scientific outlook, it seems like it would be too hot, with too much solar radiation.
And heck, while I am at, I will throw in trinary stars, for good measure. Can life exist on a trinary star?
BTW, I did hear once, on a scientific special on PBS, that in any event, it would be hard to forecast the seasons on a binary world. Because it’s motion around the suns would be governed by chaotic physics. Weird, no?
Not in the original Star Trek. The first place I remember seeing it is Tatooine. That is in movies/tv shows. It’s been a staple of written SF for much longer.
The main issue about binary stars is that the planet needs to be in a stable orbit. Generally, there’s three ways this can happen.
When the stars are far apart and the planet orbits one of them. In this case, the stars have to be something like 11 times the distance to the planet away from each other. That is, call the astronomical unit of this system the distance from the planet to the sun it orbits. The second star has to be at least 11 astronomical units away. (Note that this will almost certainly be a different AU than the one we use in the solar system.) In this situation, the planet will get a varying amount of light throughout the year, because the distance to star B will change by two AUs during the planet’s year. The thing is, the changing light will not be the same from year to year, but rather the period of most light (and least light) will move forward by maybe a month or so every year.
When the stars are close together and the planet orbits their barycenter (the center of mass of the system). I forget how far away the planet has to be, but I’m pretty sure it’s too far for a climate like on Earth even with the double sunshine.
When the planet is at either L4 or L5 Lagrange points of the two stars. At those points, the planet will nominally equidistant from the two stars, but for it to be stable, one of the stars has to be much more massive (and thus much brighter) than the other. So if one star is as massive as the Sun, star B will have to be a red dwarf and the planet will get somewhat more (say about 1.2 times) the light. That is, it will on average. The thing about these orbits is that a body can be stable there without having to stay at the actual Lagrange point.
The Trojan asteroids at Jupiter’s L4 and L5 points are an example. There’s thousands of them and clearly they can’t all be at one of two points. Instead, their movements oscillate around the the Lagrange point, sometimes getting fairly near Jupiter and sometimes far away. The Lagrange points are 60 degrees away from Jupiter and the orbits can vary from about 30 to 90 degrees away. Replace Jupiter with a red dwarf star and you’ll find the total light varying by quite a bit.
None of these are actually chaotic, but they may be too variable to have stable seasons.
In Nightfall, Isaac Asimov explores the psychological effects of having a six star system in which there is total darkness only once every 1000 years. He does not, however, address the question of stability, which would be very difficult. Even the long range stability of our solar system cannot be proved. The premise of the OP is wrong. If there are two sun-sized stars, then just put the planet further away. If there are two red dwarfs, it would have to be a lot closer. And any distance at which water is liquid will do for some form of terrestrial life.
The magic number seems to be somewhere around three.
There are two different types of orbit possible in a binary system; planets that orbit just one star in a binary pair are said to have “S-type” orbits, whereas those that orbit around both stars have “P-type” or “circumbinary” orbits.
For an S type orbit, if the planet orbits only one star at a particular distance, so long as the other star orbits at least three times as far away then the second star would not contribute too much extra insolation to the planet, and habitability should be possible.
For a P type orbit, the planet orbits both stars at once, and so long as the planet orbits at least three times as far from the two stars as the two stars orbit each other then habitability could be possible.
These figures are only a very rough rule-of-thumb, and would not be true for planets orbiting a pair of stars that were very different in brightness, or orbited each other in very eccentric orbits (actually quite a lot of binary stars do fall into one or both of these latter categories).
Not a problem, given that the stars could be arbitrarily close to each other. There are even some systems called contact binaries, where the stars are (you guessed it) actually touching each other (and of course they’re highly distorted by tidal effects).
Given the movie stills I’m finding, it doesn’t look like we ever see the suns of Tatooine more than 15-20 stellar radii apart in the sky, which means they might actually be that close in space. A planet far enough from a pair of Gs to have a (barely) habitable temperature should easily be enough times that radius to be stable.
A couple of minor nitpicks to an otherwise excellent post:
In the case of a world at a libration point, the primary and secondary star (of masses M[SUB]1[/SUB] and M[SUB]2[/SUB] respectively) would have to have a ratio of of masses of M[SUB]1[/SUB]/M[SUB]2[/SUB] > 24.96 (and M[SUB]2[/SUB]>>M[SUB]3[/SUB]), and the stars would have to be sufficiently distant that other dynamic effects do not perturb the system. In practice, in a system stable enough to last for the several hundreds of millions of years it would take for a planet to coalesce and cool sufficient that organic chemistry could form stable compounds (much less the billion years, give or take, for life to evolve into a form complex enough to significantly modify the global environment) the stars would have to be sufficiently distant that one woud essentially be orbiting the other, e.g. the barycenter of the system lies well outside the sphere of influence (SOI) of the secondary star. In practice, the system would have to be a primary as a large B- or O-type main sequence star and the secondary a small K- or M-type star, and there remains the problem of the short lifetimes of the higher temperature stars and their relative paucity in the current stellar evolution timeframe. The scenario of a planet in a Trojan point orbit of two stars with equal apparent magnitudes and size is just not physically plausible, not withstanding the need for it to be able to support a terrestrial environment. That does still leave binary (and trinary) systems in which a planet orbits a star which is in orbit, or orbited by, a companion, or a planet co-orbiting two closely adjacent stars (though the latter is unlikely to be stable on evolutionary timeframes unless they are low mass red dwarfs, and prone to large variability and flaring), but the companion star(s) are going to be far enough away that they’ll just be relatively bright stars that move oddly against the stellar background.
The other issue, as already pointed out, is that the 60 degree Trojan orbits are islands of stability and objects within them will oscillate around the locus. While the relative motion of the planet within that orbit is not necessarily a problem (depending on how much any hypothetical life is dependant upon a certain level of luminance and how stable the atmosphere is in maintianing a thermal balance), these areas will aggregate material, potentially creating a continuing impact hazard for any world in them. Whereas the oversize moon helps to clear objects in potential periodic orbits with the Earth, a body in an L4/L5 point will tend to be bombarded more often, albeit by relatively low velocity meteors. This has the presumptive benefit of making minerals and elements from interplanetary sources more available but isn’t a great scenario for a stable enivronment for life to evolve into complex and specialized forms.
It should be noted that there is generally the assumption (often unstated) that life must evolve under Earth-like conditions, e.g. within a circumstellar habitable zone of a star such that surface water can be present. However, this makes assumptions which limit both the basis of life (organic chemistry in an ionic liquid substrate) and where those conditions could occur. Setting aside any purely speculative forms of life that could form in some kind of solid state or charged gaseous medium, there still remain bodies which can maintain a liquid substrate without solar insolation, e.g. bodies such as Ganymede, Europa, Enceledus, and Titan, all of which have liquid water or hydrocarbon reservoirs. (Titan also has a thick hydrocarbon-rich atmosphere well-suited to creating a hypothetical primordial soup.) There is a school of thought that if life is relatively common in the universe (and given that our carbonaceous-nitrogenous-based terrestrial life is formed of the most common chemically-reactive elements in the universe using compounds commonly found in the interplanetary and interstellar medium, this is at least a rational thesis) that it may be commonly found on planets and moons subjected to tidal heating, which frees hypothetical extraterrestrial life from the limitation of being a particular distance from a suitable K- or G-type star and also from the hazards that come with being on the exposed surface of a planet near a star (variable output, large coronal mass ejections, stellar flaring endemic to M-class red dwarf stars as well as thermal pulsing in giant and supergiant stars). If we posit the formation of life in such environments the potential for habitable locations where life could evolve grows dramatically as compared to just planets in a presumed circumstellar habitability zone, and especially against galactic radiation closer to the galactic core and from magnetically active stars.
But the tropes of multiple suns, very close orbiting giant moons filling some vast aspect of the sky, and visible ring systems around rocky terrestrial worlds, while visually stunning, are purely from the imaginations of artists and filmmakers and do not represent plausibly habitable (from our standpoint) worlds.
I think that if you want the visual of a celestial body filling a big chunk of the sky, you actually want to make your inhabited world itself the moon, with the sky-filling object being its primary (like the sky seen from one of Jupiter’s moons). This also lets you be sufficiently close to a relatively dim star without the planet tidally locking to the star.
This is the Cirumbinary option I mentioned earlier, and this appears to be the type of system that Tattooine is (or it would be if it existed).
But is it really such an unstable arrangement among Sun-like stars? Close-orbiting binaries are often so close together that they can only be distinguished from a single star using spectroscopy, and spectroscopic binaries of all ages are commonplace in our local volume. Chi Draconis, Mu Bootis, Gliese 67 and Mu Draconis are a few of many nearby close-orbiting binaries, and none of these are red dwarfs, so even if this arrangement is often unstable, it is wrong to say it is always so.
A nice image, but we have to also imagine that the gas giant does not have a strong magnetic field that could trap ionised particles and rain them down onto the moon. I don’t know how likely that might be.
Also if the gas giant is tidally-locked the moon’s orbit becomes unstable.
I meant stability for the planets orbiting them. Even if the planetary orbit is dynamically stable over a long period it would probably be significantly perturbed if the stars are significantly distant from one another to be widely separated, and this doesn’t readily support an environment for surface life to thrive. The stars could be close orbiting as you suggest but then they would either be prone to flaring, or would have a stable planetary orbit will outside the cocircumstellar habitable zone, at least for F, G, and K type stars. There are chaotic but dynamically stable orbits that can be deliberately created between two co-orbiting stars, but they are unlikely to come about by chance nor would they support a stable environment without some internal thermodynamic regulating mechanism, e.g. some kind of life that evolves to become periodically dormant, or that stores up thermal energy and re-releases it in time with the orbiting periods where the planet is far away from either star.
Saturn has only a pretty weak magnetic field (although the magnetosphere it develops is the second largest of any planet after Jupiter), and of course any life that arises on the moon of a gas giant or other planet with a strong magnetosphere may either be more resistant to the ionizing radiation it produces, or the moon itself may protect against radiation at the surface. And smaller “icy giants” akin to Uranus and Neptune but closer to the star it orbits, may not have the larger sustained magnetic fields developed by the motion of metallic hydrogen in heavier planets.
I’ll caution that we shouldn’t draw any generalizations about the composition of planets in general by looking at our own solar system. We’ve already observed planets around other stars that defy explanation using existing models of planetary system evolution including “hot giant” planets in closer orbits than they should be capable of evolving in, and supermassive rocky worlds that ought not exist. The general laws of mechanics apply to all systems, of course, but the details of planetary features and development are already shown to be vastly more radical than planetologists have previously widely accepted. The same is true for conditions under which life, in general form, could develop exist, including a presumptive timeline for evolution. It took us nearly three billion years to get to multicellarly life and another billion to develop primitive flora and fauna, but that should not be taken as a universal guideline; it may just reflect insuffiicent plasticity in our genome to support evolutionary change at a more rapid pace, or a relatively unstressing environment, or any of a multitude of reasons why life elsewhere could form spontaneously and evolve more rapidly.
However, humanoid alien moisture farmers on a desert planet beneath double suns whining about not being able to go to Tosche Station is pretty much pure science fantasy.
Am I right in assuming that the typical fate of contact binaries is that they eventually merge? How long can they last before this occurs? Is the merger liable to be catastrophic for any planets orbiting the pair?
I’m a bit rusty on the subject, but as I remember it, yes, they do eventually merge completely, forming what’s called a “blue straggler” (because it ends up being a blue star when all of the rest of the blue stars among its age cohort have long since died of old age). The merger itself would presumably be accompanied by some degree of violence, and even if that wasn’t enough to kill any appropriate planet, the resulting star would be much brighter, which would move the habitable zone much further out. Unfortunately, I’m not sure about lifetimes.
I don’t have time to create a simulation exactly like this but this video by astrophysicist Jackie Faherty will show some of the very cool things that are coming out of ESA’s gaia DR2.
It is the best example of “co-moving” stars that I can find and I did put it at a time stamp but the entire video is very worth watching.
At 19:00 she will also start to cover brown dwarfs and point out that the closest to us were only found less than 5 years ago.
While our exoplanet detection abilities will need more missions and data.
But ya this is one of the really fast moving parts of astrophysics and ESA’s Gaia mission is going to provide some papers with more realistic guesses over the next few years. As with some of the visualizations in that video I provided are just being created today and will take some time for publication.
Several circumbinary planets have been discovered. They appear to be rare compared to planets orbiting single stars, but they are not absent.
Here’s a list.
All three Kepler examples are orbiting in the habitable zone of their parent stars.
Note that Kepler 16b conforms to the ‘rule of three’ I mentioned earlier, as it orbits at 0.7 AU while the stars are only separated by 0.22AU, giving a ratio of 0.32. I believe that this Rule of Three was devised originally by Isaac Asimov, whereas in more recent times astronomers have been applying a ‘rule of seven’ (according to the wiki article about this planet). But it seems that the good doctor may have been right. On the other hand Kepler 16b might be a freak example of a planet in a temporary or quasistable orbit; this is only one planet among thousands of exoplanets on the books, so it is hardly typical.
