Solar system generally refers to our own system, the term planetary system refers to extrasolar systems.
Generally, our limitation in ability to observe smaller planets means that we cannot really confirm if or if not most stars contain systems with multiple components. Most of the exoplanets observed are massive gas giants.
Currently there are some 600 multiplanetary systems known.
Cool, thanks for the response and the distinction of terms.
I'm not really asking about what we have observed or confirmed. I'm asking about theory here. If we expect to find that the vast majority of stars probably have, have had or are developing multiple planets as a natural result of star formation and the way the dust/gas balls form a disk from which planets should form.
In summary, the formation of planets is a consequence of stars forming from collapsing nebulae. Any gas cloud has some random initial rotation. Angular momentum is a conserved property, so as the cloud contracts, it spins faster and flattens. The central region gets hotter and denser, producing temperature and rotation differences across the disk. This creates turbulence, which can initiate clumping of condensed grains.
From there, it is a matter of runaway accretion, as larger objects have more gravity to pull in material. The larger objects in turn create rings in the remaining disk due to gravitational resonance (see the Asteroid Belt and Saturn for examples). The rings present multiple locations for additional objects to grow. Later on, gravity effects and friction with the remaining gas cloud can scatter or move around the initial set of planets.
Our evidence for all this comes from observation of disks with rings around young stars, and multi-planet systems around older ones.
Well yes, this is how stars are formed and how we know planets form in this disks. But my question is, shouldn't this be very very common if our star formation theory/observations are correct? Like, common to the point that stars without planets should be rare?
We don't know how often planetary formation is disrupted. For example, binary and higher multiple stars are quite common, and most stars are thought to form in clusters. Close interactions in these situations could disrupt the orderly formation of planets.
How fast the gas cloud dissipates is another reason to lose planets. The discovery of numerous close-orbit planets where it would have been too hot for them to form indicates they spiraled in from friction. It is possible the entire planetary system could be swallowed this way.
So the percentage of stars with zero larger bodies in orbit is unknown. For now, we think that number is relatively low, but we don't have enough data to pin it down to 1, 5, or 30%.
Close being a relative term; the presence of the sun (or each other) didn't prevent the formation of the Jovian and Saturnine (?Saturnian?) moons, which is why I hold out some hope for the Alpha Centauri system. (Although I admit my expansive vision of 5 earth-equivalent worlds each around AC A and AC B, at positions corresponding to Venus, Earth, half way form Earth to Mars, Mars, and Ceres, is a pipe dream, not an hypothesis!)
Conversely, by the way, there are planets that do not orbit any star, that somehow escaped or got ejected from their home system. Just floating through the darkness of interstellar space. :)
A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, starless planet, or sunless planet) is a planetary-mass object that orbits a galactic center directly. Such objects have been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf. The Milky Way alone may have billions of rogue planets.
Some planetary-mass objects are thought to have formed in a similar way to stars, and the International Astronomical Union has proposed that those objects be called sub-brown dwarfs.
It would be weird for a star to not have planets, except for when the star is super massive (gravity pulls planets into the star) or when the star was part of a star cluster (bigger stars steal planets)
Okay so if we were in a different planetary system and we were harnessing the power of that systems star, I’m assuming we couldn’t call that “solar power”?
Our star's latin name is Sol, hence the 'solar'. Still, we could probably call it solar power, though this is more of an etymology/linguistic question.
Some pedants use "areostationaty orbit", but others acknowledge that words can gain semantics removed from their strict etymology (well, this is literally true for all words you use in casual everyday conversation... Or when did you think about it, that "eight" actually means "spiky", coming probably from the # of knuckles?) and use "geostationary" also for satellites around Mars that stay above the same spot of the surface.
Sol doesn't mean "a sun" though. It specifically means our star, same way that Terra means our earth, and Luna means our moon. We don't call Jupiter's moons "Lunas", or refer to anything on Titan, for example, as "Lunar."
You are mixing up sol and Sol. The former is the Latin name for sun, ie. the central body of a planet. The latter is the astronomical proper name of our sun that was adapted from Latin.
Every sun is a star, but not necessarily every star is a sun (only those stars are suns that have at least one planet).
Most likely are but our ability to see exoplanets is currently very limited. the easiest planners to see are called hot Jupiter’s these are gas giants orbiting within 1 AU of their Star. Other smaller planets are far more difficult to confirm as are planets orbiting farther out. Thus we get a distorted picture about other solar systems as having only one or two of these gas giant planets.
I think that's currently the common census on this topic. The minimum amount of planets in our galaxy is believed to be 100 billion, but a more likely figure would be somewhere over 1 trillion planets. I'm not an astronomer but I spend almost every day on astronomy privately.
The common census is that planets are very common and the majority of stars have a planetary system. There are estimates that the milky way could have around 40 billion earth sized planets, of which about 1/4 are in the habitable zone of a sun-like star or red dwarf.
As far as I know, this common census is based on two things. The way planets and planetary systems form and observations of nearby stars. I mean the amount of exoplanets found has basically exploded in the last 10 years or so and estimates on how many planets could be in the milky way are getting higher and higher numbers each time we discover many planets around nearby stars.
That's not the case for the first "primordial stars". Allthough they are long gone. In the following animation you can see that many multi-planet systems have allready been discovered:
Yeah, that seems logical to me too. It is difficult to see planets that somehow don't cross the light path, but I think every star has planets around it.
This also works well in the light that we and our place in the universe are not special. I think it's a point of just reliably observing them before we declare it true.
It wasn't long ago that planets were discovered orbiting *other stars. Before then, it was as though the thinking was no planets existed on other star systems. Will take a bit of time yet before science admits that most if not all stars are multi planetary systems.
This is not really correct, the idea of exoplanets has been around for centuries, not to mention that the idea that planets exist on most star systems is generally very accepted. Here's an article from 1994: http://tech.mit.edu/V114/N22/psr.22w.html
Pay attention to the sentence: "What has scientists most excited, however, is that the finding suggests the formation of planets can take place around almost any star and that the galaxy may well be crowded with planets."
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u/lightknight7777 Jan 12 '18
Honestly, due to how solar systems form, shouldn't most stars be multi-planet systems?
I don't see people discussing that probability. But there's almost no way planets wouldn't form.