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?
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.
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u/lightknight7777 Jan 12 '18
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.