# How did Jupiter form where it is?

Since Jupiter is the biggest planet in our solar system and is made up of mostly hydrogen and helium, (the gases the Sun uses to create energy), how come it didn't form as far out as say Uranus or Neptune since when our star formed it took most of the gases around it to create? Just a little confused on this, thanks for the help!

• Fun fact: Jupiter formed further out and "migrated" inwards after interactions with the other gas giants, whose orbits were also affected. That said, gases in the disk weren't necessarily concentrated at the center. The Solar System originated in a protoplanetary nebula; not all of that collapsed to form the Sun. Jun 14 '15 at 23:58

It's at Jupiter's distance and beyond that ices were able to form out of the disk of material surrounding the early sun. Go much further in and there is too much energy from the sun for them to stay as solids (and will sublimate into gases); this is why asteroids are principally rocks and metals. So at this distance more of the materials of the planetary disk can form the base planetesimals.

At this point, your question is answered largely by a geometric consideration (or two) and one of Kepler's laws.

First, the geometric consideration. A circle of radius $r$ has area $\pi r^2$. The bigger the radius, the more area. The material for Jupiter (or any other planet) came from an annulus: stuff outside one circle, but inside a slightly bigger circle. This annulus had a lot more area for the out planets than the inner planets, and so could contain a lot more mass.

Of course, that could makes us think that Jupiter shouldn't be the largest of the gas giants: it's the closest of them all to the sun, after all. The density of the disk needn't have been approximately constant throughout these regions, though. Quite possibly the density was such that Jupiter's region had more mass than the areas for the other planets. As HDE's answer (posted as I was finishing this) points out, these ices probably also helped stop materials from passing into the inner solar system, maintaining a higher density than you might otherwise expect in the inner solar system, as well as causing materials to sort of "dam up" right around Jupiter's orbit.

Now for the Kepler's law. The further you get from the sun, the slower your orbital period. Picking the correct units, we have $P^2=a^3$, where $P$ is the period measured in years, and $a$ is the semi-major axis of the orbit measured in AU. The further out you go, the slower you go around the sun; indeed, it's not just that it takes you a longer total time, but your actual velocity goes down. We can also see this as a Newton's law of gravity consequence. At Jupiter's furthest point from the Sun, the escape velocity is a little more than 18 km/s. At Saturn's maximum distance, the escape velocity drops to around 13.25 km/s. So things can go roughly 35% faster within Jupiter's orbit than they can closer to Saturn's, and they have less distance to travel to make a complete orbit.

What this means is that it takes longer for planetesimals to get close enough to each other to accrete together the further out you go, and there is a longer mean time between collisions.

Now, eventually, the Sun "turned on" and started blasting space with it's solar wind (before that, the heat came primarily from thermal radiation from the gravitational contraction of the sun). This ended up clearing out most of the unaccreted particles out of the solar system, stopping planetary growth (and removed portions of existing atmospheres; a very young Earth probably had a lot of H and He in its atmosphere, until the sun hit it with enough energy to knock any of it not locked up in rocks away).

So Jupiter was probably in a bit of a goldilocks situation. The average density of the region it formed in was probably higher than where the other giants formed in, it was at the perfect spot for lots of materials to start accreting early, and the accretion process would have been faster. So Jupiter is growing faster, and this gives it a competitive advantage: the bigger the growing planetesimals get the further their influence extends and the faster they can pull in more materials, and subsequently interfere with the growth of other planets (or planetesimals). Somewhere around a mass of 10-15 earth masses, the giants can start pulling in large quantities of the hydrogen and helium gasses. And, again, Jupiter likely hit this mass well before the other giants, and had more material to pull from, so it became much larger than the others could before the solar wind stopped the process.

• Nice. This explains perfectly why Jupiter is so much more massive. Jun 15 '15 at 0:11

Here's something from Wikipedia (emphasis mine):

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. Planetesimals beyond the frost line accumulated up to 4 M⊕ within about 3 million years. Today, the four giant planets comprise just under 99% of all the mass orbiting the Sun. Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced into a large embryo (or core) on the order of 10 M$_⊕$, which began to accumulate an envelope via accretion of gas from the surrounding disc at an ever increasing rate.

Quoting from the actual Wikipedia article on the frost line:

The lower temperature in the nebula beyond the frost line makes many more solid grains available for accretion into planetesimals and eventually planets. The frost line therefore separates terrestrial planets from giant planets in the Solar System.

Any giants that lie inside the frost line in stellar systems most likely migrated inwards after interactions with the protoplanetary disk or, as happened in our Solar System (but didn't cause Jupiter to move that far inwards), with other giant planets. The Nice model describes such incidents