[EvaH (OP)]

Dave wants to know:

How did Jupiter originate, considering it is only made of gas (mainly hydrogen, I believe)? Where did its gravity come from to start with?

What do you think?

[chiralSPO]

Jupiter isn't only made of gasses, though hydrogen and helium are its primary components.

Luckily, there is no need for heavier elements to condense into stellar and planetary bodies. (otherwise we would be stuck with the primordial composition of the universe). Having rocky and icy and metallic substances certainly speeds the process along (both by being dense, and by radiating energy more effectively than hydrogen and helium alone, allowing the heat of planetary or stellar formation to leave as EM radiation rather than just heating up the collapsing cloud and making it expand again), but all matter has mass, and if you get enough mass in a small enough space, it will hold itself together gravitationally.

My understanding is that Jupiter formed from the same cloud of dust and gas that the sun and the rest of the planets formed from.

[evan_au]

Where did its gravity come from to start with?

From a proto-planetary disk, with a mass higher than the Sun.
- Once the Sun ignited hydrogen fusion, strong stellar winds blew away most of the gas and dust that hadn't already condensed into planet-sized objects.
- Multiple planetary systems are thought to form from local concentrations of matter within a large, cool dust cloud

See: https://en.wikipedia.org/wiki/Protoplanetary_disk

[amateurastronomer]

Current models for Jupiter’s origin suggest instead that a solid core of about 10 Earth masses formed first as a result of the accretion of icy planetesimals. This core would have developed an atmosphere of its own as the planetesimals released gases during accretion. As the mass of the core increased, it would have become capable of attracting gases from the surrounding solar nebula, thus accumulating the huge hydrogen-helium envelope that constitutes Jupiter’s atmosphere and fluid mantle. The accumulating envelope would have mixed with the outgassed atmosphere from the core. Thus, the presently observed enrichment of the most abundant heavy elements in this envelope, compared with solar values (see table), reflects the concentration of such elements in the core. The mass spectrometer on the Galileo probe (see above The atmosphere) showed that these heavy elements are enriched by the same factor of about three. For this enrichment to include volatile substances like argon and molecular nitrogen requires that the icy planetesimals must have formed at temperatures of 30 K (−400 °F, −240 °C) or less. Just how this happened remains a puzzle, though it is possible that convective mixing caused the enrichment