[Iwonda (OP)]

If a cloud of dust and gas does not have enough gas to form a star, can planets still form or will you just end up with a lot of asteroids?

[evan_au]

if a cloud of dust and gas does not have enough gas to form a star

There are many different kinds of interstellar clouds, with different size, composition, velocity profile and temperature, all of which will affect the outcome.
- One case that fits this description is a "Planetary Nebula", formed from the escaped outer atmosphere of a dying star. Depending on the size of the star, this may have a lot of carbon or heavier elements. This gas is moderately concentrated (by interstellar standards), and has a low velocity, and could clump together into dust grains, and larger objects. The Murchison meteorite is thought to contain dust from such an environment,
- There are vast clouds of intergalactic hydrogen that have plenty of mass to form stars, but lack the heavy elements needed to form planets or asteroids (...at least, until stars form, and produce heavier elements by nuclear fusion, and then release them as a planetary nebula or supernova)
- There are clouds of gas in our galaxy large enough to form stars and planets, but the temperature is too high to allow collapse. The temperature has to be quite low (about 10 degrees above absolute zero) to allow the clouds to condense.
See: https://en.wikipedia.org/wiki/Star_formation#Interstellar_clouds

Overall, I expect that if a gas cloud has heavy elements, is dense enough and cool enough, then it will form bodies of all possible sizes.
- We see that smaller bodies (eg M-Class stars) are more common than larger bodies (eg our own G-class star), which are more common than larger O-class stars. The M-Class stars make up something like 75% of all stars.
- We could extropolate that bodies smaller than M-class stars would be even more common. This would include brown dwarf stars, Jupiter-size gas giants, Earth-sized planets, and asteroid-sized objects. However, these objects do not emit light of their own, so there is no good census of their population.
- Most common of all would be dust-sized grains
See: https://en.wikipedia.org/wiki/Stellar_classification#Harvard_spectral_classification

Perhaps the James Webb Space Telescope (if/when it finally becomes operational) may be able to conduct a census of nearby brown dwarf stars, due to its large mirror and sensitivity to infra-red radiation...(?)

will you just end up with a lot of asteroids?

Asteroids still require a process of accretion, and some are large enough to experience significant melting and density-based differentiation into a metal core and rocky surface.

How dust and and gas accretes to form larger bodies is still a matter of some debate among astrophysicists. Some theories suggest that the Solar System could not have formed within the age of the universe - and yet here it is.
- Astronomers using infra-red telescopes have peered into dust clouds, and see what looks like accretion processes in operation
- Some planets formed in these planetary systems would be ejected by gravitational interactions, to form free-floating bodies from meteorite size up to Jupiter-sized objects.
See: https://en.wikipedia.org/wiki/Accretion_(astrophysics)#Accretion_of_planets

[chiralSPO]

I don't see any reason planets couldn't form under these conditions.

There are plenty of brown dwarf "failed stars" that are essentially large gas giant planets that never quite got big enough to ignite and turn into stars.

There are thought to be many starless "orphan" planets, but they are essentially impossible to detect with current methods (most of which detect the planets indirectly through measurements of their host stars). But even if the universe is teeming with orphan planets, it is still unknown what proportion of them were born in interstellar space, and what proportion were born near stars, and then tossed out through interaction with some more massive bodies.

[chiralSPO]

oops! crossed with evan...

[Iwonda (OP)]

Thanks, great answers. I did not know that temperature played a roll in planet formation, very interesting.

[Petrochemicals]

if a cloud of dust and gas does not have enough gas to form a star

There are many different kinds of interstellar clouds, with different size, composition, velocity profile and temperature, all of which will affect the outcome.
- One case that fits this description is a "Planetary Nebula", formed from the escaped outer atmosphere of a dying star. Depending on the size of the star, this may have a lot of carbon or heavier elements. This gas is moderately concentrated (by interstellar standards), and has a low velocity, and could clump together into dust grains, and larger objects. The Murchison meteorite is thought to contain dust from such an environment,
- There are vast clouds of intergalactic hydrogen that have plenty of mass to form stars, but lack the heavy elements needed to form planets or asteroids (...at least, until stars form, and produce heavier elements by nuclear fusion, and then release them as a planetary nebula or supernova)
- There are clouds of gas in our galaxy large enough to form stars and planets, but the temperature is too high to allow collapse. The temperature has to be quite low (about 10 degrees above absolute zero) to allow the clouds to condense.
See: https://en.wikipedia.org/wiki/Star_formation#Interstellar_clouds

Overall, I expect that if a gas cloud has heavy elements, is dense enough and cool enough, then it will form bodies of all possible sizes.
- We see that smaller bodies (eg M-Class stars) are more common than larger bodies (eg our own G-class star), which are more common than larger O-class stars. The M-Class stars make up something like 75% of all stars.
- We could extropolate that bodies smaller than M-class stars would be even more common. This would include brown dwarf stars, Jupiter-size gas giants, Earth-sized planets, and asteroid-sized objects. However, these objects do not emit light of their own, so there is no good census of their population.
- Most common of all would be dust-sized grains
See: https://en.wikipedia.org/wiki/Stellar_classification#Harvard_spectral_classification

Perhaps the James Webb Space Telescope (if/when it finally becomes operational) may be able to conduct a census of nearby brown dwarf stars, due to its large mirror and sensitivity to infra-red radiation...(?)

will you just end up with a lot of asteroids?

Asteroids still require a process of accretion, and some are large enough to experience significant melting and density-based differentiation into a metal core and rocky surface.

How dust and and gas accretes to form larger bodies is still a matter of some debate among astrophysicists. Some theories suggest that the Solar System could not have formed within the age of the universe - and yet here it is.
- Astronomers using infra-red telescopes have peered into dust clouds, and see what looks like accretion processes in operation
- Some planets formed in these planetary systems would be ejected by gravitational interactions, to form free-floating bodies from meteorite size up to Jupiter-sized objects.
See: https://en.wikipedia.org/wiki/Accretion_(astrophysics)#Accretion_of_planets

Technically not correct evan as it is a pressure temperature density volume relationship in relation to star formation, the average temerature is 10k
 
https://en.m.wikipedia.org/wiki/Jeans_instability#Jeans_mass


I think whilst it is possible that planets can form outside a star system,  you have to balance the gravitational attraction of a large masses, be smaller than the minimum ammount needed for star formation, the fact that the accretion disc around planets does alot to regionalise  matter in a dense arrangement, the huge distances in interstellar space, it would have to be very unlikely.

[Blimey]

You would think matter could clumps together into planet without the need for a star

[Kryptid]

I know this isn't in the spirit of the question, but the answer is technically no. The current, internationally-accepted definition of a planet requires it to orbit a star. However, a planetary-mass object should be able to form.

[hamdani yusuf]

I know this isn't in the spirit of the question, but the answer is technically no. The current, internationally-accepted definition of a planet requires it to orbit a star. However, a planetary-mass object should be able to form.

It is possible to form a planetary-mass object without a star, but we don't call it a planet yet until it is captured by a star's gravitational field and orbit the star. Now the object can be called a planet, although it was formed without a star.