Comment by goodcanadian | Hacker News Reader

Comment by goodcanadian | Hacker News Reader

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Is this really the standard terminology?

Well, it is close enough. The definition is that a brown dwarf forms like a star through gravitational collapse within a protostellar nebula, but it just wasn't massive enough to ignite. However, a planet forms in an accretion disk around a star that is forming out of the larger cloud. Of course, nature does not always perfectly align itself within our neat categories, and it is not really possible to distinguish between a brown dwarf and a supermassive planet that happened to be ejected from its star. Anyway, perhaps such a supermassive planet that is not ejected should really be considered a brown dwarf in a binary with the "parent" star. For gravitational collapse to occur in the first place, however, requires a minimum density which sort of puts a lower limit on the mass of a brown dwarf, so smaller objects are not really going to form independent of an accretion disk, but they may form in one and be ejected as you say. So yes, there is kind of a practical line between planet and brown dwarf, but it is a bit of a fuzzy line.

> The definition is that a brown dwarf forms like a star through gravitational collapse within a protostellar nebula

Do you have a cite on that? (It conflicts with Wikipedia, for example.) In particular, I suspect that many people read something that describes how a certain type of star is thought to typically form and then interpret that as a definition.

Do you have a cite on that?

Off the top of my head, no, but I do have a degree in astronomy and an interest going back to childhood.

shrug I have a degree in physics and have always heard people define brown dwarfs by size (and occasionally composition) rather than formation mechanism.

Does this also hold for rogue planets? I was under the impression that some rogue planets could form outside of any star's accretion disk.

But they did form in some star's accretion disc, just not the one they are roguing their way through. In theory, Jupiter could have caused a planet to go rogue when it made it's way to the inner solar system.

My understanding is that smaller rouge plants do not form in isolation and are always the product of ejection.

However, the categorization again is fuzzy, with definitional overlap between "planet", "sub-brown-dwarf", and "brown dwarfs"

Planets cover spherical objects ranging from 0.001 Jupiter masses to stars at ~80 Jupiter masses. Brown dwarfs range from 3-80 Jupiter masses.

Brown Dwarfs are planets and can form outside of an accretion disk. Planets smaller than brown dwarfs are not thought to be able to form outside of accretion disks.

> My understanding is that smaller rouge plants do not form in isolation and are always the product of ejection.

Wikipedia says "Rogue planets may originate from planetary systems in which they are formed and later ejected, or they can also form on their own, outside a planetary system."

The ones "forming on their own" would be essentially brown dwarfs. As myself and others have said, the line is fuzzy. However, to form on their own there needs to be sufficient density for them to coalesce. It is not a hard limit, but tends to mean only objects larger than about 10 Jupiter masses.

No. That’s not what wikipedia says, and I feel confident asserting that an Earth-mass object forming on its own would not be considered a brown dwarf by the vast majority of astronomers. (Maybe it would also not be considered a “planet”, due to some Pluto-adjacent technicalities, but then what do people call it? I’m betting they use “rogue planet” in practice.)

I think you are missing the size distinction and range for rogue planets, which I specifically highlight.

Claims about self formation pertain to rouge planets as a class, which includes brown dwarfs. That does not mean an earth mass object IS a brown dwarf.

If you want to want to focus on Wikipedia text, it also says:

>Pre-JWST observations have shown that objects below 3-5 MJ are unlikely to form on their own.[4] Observations in 2023 in the Trapezium Cluster with JWST have shown that objects as massive as 0.6 MJ might form on their own, not requiring a steep cut-off mass[1]

Those limits are ~1,000 earth masses and ~200 earth masses, respectively

https://en.wikipedia.org/wiki/Rogue_planet#Formation_like_a_...

No, an Earth-mass object forming on its own would not be considered a brown dwarf, but an Earth-mass object is incredibly unlikely to form on its own. An object that small will have almost certainly been formed in an accretion disk and been ejected.

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