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Comment by jessriedel | Hacker News Reader

> But distinguishing between smaller brown dwarfs and planets requires looking at how they form. Even though they can end up about as massive as 10 Jupiters, planets always arise around a star, from its surrounding disk of gas and dust.

> In contrast, stars, including brown dwarfs, form on their own within giant collapsing clouds of gas.

Is this really the standard terminology? It’s not how I remember it, and it doesn’t make much sense. There are tons of binary star systems, and while a some are three-body capture events, aren’t most formed from the same gas cloud? (I.e., not “on their own”.) Likewise, rogue planets (i.e., not bound to a star) can be formed in a stellar system and be ejected, but can’t they also form on their own, e.g., a dust cloud with less than enough total mass to form a star? Surely you wouldn’t call a sub-Jupiter-mass body a “brown dwarf” just because it formed in isolation?

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There's a continuum from tiny planetesimal to a huge star of 50+ solar masses, and sometimes the distinction is blurred.

Many red dwarf stars are hardly larger than Jupiter despite being more than 80 times as massive (this is commonly cited as the lower limit for protium fusion, which is the definition of a star[1]).

> Surely you wouldn’t call a sub-Jupiter-mass body a “brown dwarf” just because it formed in isolation?

In my view, the criteria for what gets classified as brown dwarf stars isn't the circumstances of their formation, but only their mass and hence the nature of fusion (if any) in their interior. So if a gas cloud collapsed into a single sub-Jupiter-mass body, it is a planet. The article says a 7-Jupiter-mass star could be a brown dwarf, and I believe the lower limit is unclear because there could be deuterium/tritium fusion at such low masses, and even at higher masses there could be no fusion at all[2].

I think the article was trying to make the distinction between gravitational collapse and accretion, but honestly, accretion can also sometimes go runaway and produce a body that is about brown-dwarf mass (i.e. 13-80 Jupiter masses).

[1]: https://coolcosmos.ipac.caltech.edu/page/low_mass_stars_brow...

[2]: https://iopscience.iop.org/article/10.1088/0004-637X/770/2/1...

Not deuterium/tritium fusion, but rather deuterium/protium fusion: D + p --> 3He + photon. There is no tritium. The star is not hot enough for the 3He to then fuse with itself, as it does in our Sun or to some extent in red dwarf stars. In the center of our Sun, the Dp reaction goes so fast that a D nucleus would be consumed in about 1 second.

A lot of sources just say "deuterium burning", and people assume this is DD. But at low temperature, the reaction rate is strongly affected by barrier penetration, and this is strongly influenced by the reduced mass of the two nucleus system.

The 3He3He reaction is slow in red dwarfs, and I understand the concentration of 3He in them builds up to about 1% before plateauing. I don't think any red dwarf is yet old enough to reach that stage. Red dwarfs are fully convective; unlike our Sun their entire mass circulates through the core and becomes available to undergo nuclear fusion. This (and their low luminonsity) makes them very long lived, up to a trillion years, far longer than the universe has yet existed.

When no category really fits the best option is to just come up with a new name.

Planets were initially defined by their motion in the nights sky. That’s continued to this day by saying they must have cleared their orbit. So if a large mass formed alone it really doesn’t fit the ancient or modern definition of a planet.

Further, many of these things may eventually become stars as they attract enough mass. Calling something that turns into a star in a 10 billion years a planet until suddenly swapping to young stellar object when the conditions change, just doesn’t fit IMO.

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.

As the article clearly points out several times by referencing the potential "upending of star formation theory" and the list of "brown dwarfs that shouldn't exist", current star and planetary formation theories are severely lacking, so you're right to question definitive statements such as stars forming "on their own".

I am questioning whether the article is accurately describing the scientific nomenclature. The author of course wants to say things are being “upended”, but I suspect the author just didn’t understand, or is inaccurately describing, the previous state of affairs.

The nomenclature has shifted as we have found stars are make our "average white star," less average, less white, and less of a star.

The classification nomenclature is definitely in need of a refactoring.

I think its getting at the difference in formation (i.e. gravitational collapse vs accretion). I'm not sure you always need a parent star to form planets by accretion though, perhaps they could emerge from dust clouds spontaneously?

As I remember it there is supposed to be some gap in size between the objects produced by these two different methods, so that nothing produced by accretion can be larger than something formed by collapse. But since that gap doesn't seem to exist in practice it really looks like we are missing something.

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