Astronomers may have spotted the smallest possible stars
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Re: Astronomers may have spotted the smallest possible stars
#2Saturn-size brown dwarfs? Who would’ve thought!
Re: Astronomers may have spotted the smallest possible stars
#3> 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?
Re: Astronomers may have spotted the smallest possible stars
#4> 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 does…
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.
Re: Astronomers may have spotted the smallest possible stars
#5> 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 does…
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".
Re: Astronomers may have spotted the smallest possible stars
#6> 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 does…
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...
Re: Astronomers may have spotted the smallest possible stars
#7> 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 does…
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.
Re: Astronomers may have spotted the smallest possible stars
#8> 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 does…
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 isolatio…
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.
Re: Astronomers may have spotted the smallest possible stars
#9> 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 does…
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 isolatio…
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.
Re: Astronomers may have spotted the smallest possible stars
#10> 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 does…
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…
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.