this post was submitted on 27 Jan 2025
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This doesn't seem to be the case, at least according to the brown dwarf wikipedia page which seems to use Jupiter as the yardstick for what isn't a brown dwarf.
Not by my ctrl-f "Jupiter" on that page.
Jupiter is, however, the top of the list on wikipedia's page for Y-type brown dwarfs.
Spectral type Y objects are not all brown dwarfs, they just have a similar elemental composition. Jupiter doesn’t have the mass to be considered a brown dwarf, they are 13-80 times the mass of Jupiter by definition.
That mass-based definition is outdated and does not consider recent observations of the interiors of Jupiter and Saturn made by the Juno and Cassini spacecraft. It is a reflection of cold-war era fusion chauvinism and is due to get an update. Jupiter is a star, Saturn straddles the boundary between star and planet.
I would suggest that a brown dwarf straddles the line between star and planet (the Wikipedia page begins with ("Brown dwarfs are substellar objects") and that therefore Jupiter is, at best, straddling the line between star and planet, and therefore Saturn is solidly a planet.
I believe there is an object called a brown sub-dwarf which jupiter would clasify IF it wasn't part of a planetary system that basically represents the smallest type of failed star, however since jupiter formed from a protoplanetary disc it is indeed a planet. It really is a bit of an issue with our classifications that they're context dependant though. E.g the moon on its own could be a dward planet, earth orbiting at the same distance as pluto would also be a dwarf planet even with no other changes.
I like what you're trying to do, but I disagree with merging brown dwarfs with planetary class objects because their interior structures and evolution are so different. Brown dwarfs are closer to stars than planets. The only difference between brown dwarfs and fusing stars is whether fusion occurs at the core. Planets are very very different in structure, morphology, and evolution.
This is how I suggest we classify things:
Let's start by splitting things into two classes: planetary class and stellar class with Saturn at the boundary. This is a separation based on internal morphology and dynamics.
Stellar class objects then get split into two further subclasses: fusing stars (suns) and non-fusing stars (brown dwarfs).
Saturn exists at the boundary between the planetary class and the stellar class. Jupiter is solidly within the "brown dwarf" non-fusing stellar class of objects. The sun is a "fusing star", which is also within the stellar class.
I mean, you can choose to define things however you want for your personal headcanon.
But for communication to work, people need to agree upon meanings. I'm guessing you don't have a PhD in astrophysics, so your opinions are very unlikely to sway the consensus opinion on how these things are defined. And it's their definitions that most lay people are going to take our cues from.
But even from the perspective of trying to come up with your own definitions...it's rather poor practice to define things by presupposing your desired outcome. They didn't define a planet vs dwarf planet by reference to Pluto, even though their desired goal was to exclude Pluto. They found actual criteria and used those. The definitions you're giving, by stating "stellar class with Saturn at the boundary" does not work as a very good definition. Though again, you're free to use that for yourself if you want...so long as you understand you will have severe difficulty communicating with others.
Most of the thesis is written, and the definitions I am giving are common among my colleagues. This is the growing consensus post-Cassini/Juno.
I'm not choosing these definitions with any presuppositions. I'm using Saturn as a useful marker of the boundary because of its hybrid internal structure as revealed by kronoseismology.
Still no, from the first sentence of your article
That's mostly because of the outdated IAU definition of the boundary between those genres being at deuterium fusion. Deuterium has a low abundance, and its fusion happens very briefly and early in the lifecycle of these larger dwarfs. That flash is not significant in the grand scheme of what these objects fundamentally are, is highly theoretical and has never been observed, and is more a reflection of cold-war era fusion chauvinism than an actual morphological boundary between object classes.
See that also seems to be using it as a point of reference? It labels it a "Y-class analog", and every other entry on the list is much heavier and hotter. I'm just not sure.
That's mostly because the heavier and hotter the object, the easier they are to detect by various means. We've only recently been able to detect Y-dwarfs and measure their spectral/chemical properties. We still cannot detect Jupiter-size Y-dwarfs beyond our solar system. Jupiter is analogous to the chemical and spectral properties that we've seen in these larger dwarf stars. However, that's only the outer atmosphere, and that alone isn't enough to conclude that Jupiter is a star.
The Juno mission has used gravity data to confirm that Jupiter's hydrogen plasma has fully dissolved what was once the planetary nucleus around which the hydrogen accreted. This is the interior transition to stellar morphology.
Similarly, the Cassini mission has used ring seismology (studying waves raised in the rings by planetary seismology and mass anomalies) to confirm that Saturn has a partially dissolved planetary core. Saturn is an object that can be classified as neither a planet nor a star, and represents a class of transition objects straddling the non-binary border between these genres of objects.
See the following previous comment threads where I've fleshed out this argument in more detail and with references:
https://sh.itjust.works/comment/14345115
https://sh.itjust.works/comment/15877252