Astronomers may have found the first exomoon

But... he literally said in the following sentence that adding more mass does not change size, implying that he is talking about diameter, not mass.

Yes; I was under the impression that from context it was clear what I was talking about. In astrophysics, to avoid precisely this confusion, the convention is to use the adjective 'massive' when comparing, well, masses, and 'largest'/'smallest' for the dimension of length and its higher powers (radius/diameter/area/volume). For instance:

> the Sun is 3.33e5 as massive as Earth.

Who uses the word size to mean mass? The typical colloquial analogue to mass is "weight" - which nobody would ever conflate with "size".

Ugh I got that wrong. Escape energy increases linearly with mass, so the temperature goes up linearly as well.

There's really a lot to unpack here. (I rewrote this response thrice...) To spoil the answers straight away, and perhaps to address some misconceptions upfront:

> If a brown dwarf is right on the edge of stardom would it start protium fusion in only the part of itself that is the right pressure

The nuclear cross-section of protium fusion in astronomical bodies is determined by temperature, pressure, and density. These variables are in turn dictated by the total mass of the object in question. Brown dwarfs never have sufficient mass for protium fusion, so they never undergo any protium fusion whatsoever. This is a hard-and-fast boundary for stardom.

> or would that ignition precipitate a pressure wave through the entire body forcing fusion to begin everywhere

The three variables above are not uniformly distributed throughout the body; there is a maximum at the centre, and fusion only happens here. Even in the Sun, about 99% of the fusion happens within about a quarter of the radius from the centre. The temperature drops rapidly thereafter, reducing the nuclear cross-section of the proton-proton chain to essentially zero. At the photosphere (surface), the temperature is ~5777 K, which is a decidedly Earthly temperature (lightning bolts are ~ 30000 K). So no, fusion does not happen everywhere.

Now, we need to discuss star formation and why brown dwarfs have never experienced protium fusion at any time in their lives. Star formation is still a very active area of research, debate, and fitting models to empirical study, and this is especially true for the detailed interior and structure of protostars and pre-main-sequence stars; hence, this is going to be quite surface-level (pun not intended).

Collapsing molecular clouds form stars. The total mass of a given cloud (or a particular region of it) sets an upper bound on the resultant object, because the total mass dictates the gravitational potential energy and hence the terminal velocity of the matter, and hence the rate of matter infall at the centre before the cloud dissipates. If the mass is low enough, the central object will become a brown dwarf, or even a large gas giant and a 'rogue planet'.

As the gas cloud collapses, the central region increases in density, temperature, and pressure, but no fusion occurs yet. Conservation of angular momentum forms a circumstellar disc, and material continues to fall onto the central region. As long as this infall continues, the central region is called a protostar. At some point the mass of protostar crosses the boundary needed for deuterium fusion; if the infall stops here, the result is a brown dwarf. If this infall continues, the mass increases beyond the boundary (~80 Jupiter masses) needed for protium fusion, and protium fusion can begin. When the infall stops and the circumstellar disc largely dissipates, the result is a pre-main-sequence star.

Note that both these very young pre-stellar objects are not yet at hydrostatic equilibrium, and are still comparatively rarefied (or 'puffy') compared to main-sequence stars; they are still collapsing, and the temperature, density, and pressure at their cores continues to increase. Only when this equilibrium is achieved and gravitational collapse is halted do stars begin life on the main sequence.

Now, it should be evident why brown dwarfs never experience protium fusion: at no point in their lives have they ever had any region in their interior hot, dense, or hyperbaric enough to have a high enough nuclear cross-section for protium fusion. At their formations, they were simply not massive enough; they continue to collapse, which admittedly provides a considerable power output—surface temperatures are ~1000 K. The largest brown dwarfs experience deuterium/tritium/lithium fusion into helium, but this also stops over time.

(Side note: in my opinion the word 'brown dwarf' is a bit of a misnomer, because look at how bright molten iron (~ 1500 K) is even in broad daylight[1]; now imagine an object ten to twenty times the radius of Earth, emitting this much heat from every square millimetre. If you approached a 'new' brown dwarf it would cast a lot of light.)

[1]: https://commons.wikimedia.org/wiki/File:Scunthorpe_Molten_St...

The other reply gave good info, but didn't quite answer this. Once an object's core acquires enough temperature/pressure/density to start protium fusion, it will continue, not burn out. The fusion itself increases the temperature, and the produced helium is denser than hydrogen, so the core compresses more under gravity thus increasing the pressure too. So protium fusion creates its own conditions to continue, so there's no such thing as borderline stardom; once protium fusion starts at all, it will continue and it's now a red dwarf. Fusion doesn't begin or occur everywhere, it's still only at the core, but convection will eventually (10^12 year timescale) cause all the hydrogen to have moved through the fusing region.

The skies are so dark and clear that some of the indigenous people had "constellations" made up of the dark patches in the sky instead of connecting stars.

Simple stargazing seems to be selling it short. It's the best seeing on the planet. I can do simple skygazing from my backyard. With a four hour drive I can get to a spot for pretty advanced skygazing. The ESO is building the largest telescope man has ever created in a spot you describe as good for "simple skygazing".

It's a problem of useful/meaningful labeling.

'Planet' comes from the Greek word for 'wanderer' which was very useful for labeling the handful of bright stars that moved through the heavens in a pattern. Then we got to the Space Age and kept calling them (and similar new additions) 'planets' even as we learned far more about them. And immediately there's a problem. Setting aside Pluto (and the Sun), you've still got Mercury on one end (5% of Earth's mass, and smaller than several moons) and Jupiter on the other end (320% of Earth's mass - more than everything else in the solar system put together, and over 1000 times Earth's volume). "Planet" stops being a word you can use with any sort of detail. (Thus the IAU definition battle and Pluto's "demotion".) Astronomers have pretty good terminology for stars, but once you go sub-stellar, the labeling rapidly gets difficult, and for good reason.

Small-statured-person planets.

Because a brown dwarf is a star? https://en.wikipedia.org/wiki/Brown_dwarf

Granted, makes sense...

Exactly. Chile has a special status as host country of the observatory!

Obviously, how else would you do it rather than the standard left-margin notation for the host country :)

Redefined. We can point to at least 4 distinct definitions and lists of “planets” throughout history.

Neutron stars are held up by neutron degeneracy pressure, btw, not electron degeneracy pressure. White dwarfs stars are held up by that (even though they can be very hot; the temperature needed to "lift" degeneracy increases with the mass of the object.) Even the cores of some very large stars are electron degenerate; this is why they remain stable against the "pair production instability": there aren't any states into which newly created electrons can go, suppressing pair production.

It might be helpful to describe gravitational relationships with human generational terms of parent-child, grandparent-grandchild, sibling, cousin, etc. So the Sun is the Moon's gravitational grandparent and Jupyter is its parent-sibling (Aunt/Uncle, maybe pibling upwards and nibling downwards).

I had only the vaguest notion up until 15 minutes ago of how these are being categorized, this one is described as either a brown dwarf or giant planet? https://en.wikipedia.org/wiki/CoRoT-3b

see the Classification header of the page

It is from the Moon's perspective. On the visual spectrum the Earth is much larger and more reflective than the Moon. You're looking at something like 40x greater illumination during a 'full Earth' than you get during a full Moon on Earth. Earth is also extremely noisy in the radio spectrum for radio telescopes, while the Moon itself blocks most of all of that out on the far side.

Sorry my dark side comment was not in reference to the light or darkness of the moon it was in reference to the more "radio silenced" aspect of it. But just a solid platform for building a ultra large scale machine would be a start off planet. I am stoked about the discoveries JWST has driven with its jump in res and reach!

By the time we have a radio telescope on the far side of the Moon, I assume we can just lay a couple hundred kilometers of fiber to the nearest settlement. The border of the disk, from Earth's perspective, is prime scientific real estate - the border of the area that's always visible is not too far from the area that's never visible (and, therefore, shadowed from our radio noise).

Maintenance crews can do quick hops to the telescope, while their offices are always in contact with Earth.