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Astronomers may have found the first exomoon

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Re: Astronomers may have found the first exomoon

#71

Post by lucideer »

walrus01 wrote: post #28
delta_p_delta_x wrote: post #22

Worth noting that the artist's impression is... not accurate. Both CD-35 2722 b (the brown dwarf orbiting the primary star) and CD-35 2722 b I (the exomoon orbiting the secondary) should be much closer in size. It is estimated that Jupiter is essentially the largest any gas giant can get; adding more mass will simply increase density and interior temperature until deuterium and lithium fusion and brown dwarfdom, and…

> It is estimated that Jupiter is essentially the largest any gas giant can get That does not appear to be the case if we mean mass, not diameter. https://en.wikipedia.org/wiki/Super-Jupiter https://en.wikipedia.org/wiki/CoRoT-3b

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

Re: Astronomers may have found the first exomoon

#72

Post by ButlerianJihad »

renegade-otter wrote: post #43
HelloUsername wrote: post #10

An important phrase from the article to consider before commenting: "This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’"

We don't really have that nomenclature. We just recently decided what a planet it for the solar system, and even that is contentious.

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

Re: Astronomers may have found the first exomoon

#73

Post by nkrisc »

delta_p_delta_x wrote: post #22

Worth noting that the artist's impression is... not accurate. Both CD-35 2722 b (the brown dwarf orbiting the primary star) and CD-35 2722 b I (the exomoon orbiting the secondary) should be much closer in size. It is estimated that Jupiter is essentially the largest any gas giant can get; adding more mass will simply increase density and interior temperature until deuterium and lithium fusion and brown dwarfdom, and…

It’s a perspective rendering, not orthographic. I don’t think you can reliably judge relative volume based on this image.

Re: Astronomers may have found the first exomoon

#74

Post by ButlerianJihad »

replatformradar wrote: post #42

About time!, I cant wait till we start putting large hardware on the dark side of the moon. That will make JST look like a toy.

The radio silence which makes the far side so attractive will also present our most significant challenge in communicating with that hardware.

Re: Astronomers may have found the first exomoon

#77

Post by vikingerik »

hparadiz wrote: post #56
delta_p_delta_x wrote: post #22

Worth noting that the artist's impression is... not accurate. Both CD-35 2722 b (the brown dwarf orbiting the primary star) and CD-35 2722 b I (the exomoon orbiting the secondary) should be much closer in size. It is estimated that Jupiter is essentially the largest any gas giant can get; adding more mass will simply increase density and interior temperature until deuterium and lithium fusion and brown dwarfdom, and…

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 and then slowly burn out or would that ignition precipitate a pressure wave through the entire body forcing fusion to begin everywhere?

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.

Re: Astronomers may have found the first exomoon

#78

Post by vikingerik »

walrus01 wrote: post #13

> Instead, it circles a brown dwarf, an object larger than a planet, that orbits the CD-35 2722 star. Isn't the dividing line between the largest possible gas giants and the smallest brown dwarfs a bit fuzzy? https://en.wikipedia.org/wiki/Substellar_object

The definition isn't fuzzy, we define a brown dwarf as an object that has undergone deuterium fusion.

The fuzziness is in the physical properties needed to achieve that. 13 Jupiter masses is generally the threshold where deuterium fusion starts, but that can vary: a brown dwarf composed of only primordial H and He would need a bit more, while one with some heavier elements from previous generations of stars will be denser and need slightly less. Rotation speed also matters: a faster rotator will experience centrifugal force and thus have less interior pressure.

And there is fuzziness in what we can observe, so sometimes we're not sure if an object is a brown dwarf. We may not know its exact mass and composition, and we might not be able to tell if it's radiating energy from deuterium fusion, or just from gravitational compression, or (in a binary system) if it's just reflecting light from its companion star.

Re: Astronomers may have found the first exomoon

#79

Post by delta_p_delta_x »

vikingerik wrote: post #77
hparadiz wrote: post #56

Earlier quoted context omitted.

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 and then slowly burn out or would that ignition precipitate a pressure wave through the entire body forcing fusion to begin everywhere?

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 the…

Thanks; this is much more concise than I could manage.

I think the key idea is fixed minimum/maximum boundaries for a lot of astrophysical phenomena—stellar protium fusion, the Chandrasekhar limit, Tolman–Oppenheimer–Volkoff limit, etc.

Re: Astronomers may have found the first exomoon

#80

Post by pfdietz »

pfdietz wrote: post #40
delta_p_delta_x wrote: post #22

Worth noting that the artist's impression is... not accurate. Both CD-35 2722 b (the brown dwarf orbiting the primary star) and CD-35 2722 b I (the exomoon orbiting the secondary) should be much closer in size. It is estimated that Jupiter is essentially the largest any gas giant can get; adding more mass will simply increase density and interior temperature until deuterium and lithium fusion and brown dwarfdom, and…

One thing this implies is that the escape velocity of such objects increases linearly with mass, so the surface temperature they can sustain without losing mass increases quadratically with their mass. Massive super-jovian planets can orbit close to their star, limited only by tidal disruption. Some could even orbit within the outer envelope of the star for quite some time.

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