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Astronomers may have found the first exomoon

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Re: Astronomers may have found the first exomoon

#31

Post by delta_p_delta_x »

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

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.

Re: Astronomers may have found the first exomoon

#32

Post by walrus01 »

delta_p_delta_x wrote: post #18
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 of a brown dwarf is that there is enough heat and compression for deuterium and/or lithium fusion, but not protium fusion. The exact model limit is unclear, but the physics modelling is relatively straightforward.

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

Re: Astronomers may have found the first exomoon

#33

Post by walrus01 »

Foskya wrote: post #29
walrus01 wrote: post #28

Earlier quoted context omitted.

> 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

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.

Correct, but the link for "super jupiter" also goes to something that astronomers are fairly sure is 1.38x the diameter of Jupiter. Much less massive of course. Meaning Jupiter's diameter is not an absolute hard limit nothing can be larger than.

https://en.wikipedia.org/wiki/HAT-P-1b

It also orbits its star in only under 5 days, so a few theoretical visualization/renderings of it would probably be quite spectacular.

Re: Astronomers may have found the first exomoon

#36

Post by GolfPopper »

selectodude wrote: post #14
Foskya wrote: post #9

I guess it is more of a derivative of the question: "in which bucket should we put the brown dwarf? Is it a star or a planet?". The answer is neither of course; but since it is more closely related to stars than to planets, I'm more inclined to call this satellite an exoplanet (instead of exomoon). That being said it is still a nice finding, way more difficult to discover than "normal" exopanets.

I thought we’re supposed to call them dwarf planets now. Dwarf star, dwarf planet.

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.

Re: Astronomers may have found the first exomoon

#38

Post by foolfoolz »

Foskya wrote: post #9

I guess it is more of a derivative of the question: "in which bucket should we put the brown dwarf? Is it a star or a planet?". The answer is neither of course; but since it is more closely related to stars than to planets, I'm more inclined to call this satellite an exoplanet (instead of exomoon). That being said it is still a nice finding, way more difficult to discover than "normal" exopanets.

Planet++

Re: Astronomers may have found the first exomoon

#40

Post by pfdietz »

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.
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