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As far as I am aware, the latest spectral types that have been assigned are around Y2, for objects like WISE 0855-0714 that have temperatures around 250 K or so. I've also seen several directly-imaged exoplanets have had their spectra classified as L- or T-type (e.g. Bonnefoy et al. 2016 who find that the giant planets HR 8799 d and e are good matches for L6–L8 dwarfs).

Jupiter is colder and less massive than the observed Y dwarfs and differs from isolated brown dwarfs because it is being illuminated by the Sun, but there is thermal emission which could be observed from the nightside of the planet. Has Jupiter's nightside spectrum (i.e. the spectrum of Jupiter excluding contributions from reflected sunlight) been observed in sufficient detail to compare it to the spectra of Y dwarfs, and if so does it fit the trends observed in spectral type Y or is it so different that it cannot be spectrally classified as a Y dwarf?

For clarity: I am asking purely in terms of spectral classification, i.e. the classification of the spectrum. I am not asking about whether Jupiter is a brown dwarf making the Sun+Jupiter a binary system. That has been asked elsewhere.

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According to this article:

(https://iopscience.iop.org/article/10.3847/1538-4365/ac886a#:~:text=The%20visual%20and%20near%2Dinfrared,and%20ammonia%20(NH3).)

"The visual and near-infrared (below 3.5 μm) spectrum of Jupiter is dominated by reflected solar light with absorption bands primarily from methane (CH4), phosphine (PH3), and ammonia (NH3)."

Now, Wikipedia states that water, methane, carbon monoxide, carbon dioxide, and ammonia, were detected in a class Y brown dwarf (WISE 0359−5401). We see that there are many similarities with the spectra of WISE 0359−5401 and Jupiter, in particular the methane and ammonia. So there are many similarities between the spectra of Jupiter and the spectra of brown dwarfs. One main difference is that Jupiter would be colder, so its peak wavelength would be longer. (0.01776 millimeters), as opposed to 0.005796 millimeters for class Y brown dwarfs assuming a temperature of 500 K (I used Wein's Law). So Jupiter and class Y brown dwarfs do have many similarities, but the temperature difference and the absence of deuterium fusion in Jupiter cause differences.

Edit: "A planet absorbs energy from the Sun in the form of light and converts the energy into heat. The heat is then reradiated back into space (mostly from the nightside of the planet). Based on how much energy Jupiter absorbs from the Sun, then its mean temperature should be 105 K (about -280 F). However, IR and radio measurements of Jupiter show that it has a mean temperature of 125 K, or 20 degrees too warm. In other words, Jupiter radiates about twice as much energy as it receives. Conservation of energy requires that this heat come from someplace and the only reservoir is the core of Jupiter. Thus, this extra heat is leftover energy from the time of Jupiter's formation." This article from http://homework.uoregon.edu/pub/emj/121/lectures/jupint.html does indicate similarities from Jupiter and Class Y brown dwarfs. They both gain energy from gravitational contraction. However, Jupiter's emission is mostly from remitted radiation from the Sun, unlike Brown dwarfs which undergo deuterium fusion.

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  • $\begingroup$ The coolest Y dwarfs have temperature similar to Jupiter. Why are you discussing a 500K object? The question also specifically asks about the night side Jupiter spectrum, which of course is not dominated by reflected light from the Sun. Deuterium fusion does take place in brown dwarfs. $\endgroup$
    – ProfRob
    Mar 9 at 21:14
  • $\begingroup$ I used the average temperature of class Y dwarfs, and analyzing the absorption lines from reflected sunlight after being reflected allows us to detect the composition of Jupiter's atmosphere. $\endgroup$
    – Astrovis
    Mar 10 at 0:19
  • $\begingroup$ I added more information about Jupiter's emission on its night side $\endgroup$
    – Astrovis
    Mar 10 at 0:21
  • $\begingroup$ Pretty sure that deuterium fusion is not happening in Y dwarfs though. That phase of the evolution takes place whilst they are class M or L. How would that affect the spectrum anyway - are deuterium features obvious in Jupiter's spectrum? $\endgroup$
    – ProfRob
    Mar 10 at 7:38
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This Wikipedia page lists Jupiter as a disputed sub-brown dwarf because its mass reportedly is sufficient to qualify for one. Distance to another star like the Sun doesn't matter, nor does the visibility in the infrared spectrum either since every planet is emitting some infrared light. The fact that Jupiter emanates more energy than it gets from the Sun can't be accounted for either, since that's also a matter of distance from the Sun, and Saturn and Neptune do so too. But, as the article states, Jupiter obviously has enough mass to possibly count as a sub-brown dwarf. Sub-brown dwarfs are still planets because they don't undergo nuclear fusion unlike brown dwarfs and hydrogen-fusing stars and never did.

Edit: There is no reason to only classify the night side just because the day side is illuminated by a(nother) star. Spectral classifications are given to the entire object. Since sub-brown dwarfs reportedly have a mass of at least one Jupiter mass, you may want to classify Jupiter a Y9V sub-brown dwarf.

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    $\begingroup$ This doesn't really answer the question, which is about the classification of Jupiter's spectrum. It isn't about whether or not Jupiter qualifies as a brown dwarf. $\endgroup$
    – user24157
    Jul 24, 2020 at 15:51
  • $\begingroup$ @antispinwards We don't know where a lower limit of Y-type objects might be set and therefore this is hard to answer with a number. Reportedly, sub-brown dwarfs have a mass of at least a Jupiter mass, so Jupiter would qualify as a sub-brown dwarf. In my opinion, the entire Y-type sub-brown dwarf class is unnecessary since they're actually planets (gas giants) if they don't undergo nuclear fusion. $\endgroup$
    – Ioannes
    Jul 24, 2020 at 16:20
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    $\begingroup$ Once again, this is not about whether or not Jupiter is a (sub-)brown dwarf. It's about its spectrum and how it compares to objects that have been classified as spectral type Y. $\endgroup$
    – user24157
    Jul 24, 2020 at 16:44
  • $\begingroup$ @antispinwards You already stated "Jupiter is colder and less massive than the observed Y dwarfs". So the spectrum is obviously less intensive than that of the observed ones. When looked from outer space on the objects, all of them have a pitch dark night side or are entirely like this when not illuminated by a star. No visible light is emitted on the exterior. $\endgroup$
    – Ioannes
    Jul 24, 2020 at 17:10
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    $\begingroup$ No, I'm specifically asking whether it fits into spectral class Y, which is what is in the question title and the body of the question. If I were asking about whether Jupiter is a brown dwarf or a planet (which is another question entirely), I would have asked that. Spectral classification is the classification of spectra. It is not the classification of the objects that produce those spectra. To take a different example, some late M-type dwarfs are hydrogen-burning stars, some are young brown dwarfs, but the spectra are classified as type M regardless. $\endgroup$
    – user24157
    Jul 24, 2020 at 18:21

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