Can Jupiter's nightside be classified as spectral type Y?

Question

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.

Answer 1

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.

Answer 2

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.