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This answer to Compute Planet's Apparent Visual Magnitude reports the new work on planetary magnitudes, and contains an intriguing blurb about Neptune:

  • Neptune keeps getting brighter. No one knows why.

So I'd like to ask: How fast is Neptune getting brighter? When was was this first noticed and reported?

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    $\begingroup$ Although the precise reasons aren't well understood, I don't think people find it especially anomalous. We have accurate photometry for Neptune for only about 60 years or so, I think, so it's quite plausible that the variation is seasonal (Neptune's orbit is 165 years). Keep in mind also that the brightening is fairly subtle by ordinary standards, maybe a couple of tenths of a magnitude (about 15 percent brighter than before 1980). $\endgroup$ – Brian Tung Jul 30 at 3:58
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    $\begingroup$ The Wikipedia plot summary for Neptune has the magnitude change. Schmude et al. "The Secular and Rotational Brightness Variations of Neptune" has more details, but at any rate, its first paragraph states that the brightness has increased 10 percent in "the past several decades." $\endgroup$ – Brian Tung Jul 30 at 4:41
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    $\begingroup$ I don't know how steady the secular changes are; I haven't read the paper. (Note that "secular" is probably relative to how long we've been observing Neptune; they might still be periodic wrt Neptune's orbit.) $\endgroup$ – Brian Tung Jul 30 at 4:43
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    $\begingroup$ This article says "In 2002 images, Neptune is clearly brighter than it was in 1996 and 1998,” Sromovsky says, “and is dramatically brighter at near infrared wavelengths. The greatly increased cloud activity in 2002 continues a trend first noticed in 1998.”" That's impressive, though it doesn't give any numbers. scienceblog.com/1643/… I'm just going to say it. Neptune's wierd. $\endgroup$ – userLTK Jul 30 at 6:29
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    $\begingroup$ @BrianTung Please write an Answer so you can get credit and we can see something other than "0 answers." $\endgroup$ – Mike G Jul 30 at 10:19
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This is an incomplete answer—a placeholder for a more complete answer that I'm adding here at the request of the OP (who, I will say, has been very patient with me!).

Although the precise reasons aren't well understood, I don't think people find the observed brightening of Neptune to be especially anomalous. We have accurate photometry for Neptune for only about 60 years or so, I think, so it's quite plausible that the variation is seasonal (Neptune's orbit is 165 years). Keep in mind also that the brightening is fairly subtle by ordinary standards, maybe a couple of tenths of a magnitude (about 15 percent brighter than before 1980).

One source is Schmude et al. "The Secular and Rotational Brightness Variations of Neptune"; its first paragraph states that the brightness has increased 10 percent in "the past several decades."

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