Bright stars outshining faint nebulae is nothing new: the so-called "hidden planetary nebula" Abell 12 was discovered only 40 years ago on photographic plates, because it's outshined by 4.1 magnitude star μ Orionis. Aldebaran is much brighter - 0.9mag - so it seems plausible that a faint nebula has been hidden from our view for this long time.
Let's look at the evidence. The object appears at RA 04h36m30s Dec +16°26'15", 9' SE of Aldebaran on images like this one from DSS2 (WikiSky):
|Screenshot of wikisky.org. White arrow denotes the object.|
It's more obvious on this high-res POSS II image:
|Credit: Second Palomar Observatory Sky Survey|
|Credit: HST Guide Star Cataloge 1|
There might be one: Someone in this German forum dug up a photo of Aldebaran taken by another British amateur, Prof. Greg Parker. The image is deeper than most other shots of this particular star. It is shown here with permission by the photographer. Although not as high-res as the Palomar images above, I think one can find a blurry spot right where the object should be:
|12x 5 minute exposures, Sky 90 refractor (f/4.5), Starlight Xpress M26C 10 Megapixel OSC CCD|
©Greg Parker and Noel Carboni
While some folks are already speculating whether the object might be a planetary nebula or a galaxy, I am not convinced it is anything real at all at this moment. More deep photographs, visual observations and - if these are positive - a spectrum are required to determine the nature of this ghostly "something". Unfortunately, this will not be easy to do - some amateurs fear their CCD might get damaged by the bright light of Aldebaran.
Update: Via Twitter came the information that the object is also clearly visible in all three 2MASS bands. More precise position: 04:36:29.9 +16:26:01.1. Different instrument, different wavelength - this is getting interesting!
Update 2: I sifted through some more images in Aladin Lite, and found the object not only in 2MASS but also WISE infrared images. So I am much more inclined to think it is a real thing:
|2MASS (Aladin Lite screenshot)|
|WISE (Aladin Lite screenshot)|
A challenging observation. At least, Aldebaran is situated perfectly for observation in the winter sky.
Update, Dec. 24
Jost Jahn, an amateur astronomer from Germany did some imaging with a remote 0.6m telescope in southern France yesterday (see comments) - the best image of the object I know so far! Thanks!
|The best image of the image so far to my knowledge) - thanks to Jost Jahn, Germany!|
I took the liberty of playing around with the image and applied a Larson-Sekanina filter in Fitswork. The filter enhances both radial and circular symmetric structures. It seems the object is something like a shell, with two opposing lobes NE and SW of a central brightening (which does not look much like a star, maybe a faint one). As a visual observer, this looks very much like a Planetary Nebula to me, but, as said above, a spectra is needed to confirm this.
|Larson-Sekanina analysis of above image.|
Update, Dec. 26:
Another image and opinions towards the "background galaxy" explanation. No spectrum so far.