The expression comet wine (or comet vintage) denotes a wine (or a vintage) produced in a year in which a notable comet appeared, and therefore thought to be of superior quality.

The expression comet vintage occurred, for example in The Adventure of the Stockbroker’s Clerk, by the British author Arthur Conan Doyle (1859-1930), published in The Strand Magazine: An Illustrated Monthly (London, England) of March 1893:

Sherlock Holmes cocked his eye at me, leaning back on the cushions with a pleased and yet critical face, like a connoisseur who had just taken his first sip of a comet vintage.

The following two authors mentioned the connection between a comet and a wine of superior quality:

1-: The British amateur astronomer Patrick Moore (1923-2012) in Patrick Moore’s Data Book of Astronomy (Cambridge (UK): Cambridge University Press, 2011):

The Great Comet of 1811, discovered by Honoré Flaugergues on 25 March, was also a daylight object; it had a coma about 2 000 000 km in diameter, and a tail which extended for 160 000 000 km. En passant, the wine crop in Portugal was particularly good, and for years afterwards ‘Comet Wine’ appeared in the price lists of wine merchants.

2-: The U.S. physician Robert Tomes (1817-1882) in The Champagne Country (New York: Hurd and Houghton, 1867):

The knowing ones said that the wine of 1865 would compare with that of 1822 for precocity, with that of 1849 for quantity, and with that of 1846 for quality. There had never been, since 1811, a grape so ripe, so sugary, and one harvested under such favorable circumstances of weather. It was agreed, however, that the juice of 1865, excellent as it was, would never be such as was obtained in the exceptional comet year of 1857, and which is the basis of the celebrated wine of 1858, the best ever made. […]
[…]
The cork—already branded on the lower end, generally with the image of a comet, ¹ and on the side with the name of the manufacturer, by an iron die heated over an alcohol lamp,—is at least one third too large.
¹ This in honor of the “comet year,” which produced so fine a vintage.

The earliest occurrences of the expression comet wine that I have found are as follows, in chronological order:

1-: From a letter that the British naturalist and patron of science Joseph Banks (1743-1820) wrote to the British horticulturalist and botanist Thomas Andrew Knight (1759-1838) on Friday 26^th September 1817:
—as published in The Letters of Sir Joseph Banks: A Selection, 1768-1820 (London: Imperial College Press, 2000), edited by Neil Chambers:

The newspapers during the whole summer & autumn have been full of inundations in all Countries where Rivers are fed from Icy mountains, & have told us that the Coast of West Greenland, which has not been free from Ice for 150 years, has been this year seen & found clear of all obstruction for many leagues. The Atlantic has been unusually clogged with Islands of Ice.
Possibly I am too sanguine, but as I have always attributed the increasing Coldness of our Climate to the increase of Polar Ice, I feel a hope that we shall be indulged with better Springs than have lately been provided for us. It is now 16 years, I am told, since the Cyder harvest was too large for the Casks provided to contain it. In France there has been no good Vintage since the Comet year, & they are now all drinking Comet-wine.

2-: From The Star (London, England) of Friday 17^th July 1818:

Such has been the favourable effect of the peculiarly fine weather enjoyed during the whole of this summer, on the vineyards on the Rhine, that a triple vintage is expected. On many stems there are 90 bunches of grapes, and the grapes are already of the size of peas. If nothing happens to blight this fair prospect, “we may safely prophecy,” says a Rhenish Journal, “that the wine of this year will be equal to what is called Comet wine.”

The earliest occurrence of the expression comet vintage that I have found is from Sales by Auction, published in The Morning Herald (London, England) of Monday 1^st February 1819:

Five Pipes of Port in the Wood, East and West India Madeira, Sherry, &c.
By Mr. PHILLIPS,
At his Great Rooms, New Bond Street, on SATURDAY, the 13^th of February, at One for Two precisely,

FIVE PIPES of excellent and highly-flavoured PORT, of the Comet Vintage, 1812, selected by the Proprietor (for his own use), with unquestionable judgment and liberality, of one of the of the most respectable houses at Oporto.