Are there any other planets besides Arda in Ea?

Question

Did Tolkien mention anything about other planets in his mythology or did he invent just the one planet for his stories?

Answer 1

No other planets are visited in the Legendarium¹, but several of Tolkien's dictionaries (published in History of Middle-earth in a number of places) include Quenya and Sindarin names for what we now know of as planets²; whether they were known to be planets by the inhabitants of Middle-earth is unknown.

Some examples:

• Morwen (Gnommish) and Silindo (Quenya): Jupiter (from The Book of Lost Tales, Part 1)
• Carnil (Mars)
• Luini (possibly Neptune)
• Nénar (possibly also Neptune)
• Lumbar (Saturn)
• Alcarinquë (Jupiter)
• Elemmírë (possibly Mercury)

The final examples are from the Index of Morgoth's Ring.

As TGnat reminds me in comments, the star known as Eärendil corresponds to what we would call the planet Venus. The complication here is that we know what Eärendil is: it's a half-elf (named Eärendil, funnily enough) flying through the air with a Silmaril strapped to his forehead, so it's definitely not a planet.

Since no further details about the other "planets" I've named above are given, it's entirely possible that they are also not actually planets in Tolkien's cosmology, but some other kind of thing. Unfortunately we don't know for sure.

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¹ Except Aman, which is sort of another planet after the Fall of Númenor

² This pre-supposes that Middle-earth is in our solar system, which was Tolkien's intention. We have a number of questions about that on this site, including How exactly is Arda supposed to be an ancient Earth? and Is Tolkien's Middle-earth in our Universe? and In what time period does The Lord of the Rings take place in the real world?

Answer 2

From the Silmarillion, when Varda creates the Stars:

Then Varda went forth from the council, and she looked out from the height of Taniquetil, and beheld the darkness of Middle-earth beneath the innumerable stars, faint and far. Then she began a great labour, greatest of all the works of the Valar since their coming into Arda. She took the silver dews from the vats of Telperion, and therewith she made new stars and brighter against the coming of the Firstborn; wherefore she whose name out of the deeps of time and the labours of Eä was Tintallë, the Kindler, was called after by the Elves Elentári, Queen of the Stars. Carnil and Luinil, Nénar and Lumbar, Alcarinquë and Elemmírë she wrought in that time, and many other of the ancient stars she gathered together and set as signs in the heavens of Arda: Wilwarin, Telumendil, Soronúmë, and Anarríma; and Menelmacar with his shining belt, that forebodes the Last Battle that shall be at the end of days. And high in the north as a challenge to Melkor she set the crown of seven mighty stars to swing, Valacirca, the Sickle of the Valar and sign of doom.
_{The Silmarillion - Chapter 3 - "Of the Coming of the Elves and the Captivity of Melkor"}

In particular note the phrase "Carnil and Luinil, Nénar and Lumbar, Alcarinquë and Elemmírë she wrought in that time".

A c.1951 manuscript of this passage which is kept in the Marquette collection (MS 3/9/36), has the following letters above each name:

Carnil = M
Lumbar = S
Luinil = (no letter)
Nénar = N (crossed out)
Alcarinquë = Jup
Elemmírë = M
_{Vinyar Tengwar #12, "Nole i Meneldilo: Lore of the Astronomer"}

In Vinyar Tengwar #12, editors Jorge Quinonez and Ned Raggett reason that this is likely a list of planet names, and so the "S" would be Saturn, and the two "M's" Mars and Mercury, distinguished by fact that one of them includes the root carn- 'red'. Excluding Pluto (which may have been too small for the elves to see), Earth (which is Arda), and Venus (which is Eärendil), that leaves Neptune and Uranus for the two remaining ones. They point out that luin- means blue and so associate Luinil with Neptune, and thus arriving at the following list:

• Elemmírë = Mercury
• Carnil = Mars
• Alcarinquë = Jupiter
• Lumbar = Saturn
• Nénar = Uranus
• Luinil = Neptune

In the introduction to the index of Morgoth's Ring, Christopher Tolkien discusses this manuscript page and the conclusion reached in Vinyar Tengwar, saying that "this conclusion appears to be no mere ingenious deduction but to derive from my father's own clear indications". Christopher admits that he finds it strange that Uranus and Neptune should appear on a list of bright celestial objects, but supposes that his father must have previously independently decided on names for all the planets prior to drafting that passage.