It is most likely that he saw the solar system since the verse says that he saw the sun, the moon, and eleven planets. We know that the sun and moon are real. Therefore, it is most likely the eleven planets are real and are the planets in the solar system. Based on this statement of faith, I would say there is an eleventh planet.

"Ibn Kathîr writes: In Joseph’s dream the eleven stars represent his brothers, who were eleven, and the Sun and the Moon represent his father and mother. This explanation was given by Ibn `Abbâs, al-Dahhâk, Qatâdah, Sufyân al-Thawrî and `Abd al-Rahmân b. Zayd b. Aslam. Joseph’s vision became a reality forty years later – or as some say, eighty years – when Yusuf raised his parents to the throne while his brothers were before him. This is what the dream represented. It was in no way detailing a point of astronomy. Moreover, even if we were to assume that the eleven planets that Joseph (peace be upon him) saw in his dream were the images of real heavenly bodies, Joseph mentions them in an absolute, unqualified sense. There is nothing in the verse to indicate that those eleven are the only planets in existence, or that those planets refer specifically to objects within our Solar System, or even that they refer to planets as astronomers define them today. We must also realize that at the time the Qur’ân was reveled, the Arabic word used in the verse – kawkab – was used to refer to stars as well as to what we consider planets. Restricting the word kawkab to mean planets as opposed to stars was a much later development. In the dictionary al-Qamûs al-Muhît (131), al-Fayrûzabâdî defines a kawkab as a “star” (najm), and indeed the word kawkab was traditionally used interchangeably with najm. Therefore, we can safely conclude that the verse is not at all addressing the question of how many planets are in the Solar System. The verse is simply referring to Joseph’s dream vision where his brothers appear as celestial bodies. Islam has no doctrinal stance on the matter. " "The Number of Planets in the Solar System is a Question of Terminology In the field of Astronomy, the number of planets in the Solar System is primarily a question of nomenclature. It depends on two things: (1) What we observe in space and the discoveries that we make (2) How we wish to define a planet." "Therefore, the present grand total of eleven (planets + dwarf planets together) is only temporary, since the official classification of other known dwarf planets is simply a matter of procedure. Also, astronomers expect to discover many other large, round objects beyond Neptune’s orbit, since such discoveries have been considerable in recent years. Conclusions This shows us quite clearly how dangerous it is to try and interpret verses of the Qur’ân to express meanings that those verses do not express. Unfortunately, some people today have the habit of advancing such interpretations in hopes of establishing the existence of certain scientific facts or recent theories in the Qur’ân. " Source and further information: http://www.islamtoday.net/english/showme2.cfm?cat_id=29&sub_cat_id=870 Further information: http://answers.yahoo.com/question/index?qid=20071116184257AABZ7Ap

No.

it could be the planet of 2012, when it arrives, everyone will say Prophet Joseph predicted it, when the Maya were the first.