Edit: As @msj121 's answer says, it's the GLGLIM (spheres, orbits, ... ) that Rambam refers to as being clear, appearing blue etc.
I just want to add that there is also much more than empirical physical characteristics; the ככבים (planets and stars) have souls etc. .. see just further down in parag. 9:
https://www.sefaria.org/Mishneh_Torah%2C_Foundations_of_the_Torah.3.9?lang=bi
כָּל הַכּוֹכָבִים וְהַגַּלְגַּלִּים כֻּלָּן בַּעֲלֵי נֶפֶשׁ וְדֵעָה וְהַשְׂכֵּל הֵם. וְהֵם חַיִּים וְעוֹמְדִים וּמַכִּירִין אֶת מִי שֶׁאָמַר וְהָיָה הָעוֹלָם. כָּל אֶחָד וְאֶחָד לְפִי גָּדְלוֹ וּלְפִי מַעֲלָתוֹ מְשַׁבְּחִים וּמְפָאֲרִים לְיוֹצְרָם כְּמוֹ הַמַּלְאָכִים. וּכְשֵׁם שֶׁמַּכִּירִין הַקָּדוֹשׁ בָּרוּךְ הוּא כָּךְ מַכִּירִין אֶת עַצְמָן וּמַכִּירִין אֶת הַמַּלְאָכִים שֶׁלְּמַעְלָה מֵהֶן. וְדַעַת הַכּוֹכָבִים וְהַגַּלְגַּלִּים מְעוּטָה מִדַּעַת הַמַּלְאָכִים וּגְדוֹלָה מִדַּעַת בְּנֵי אָדָם:
All of the stars and spheres are beings, endowed with a soul, intelligence and understanding; they continue a purposeful life and are conscious of the existence of Him Who spoke and the universe sprang forth. Each and every one, according to his greatness and degree, extol and glorify the Creator as do the angels; and to the extent that they recognize the Holy One, blessed is He! they also know their own selves and the angels above them. But the intelligence of the stars and spheres is lesser than the intelligence of the angels, but greater than the intelligence of man.
And even here on earth, a rock or some earth in Eretz Yisrael is entirely holy, compared to an "identical" rock or dirt in Chu"l. See Likutei Moharan 116
סִפֵּר שֶׁכְּשֶׁהָיָה בְּאֶרֶץ־יִשְׂרָאֵל, סִפְּרוּ לוֹ שָׁם הַחֲשׁוּבִים שֶׁהָיוּ שָׁם, שֶׁבָּאוּ מִמְּדִינוֹת אֵלּוּ וְקָבְעוּ דִּירָתָם שָׁם בְּאֶרֶץ־יִשְׂרָאֵל, כַּמְפֻרְסָם. וְסִפְּרוּ לוֹ, שֶׁקֹּדֶם שֶׁהָיוּ בְּאֶרֶץ־יִשְׂרָאֵל, לֹא הָיוּ יְכוֹלִים לְצַיֵּר לְעַצְמָן, שֶׁאֶרֶץ־יִשְׂרָאֵל הִיא עוֹמֶדֶת בְּזֶה הָעוֹלָם, וְהָיוּ סוֹבְרִים שֶׁאֶרֶץ־יִשְׂרָאֵל הוּא עוֹלָם אַחֵר לְגַמְרֵי, לְפִי גֹּדֶל קְדֻשַּׁת אֶרֶץ־יִשְׂרָאֵל הַמְבֹאָר בַּסְּפָרִים. וּמְפֹרָשׁ בְּתוֹרָתֵנוּ הַקְּדוֹשָׁה מַעֲלַת קְדֻשַּׁת אֶרֶץ־יִשְׂרָאֵל כַּמָּה וְכַמָּה פְּעָמִים, וְהַתּוֹרָה בְּעַצְמָהּ מְפָרֶשֶׁת כָּל גְּבוּלֵי אֶרֶץ־יִשְׂרָאֵל לְגֹדֶל מַעֲלָתָהּ וּקְדֻשָּׁתָהּ הֶעָצוּם וְהַנּוֹרָא מְאֹד מְאֹד. עַל־כֵּן לֹא הָיוּ יְכוֹלִין לְצַיֵּר לְעַצְמָן בְּעֵינֵיהֶם, שֶׁאֶרֶץ־יִשְׂרָאֵל יִהְיֶה בְּזֶה הָעוֹלָם, עַד אֲשֶׁר בָּאוּ לְשָׁם וְרָאוּ, שֶׁאֶרֶץ־יִשְׂרָאֵל הִיא בְּזֶה הָעוֹלָם מַמָּשׁ.
Rebbe Nachman related that when he was in the Land of Israel, he spoke with distinguished figures there who had originally been from this locale and settled in the Land of Israel, as is known. They told him that before being in the Land of Israel, they could not picture for themselves that it was located in this world. Considering the way the holy books depict the great sanctity of the Land of Israel, they assumed it was in an entirely different world. Its sanctity is described in our holy Torah numerous times, and the Torah itself specifies all its boundaries on account of its loftiness and sanctity, which are extremely immense and awesome. Therefore, they could not picture for themselves, with their own eyes that the Land of Israel exists in this world, until they came there and saw that it is actually in this world.
כִּי בֶּאֱמֶת אֶרֶץ־יִשְׂרָאֵל הוּא כְּמוֹ מְדִינוֹת אֵלּוּ מַמָּשׁ, וַעֲפַר אֶרֶץ־יִשְׂרָאֵל הוּא בְּמַרְאֶה וּדְמוּת כְּמוֹ עֲפַר מְדִינוֹת אֵלּוּ מַמָּשׁ. כִּי מַה שֶּׁמְּבִיאִין לִפְעָמִים עָפָר לָבָן מִשָּׁם זֶהוּ רַק בִּמְקוֹמוֹת פְּרָטִיִּים, וְגַם בִּמְדִינָה זוֹ נִמְצָא עָפָר לָבָן וְעָפָר שֶׁל נֶתֶר בְּכַמָּה מְקוֹמוֹת, אֲבָל עִקָּר הֶעָפָר בְּאֶרֶץ־יִשְׂרָאֵל הוּא כְּמוֹ עֲפַר מְדִינוֹת אֵלּוּ מַמָּשׁ. כִּי בֶּאֱמֶת בִּדְמוּת וּתְמוּנָה אֵין חִלּוּק כְּלָל בֵּין אֶרֶץ־יִשְׂרָאֵל וּבֵין שְׁאָר מְדִינוֹת, לְהַבְדִּיל, וְאַף־עַל־פִּי־כֵן הִיא קְדוֹשָׁה מְאֹד מְאֹד בְּתַכְלִית הַקְּדֻשָּׁה עֲצוּמָה וְנוֹרָאָה מְאֹד. אַשְׁרֵי הַזּוֹכֶה לַהֲלֹךְ שָׁם אֲפִלּוּ אַרְבַּע אַמּוֹת וְכוּ', כַּאֲשֶׁר הִפְלִיגוּ רַבּוֹתֵינוּ זִכְרוֹנָם לִבְרָכָה וְכָל הַסְּפָרִים בְּעֹצֶם קְדֻשָּׁתָהּ הַגְּדוֹלָה וְהַנּוֹרָאָה:
For, in fact, the Land of Israel is exactly like these lands, and in look and appearance its soil is exactly like that of these lands. Although people occasionally bring white soil from there, that is only from particular locations, and even in this land white soil and natron are found in some places. However, the majority of the soil in the Land of Israel is exactly like the soil of these lands. The fact is that in outward appearance there is no difference at all between the Land of Israel and other lands, l’havdil. Nonetheless, it is extremely holy, supremely sanctified by an immense and awesome holiness. Praiseworthy is he who merits taking even four steps there…—as our Sages, of blessed memory, and all the holy works say in extolling its immensely great and awesome holiness.