Learn Spanish (Latin America): Este exoplaneta es lejanísimo.

The root word is the adjective lejano (distant). To form this absolute superlative, we drop the final vowel (-o) and add the suffix -ísimo. Because the noun it modifies (exoplaneta) is masculine and singular, the adjective also takes the masculine singular form: lejanísimo.

Is there a difference between saying es muy lejano and es lejanísimo?

Yes, there is a difference in intensity. While muy lejano just means "very distant," the -ísimo suffix creates an absolute superlative. This means the quality is pushed to its absolute limit, translating closer to "extremely distant," "incredibly distant," or "distant to the highest possible degree."

We usually use estar for location and distance. Why does this sentence use es?

You are right that expressing a physical location usually requires estar (as in El exoplaneta está lejos). However, here we are using the adjective lejano (distant) to describe an inherent characteristic or category of the planet, which requires ser. If we wanted to use the adverb lejos (far) with a superlative, we would switch back to estar: Este exoplaneta está lejísimos.

Exoplaneta ends in -a, so why do we use the masculine instead of ?