Planet FM, as it appears in the third game.

Planet FM (FMプラネット) is a fictional planet from the Mega Man Star Force series. It is home to the FM-ians, also known as FM lifeforms (FM星人), a race of aliens who have electromagnetic bodies who usually bear the names of constellations and feed on human negative emotions. They are the main villains in the first installment, having destroyed the similar Planet AM and planning to destroy planet Earth due to their king Cepheus' paranoia. However, some of them are not evil, and by the end their king decides to halt his actions and make up for his crimes.

The planet itself appears to be covered in EM Waves, which it emits into space.

List of FM-ians

Taurus Taurus (Ox in Japan) is one of the FM-ian life forms that appear on Earth. He is named after the constellation Taurus the Bull or Ox, and takes on the form of a flaming bull. He is said to favor size and brute force, and due to this Wave Changes with Geo's rather strong friend Bud Bison to become Taurus Fire (Ox Fire in Japan). Early in the third game, he becomes Bud's Wizard.
Cygnus Cygnus is an FM-ian who favors grace. He takes the form of a blue swan as he's named after the constellation Cygnus the Swan. He takes advantage of Aaron Boreal's assistant Tom Dubius's paranoia to take him over to become Cygnus Wing. In the third game, his residual wave data is recovered and converted to Wizard format in order for Dubius to freely Wave Change.
Lyra Lyra (Harp in Japan) is a female FM-ian who usually gives Omega-Xis a hard time in conversations due to her femininity. She's named after the constellation Lyra the Lyre (or Harp) and thus takes on the form of a lyre. She uses young singer Sonia Strumm's (Sonia Sky in the anime) loneliness and misery to Wave Change with her to become Harp Note (Lyra Note in the English anime dub). Oddly, she chooses to stick with Sonia rather than abandon her like the rest of the FM-ians do, due to her liking both Earth and Sonia.
Libra Libra is an FM-ian who balances fire and water on his two scales for arms. He gets his name and appearance from the constellation Libra the Scales. In order to Wave Change, he takes over the body of Geo's teacher Mitch Shepar and becomes Libra Scales (Libra Balance).
Ophiuca Ophiuca (Ophiuchus in Japan) is an FM-ian named after the constellation Ophiuchus the Serpent Holder. Her serpentine-like appearance doesn't become very pronounced until she enters Luna's body and becomes Queen Ophiuca (Ophiuchus Queen). When she is defeated, her residual body wave data remained inside Luna, which Dark Phantom abused to force her to Wave Change.
Gemini Gemini is named after the constellation Gemini the Twins, and has two heads in accordance with this. He forms an alliance with Geo's classmate Patrick Sprigs and becomes Gemini Spark, an entity of two beings: Gemini Spark White and Gemini Spark Black, White being controlled by Pat and Black by Pat's alternate personality, Rey.
Cancer is the crab FM-ian, named after the constellation Cancer the Crab. He Wave Changes with the 3rd grader Claud Pincer and becomes Cancer Bubble. Unlike other FM-ians, he was not inherently "evil", and rather, decides to stay along with Claud afterward.
Wolfmugshot.png Wolf, named after the constellation Lupus the Wolf, is a werewolf FM-ian who merges with the gardener Damian Wolfe and becomes the savage Wolf Woods (Wolf Forest). He glows orange and goes insane when he sees a full moon. Wolf is also generally not an "evil" FM-ian. Almost ironically, he is rather calm minded, compared to Wolf Woods' transformed state. He remains Damian's Wizard in Star Force 3.
Crown is themed after two constellations, Corona Australis the Southern Crown and Corona Borealis the Northern Crown, and as a result wears a crown the size of his original body. He merges with the skull and ghost of a deceased king, Couronne XIV, and becomes Crown Thunder.
Goat-Foo is a Goat FM-ian named after Capricornus the Goat. He meets with the boy Kidd Gruff and partners with him to become Kung Foo Kid (Goat Kungfu).
Virgomugshot.png Virgo is an FM-ian named after the constellation Virgo the Virgin. She appears in MegaMan Star Force 3 and with her partner, Queen Tia, the two transform to become Queen Virgo. She is a criminal that escapes to Earth and almost tries to break Ace and Queen Tia apart.
Corvusmugshot.png Corvus is an FM-ian named after Corvus the Crow/Raven. He appears in MegaMan Star Force 3 and with his partner, Jack, they transform to become Jack Corvus. He is a criminal that escapes to Earth along with Virgo. He has a past relationship with Omega-Xis as they used to engage in duels back on Planet FM.
Cepheus FM King Cepheus is the ruler of Planet FM. He is shown to be very paranoid and is easily manipulated by Gemini because of this. He is named after the constellation Cepheus, a constellation named after a king from Greek mythology.
Andromeda is an FM-ian planet destroyer. It destroyed Planet AM and is the final boss of the first installment of the series, as it attempts to destroy Earth. It has a ball of loneliness at its center and devours other EM beings. It's possibly named after Andromeda, a constellation of a mythological princess; although Andromeda's gender is never revealed, as everyone refers to Andromeda simply by "it". However, it's more likely named after the Andromeda Galaxy than the constellation, as it is themed after a planet-eating space monster, which transforms into a space-themed robot. Whether it's originally an FM-ian or it was built by the FM-ians, its origins remain unknown, although it seems to have a legendary reputation among the FM-ians.