Story highlights Australian astronomers have discovered a planet orbiting a star 500 light years away

The planet is unusually close to the star -- HATS-6 -- orbiting it every 3.3 days, they say

An amateur astronomer helped confirm the planet's presence using a backyard telescope

(CNN) A huge "puffed up" planet "too big for its star" some 500 light years from Earth has been discovered by Australian researchers -- with the help of a backyard astronomer.

The planet's size in relation to the star it orbits challenges theories about how planets are formed, researchers from the Australian National University said.

"We have found a small star with a giant planet the size of Jupiter orbiting very closely," said George Zhou from the Research School of Astrophysics and Astronomy.

"It must have formed further out and migrated in, but our theories can't explain how this happened."

Astronomers believe that planets are created from a "disc" of dust and gas around stars, Zhou told CNN.

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