Which spiral arm of the Milky Way lies just outside the Orion Arm?

The spiral arm of the Milky Way just outside the Orion Arm is the Perseus Arm.

The Sun lies in the Orion Arm, more precisely in a smaller structure often called the Orion Spur or Local Arm. Looking outward from the Solar System, astronomers identify the Perseus Arm as the next major spiral feature. Looking inward toward the Galactic Center, the Sagittarius–Carina Arm is the closer neighbouring arm.

The Milky Way’s structure is difficult to map because Earth is inside its dusty, star-filled disk. Astronomers use radio observations, infrared surveys, stellar distances and star-forming regions to trace the arms. The Perseus Arm is considered one of the galaxy’s two major arms, although different maps classify and label the Milky Way’s arms somewhat differently. It should not be confused with the Perseus constellation, which is simply a direction in the sky and is not the arm’s physical location.