mass = $mass; $this->radius = $radius; } public function mass() : float { return $this->mass; } public function radius() : float { return $this->radius; } public function surfaceGravity() : float { return self::G * $this->mass / ($this->radius * $this->radius); } public function surfaceWeight(float $otherMass) : float { return $otherMass * $this->surfaceGravity(); } }