Predators

Royce (Adrien Brody) awakens to find himself parachuting into an unfamiliar jungle. He meets several others who arrived in the same manner: Mexican drug cartel enforcer Cuchillo (Danny Trejo), Spetsnaz soldier Nikolai (Oleg Taktarov), Israel Defense Forces sniper Isabelle (Alice Braga), Revolutionary United Front officer Mombasa (Mahershala Ali), death row inmate Stans (Walton Goggins), Yakuza enforcer Hanzo (Louis Ozawa Changchien), and a general practice doctor Edwin (Topher Grace).
Upon landing, they discover that all are armed and lethal killers, with the apparent exception of Edwin, though none know where they are or how they got there. The group follows Royce, who Isabelle suspects is a former black operations soldier turned mercenary. In the jungle they find empty cages, plants with a neurotoxic poison that Edwin collects on a scalpel, and a deceased US Special Forces soldier. Arriving at higher ground, they find themselves staring at an alien sky and realize that they are not on Earth.
The party is attacked by a pack of quadruped alien beasts. Cuchillo is killed, and his body is then used as bait to lure the survivors into a trap, but they avoid it. Royce deduces they are on a planet used as a game preserve, where humans are hunted as game. The group follows the quadruped's tracks to a camp and find a captive Predator. The "hunters", three larger Predators, attack the group. Mombasa is killed and the rest of the group escapes. Isabelle reveals that she has heard of the Predators before, from a report by the only survivor (Major Alan "Dutch" Schaefer) of a Special Forces team who encountered one in Guatemala in 1987.
The group next meets Noland (Laurence Fishburne), a lone soldier who has survived on the planet for "10 seasons" by hiding and scavenging from the Predators and their victims. At his hideout, he explains that the Predators sharpen their killing skills by collecting warriors and dangerous beasts from other worlds and bringing them to the planet to hunt. Noland also reveals that there is a blood feud between the larger Predators and the smaller ones. Royce hopes that if the group can free the smaller Predator being held prisoner in the encampment, it may take them home using the other Predators' spaceship.
When the group fall asleep, Noland traps them in a room and attempts to use smoke to suffocate them. Having gone mad, he plans to kill them for food. Royce uses an explosive to break out of the room, attracting the Predators to the hideout. Noland tries to escape, but a Predator kills him. In the ensuing chase, Nikolai uses a claymore mine to kill a Predator, sacrificing himself. Stans savagely attacks another Predator with his shiv, buying time for the others to escape, and is killed. Hanzo duels the third Predator with a katana, killing it at the cost of his own life.
As Royce, Isabelle and Edwin head for the Predators' camp, Edwin is injured by a trap. When Isabelle refuses to abandon him, Royce leaves them both behind and they are caught by the remaining hunter. Royce frees the smaller Predator and heads for the ship as the two Predators confront each other. The larger Predator kills the smaller one and destroys the ship as it takes off. Meanwhile, Edwin paralyzes Isabelle with the neurotoxic poison on his scalpel and reveals that on Earth he was a murderer, and feels that he fits in on this planet among the monsters. Royce appears, never having boarded the ship, and stabs Edwin with his own scalpel, paralyzing him.
Royce booby-traps Edwin with grenades, using him as bait to injure the Predator. While Isabelle covers him with her sniper rifle, he proceeds to fight the Predator with an axe and eventually decapitates it. As Royce and Isabelle rest, they see parachutes opening at a distance. Royce, surmising that more Predators will soon arrive to hunt the new prey, tells Isabelle they must find another way to get off the planet, and they make their way back towards the jungle.

Question: Where do they realize they are not at when staring at the alien sky?
Answer: Earth