The U.S. Supreme Court has ruled that police conducted a search within the meaning of the Fourth Amendment when they used a police dog to sniff for drugs on the porch of a home.

Conservative Justices Antonin Scalia and Clarence Thomas joined with three of the court’s liberals to form a majority. The case is “a straightforward one,” Scalia said in his majority opinion (PDF), because police had gathered evidence in the vicinity immediately outside the house—the area known as the curtilage.

Justice Elena Kagan wrote a concurring opinion joined by Justices Ruth Bader Ginsburg and Sonia Sotomayor. Kagan said the use of the drug dog was a search based on privacy as well as property grounds. “The court today treats this case under a property rubric,” Kagan said. “I write separately to note that I could just as happily have decided it by looking to [the homeowner’s] privacy interests.”

The defendant did not fare as well in another drug-dog case last month. In a unanimous decision, the U.S. Supreme Court ruled in Florida v. Harris that an alert by Aldo the drug-sniffing German shepherd gave a Florida police officer probable cause to search a truck with an expired license plate.

The drug sniffing dog in the porch-sniff case, Florida v. Jardines, was named Franky. Based on Franky’s alert, officers obtained a warrant to search the home of Joelis Jardines and found marijuana plants. Jardines had contended the evidence should be suppressed because the use of the dog outside his home was a search not supported by probable cause.

Scalia’s opinion stressed the physical nature of the intrusion. “When it comes to the Fourth Amendment, the home is first among equals,” he wrote. The right to be free of unreasonable governmental intrusion inside the home “would be of little practical value if the state’s agents could stand in a home’s porch or side garden and trawl for evidence with impunity.”

Scalia said two officers who conducted the search “had all four of their feet and all four of their companion’s firmly planted on the constitutionally protected extension of Jardines’ home.” Police officers may come to the porch and knock on the door, as could any private citizen, he said. “But introducing a trained police dog to explore the area around the home in hopes of discovering incriminating evidence is something else,” he wrote. “There is no customary invitation to do that.”

Scalia cited his 2012 opinion United States v. Jones, which held that police conducted a search within the meaning of the Fourth Amendment when they attached a GPS device to a car to monitor the driver’s movements. “Because the GPS receiver had been physically mounted on the defendant’s automobile (thus intruding on his ‘effects’), we held that tracking the vehicle’s movements was a search,” Scalia said.

Justice Samuel A. Alito Jr. wrote a dissenting opinion joined by Chief Justice John G. Roberts Jr. and Justices Stephen G. Breyer and Anthony M. Kennedy. Alito argued that the majority had based its decision “on a putative rule of trespass law that is nowhere to be found in the annals of Anglo-American jurisprudence.”