The Supreme Court handed down a number of opinions Thursday as it races towards the end of the Term. At first glance, Davis v. Ayala, a criminal habeas case, might seem worth skipping over in favor of sexier opinions like Reed v. Town of Gilbertand Walker v. Sons of Confederate Veterans. Yet buried in Ayala is something extraordinary: a strong hint from the Court’s swing justice that he wants to rein in solitary confinement—and soon.

The issue in Ayala was whether the Ninth Circuit ignored statutory limits on federal habeas relief when it overturned Hector Ayala’s murder conviction and death sentence. The Ninth Circuit had concluded that the prosecutor at Ayala’s 1989 California trial struck prospective jurors on the basis of their race, violating Ayala’s constitutional rights. The Court divided five to four; the majority opinion, written by Justice Alito and joined by the Chief Justice and Justices Scalia, Kennedy, and Thomas, concluded that Ayala wasn’t entitled to a new trial. Justice Sotomayor, joined by the other three traditionally more liberal Justices, dissented. Although the Court split along traditional ideological fault lines, the opinion doesn’t break much new legal ground. Instead, it mostly wades through messy factual issues—specifically, whether the record showed that the prosecution used peremptory strikes on seven black and Latino prospective jurors for racially discriminatory reasons.

Yet for patient readers, there’s a big surprise in Ayala. It appears after the 29-page majority opinion, and it portends a coming constitutional blockbuster. (Think of it as the judicial equivalent of those teasers Marvel tacks on to the end of its superhero movies to get you to sit through the credits.) It’s a short concurrence by Justice Kennedy in which he expresses serious reservations about the widespread use of solitary confinement in American prisons and strongly suggests that courts need to step in to put limits on the practice.

Now, if this strikes you as a bit of a non sequitur, you’re not alone. As Justice Kennedy admits, solitary confinement had “no direct bearing on the precise legal questions presented by” Ayala. This, charitably, is a bit of an understatement. Solitary confinement had no bearing whatsoever—“direct” or indirect—on any of the legal issues—“precise” or otherwise—in the case. The only connection is that Ayala has spent much of his time in prison in solitary confinement. But none of the briefs in the case had even mentioned that. The only reason, as far as I can tell, that Justice Kennedy was aware that Ayala had spent time in solitary was that—apropos of nothing—Justice Kennedy asked Ayala’s lawyer, just as he was leaving the podium at the end of his oral argument, whether Ayala had “spent time in solitary confinement, and, if so, how much?” So, before even getting into what Justice Kennedy says about solitary, it’s important to stress that the very fact that he chose to say anything about it at all in this case was itself extremely unusual.

No less extraordinary, though, is the substance of what Justice Kennedy says. His concurrence reads more like a policy paper than a judicial opinion; he cites a wide range of sources, including Dickens, Dostoevsky, Amnesty International, public health and psychiatry journals, and newspaper articles about Kalief Browder—the teenager who spent more than two years in solitary confinement at New York’s Rikers Island without being convicted of a crime, and who recently committed suicide. Surveying the territory, Justice Kennedy expresses concern that long-term solitary confinement “exacts a terrible price” on the prisoner’s mind. Even more striking, he strongly suggests that courts should play more of a role in limiting the practice: “In a case that presented the issue, the judiciary may be required, within its proper jurisdiction and authority, to determine whether workable alternative systems for long-term confinement exist, and, if so, whether a correctional system should be required to adopt them.”

Justice Kennedy’s concurrence is a good example of what Richard Re has called a “Supreme Court signal”—an official statement by a justice that isn’t formally binding but that gives other courts a strong hint as to how they should address a question. By indicating his view that solitary confinement requires judicial intervention, Justice Kennedy is sending a signal to lower courts that they need to take the issue seriously. But he’s also sending a strong signal to lawyers—telling them that they should be bringing legal challenges to confinement practices—and soon, so that the Supreme Court has an opportunity to consider the issue. (Indeed, the fact that the issue hasn’t reached the Court recently is presumably why Justice Kennedy felt it necessary to use Ayala as a platform).

Short solo opinions like Justice Kennedy’s can, though not binding, change the course of the law. In a recent book, Evan Mandery has told the story of how Justice Arthur Goldberg helped launch a series of constitutional challenges to capital punishment through a four-paragraph dissent from denial of certiorari in the 1963 case Rudolph v. Alabama. As Mandery reveals, Justice Goldberg consciously strategized in writing his opinion with the goal of encouraging lawyers to launch a constitutional assault on the death penalty. He was largely successful; the challenges his dissent helped inspire culminated in the 1972 case Furman v. Georgia, which struck down every death penalty statute then on the books in the United States. If Justice Kennedy’s Ayala concurrence could be nearly as important if it actually lays the groundwork for a majority opinion imposing real limits on solitary confinement.

And while Justice Kennedy doesn’t say what kind of limits he would support, there’s reason to think he’d favor significant restrictions on solitary confinement. Justice Kennedy doesn’t always side with defendants in criminal cases, especially on procedural issues. Yet in recent years Justice Kennedy appears to have become increasingly troubled by the severity of punishment in the American criminal justice system, and has authored several major opinions seeking to rein in the system’s excesses. When he has supported intervention, he has tended (as is consistent with his jurisprudence as a whole) to paint with a broad brush. He has written opinions declaring capital punishment categorically impermissible for non-homicide crimes (Kennedy v. Louisiana) and for juveniles (Roper v. Simmons), extending the latter restriction to life without parole as well Graham v. Florida). More recently, he wrote the opinion affirming an injunction that ordered California to release thousands of prisoners (Brown v. Plata). So if Justice Kennedy is convinced that solitary confinement is a troubling practice, he’s unlikely to nibble at the edges; it’s easy to imagine him drafting an opinion that imposed broad limits on its use, though likely not banning it completely.

To do that, of course, Justice Kennedy would have to convince some of his colleagues to hear the issue and to vote his way. While he may be able to put together a majority when the time comes, Ayala made clear that not everyone will go along easily. Justice Thomas responded to Justice Kennedy’s concurrence in a brief but biting opinion of his own. He observed that “the accommodations in which Ayala is housed are a far sight more spacious than those in which his victims ... now rest. And, given that his victims were all 31 years of age or under, Ayala will soon have had as much or more time to enjoy those accommodations as his victims had time to enjoy this Earth.” When the Court weighs in on solitary confinement—and it looks as if the question is when, not if—Justice Kennedy is going to have a fight on his hands.