AMNA NAWAZ: In 2008, Supreme Court Justice Samuel Alito took an undisclosed fishing trip to Alaska with billionaire donors, including hedge fund manager Paul Singer, who later had business before the court.

Coupled with previous reporting on the relationship of Justice Clarence Thomas and Republican megadonor Harlan Crow, it raises serious ethical questions regarding the High Court.

Josh Kaplan is one of the ProPublica reporters who broke the story.

And he joins me now.

Josh, welcome back.

At the heart of your story is this July 2008 trip that Alito took to Alaska, traveling there on a private plane paid for by that billionaire, Paul Singer, his stay at the lodge also paid for by another major donor on the trip.

Neither of those things was disclosed.

So tell us a little bit about what your reporting revealed about that trip and why people think it should have been disclosed.

JOSHUA KAPLAN, ProPublica: Singer was not just another guests on the trip.

He provided Alito with a private jet to fly him across the country to Alaska.

And if Alito had chartered that plane himself, it could have cost easily more than $100,000, we were told.

And Alito didn't disclose any of this.

It's been secret until now.

And experts told us that he appears to have violated a government ethics law passed after Watergate that requires justices to disclose most gifts to the public.

AMNA NAWAZ: That man, that billionaire, Paul Singer, his hedge fund later had a case before the Supreme Court, a case that the court, as you reported, previously declined to take up, right?

So why did the experts you talk to say that, in this case, Alito should have recused himself from that case?

JOSHUA KAPLAN: Alito didn't recuse himself in any of the more than at least 10 cases that Paul Singer had after the trip.

And they said that they never -- experts said that they'd never heard of another example of a Supreme Court justice sitting on a case after receiving an expensive gift from one of the parties.

There is a law that governs when justices must recuse themselves from the case.

And it sets a very high standard, but a subjective standard.

It's that, if there's the appearance of impropriety, a justice must recuse.

But when it comes to the Supreme Court, the only person that interprets that standard, the sole arbiter of when a justice should recuse from a case is the justice him or her self.

AMNA NAWAZ: So, Alito has responded in the form of an op-ed published in The Wall Street Journal.

In brief, he basically says he doesn't believe his relationship with Paul Singer warranted recusal.

He doesn't think that the hospitality provided needed to be reportable.

He claims, in his words, ProPublica misleads its readers.

And he writes: "It was and is my judgment that these facts would not cause a reasonable and unbiased person to doubt my ability to decide the matters in question impartially."

So, Josh, this is one of the central questions, right, on impartiality.

Do we know if his relationship with Singer or this trip or anything else had any impact on his decisions?

JOSHUA KAPLAN: We don't know that they had any impact, in terms of the quid pro quo, that a case went one way or another because of this trip.

There's no evidence that.

And I should say that when Justice Alito wrote this op-ed, saying the article was misleading, the article hadn't come out yet and he hadn't read it.

But he -- in terms of recusal, it's -- one expert put it to us, as, you know, it's fairly simple.

If you were on the other side of the case in any court in this country, and you learned that the person that you're fighting against had given an expensive gift, taken the judge sitting on the case on vacation, you would have questions about that judge's ability to sit.

And they said that means that Alito clearly should recuse in this case.

AMNA NAWAZ: And we asked Kathleen Clark, who is a professor of law at Washington University in St. Louis, who focuses on government ethics, about this report, about your last report about Justice Clarence Thomas and the gifts he had received.

Here's what she had to say.

KATHLEEN CLARK, Washington University of Saint Louis: It is a systemic problem.

It's a problem for the entire institution.

These weaknesses are not about us one or two bad justices.

The court has failed to adopt and apply state ethics standards.

And until it does or until Congress imposes those standards, we will continue to have stories about unethical Supreme Court justices.

AMNA NAWAZ: Josh, what does all of your reporting say to you about the court's ability to police themselves on these issues?

JOSHUA KAPLAN: Well, I think one of the root issues in all of this is the lack of transparency and oversight on the High Court that really is a stark contrast to the other branches of government.

If you work in the executive branch, there is an ethics office that clears things for you, that analyzes these sorts of problems when they come up.

There are strict rules on what gifts you can accept.

There are extremely strict rules on what gifts you must disclose.

And the justices are more or less left to police themselves.

And they always have been.

These -- when Alito went on this vacation and accepted this private jet ride that could have cost $100,000 to charter from a person he didn't know who had just had a case before the court, that was completely within the court's rules.

He had to disclose it.

And experts say he violated the law by not disclosing it.

But there was no restrictions on this.

And then, if a conflict comes up or a potential conflict comes up, the only person who decides if a justice should recuse is the justice.

And that's a decision that can't be appealed.

AMNA NAWAZ: That is Josh Kaplan of ProPublica joining us tonight with his latest report.

Josh, thanks for making the time.

Thanks for having me.