Mike Brown is an astronomer at Caltech who's been instrumental in the discovery of more than 30 dwarf planets and asteroids at the far reaches of our solar system. Most members of the general public probably don't know about all of the planetoids he has found — but many know about the one planet he has killed.

When Brown and his team discovered Eris, a dwarf planet more massive than Pluto, it was initially referred to as our solar system's 10th planet. But that ended in 2006 when the International Astronomical Union voted to define "planet" officially for the first time. Eris was out of the running, and Pluto was a casualty — cut from the official planetary roll call.

Now Brown and his co-author, Konstantin Batygin, believe they've compiled the best evidence for a true ninth planet to take Pluto's place in the history books. We called Brown up to chat about the latest blow he has dealt to Pluto fans.

This interview has been edited for length and clarity.

Washington Post: How did you get into the planet finding (and planet killing) business?

Mike Brown: So, planetary science is something I got into when I was in grad school. I went into astronomy anticipating studying some of the most distant galaxies, but I fell in love with this field because planetary science is just so much more concrete to me. I could actually go outside at night and see this thing I was studying.

I can’t do that anymore! But it’s still much more visceral to me than some of the more abstract parts of astronomy.

And the reason why I particularly like these objects I study in the outer solar system is that they have grand importance for how our solar system came together, and how our planetary system formed, but it's also just as much about this journey of discovery. It’s exploration. For me, this is what we do as humans. We see our neighborhood out there and explore it.

WP: Tell us more about that "grand importance." What can objects like Eris, Pluto and Planet Nine teach us about the origins of the solar system?

MB: The objects in the outer solar system have the unique property of not having been messed with very much.

All the planets on the inside of the solar system have been heated, smashed, had their orbits pushed around, and so on. But on the very edge, there’s just not as much out there to mess with things. The sunlight isn’t very hot, and they can preserve these very delicate records of what happened at the beginning of the solar system.

Planet Nine is actually an example, and not one I ever thought we’d get. The fact that we can find another planet out there based on the very subtle gravitational influence it has is because everything is so pristine.

Astronomers say they have evidence of a ninth planet in our solar system. (Joel Achenbach, Gillian Brockell/The Washington Post)

WP: When you discovered the dwarf planet Eris, at what point did you realize Pluto was in trouble?

MB: Oh, within 15 seconds. I was sitting — I'm actually sitting in the same chair right now — I was looking at data, and the only reason it took 15 seconds was that for the first 14 I was thinking, no, this can't be right; there's nothing this bright in the sky.

Then I did a quick calculation to see how big this object was and — we didn't know exactly how big it was at the time, but it was clearly the size of Pluto or much bigger. Either this was going to be a new planet, or Pluto was in trouble.

WP: A lot of people got excited when New Horizons found that Pluto is probably larger than Eris, for exactly that reason. 

MB: Eris is significantly more massive — 25 percent more massive. If you take Pluto and you add every single object in the asteroid belt to it, you get to the mass of Eris. Pluto is bigger by volume, but it's just puffed up with extra ice.

WP: So size isn't really the thing to get hung up on?

MB: It's good for bragging rights, but it doesn't actually mean anything. They're both so much smaller than anything else we'd call a planet. In the end, it doesn't matter that Eris exists. Eris isn't the problem with Pluto. Pluto is the problem with Pluto.

WP: What made you take on the title of "Pluto Killer" willingly? Any regrets?

MB: It all happened kind of accidentally back in the early days of Twitter. I thought, oh, I should try it, and I needed a name and just thought being Pluto Killer would be funny.

[Laughing] Be careful what you use, because that's going to follow you forever. I can say it was not really a calculated statement at the time.

WP: Do the Pluto fans ever make it personal?

MB: I think that there are people who would still love for Pluto to be a planet, and they wish people who kept reminding them why it’s not would be quiet and go home.

Personally, I think it actually matters. I think that for people understanding the solar system as it really is, that's really important. So I have not been willing to back down and give up, and there are certainly people who don't like that.

WP: And now you haven't just killed Pluto, you're trying to replace it. 

MB: My daughter — she's still kind of mad about Pluto being demoted, even though she was barely born at that time — she suggested a few years ago that she'd forgive me if I found a new planet. So I guess I've been working on this for her.

WP: Speaking of Planet Nine, at what point can we actually say the planet has been "discovered"?

MB: In the end, discovery means it’s been seen. Someone has seen it, has seen it move, has seen the orbit. We think we know the orbit, but we don't know where on that orbit the planet is, and we haven't seen it yet. This paper we published is like handing everyone a treasure map.

It's going to depend on the luck of the person who points the telescope in the right place first. It's going to take a lot of hard work, sure, but also a lot of luck. It might not be us.

WP: But if you do get naming rights, what name are you going to propose for Planet Nine?

MB: This would only be the third planetary discovery in modern history [note: not counting Pluto!]. Naming it just seems too big for any one person. It just really seems like too big of a thing for the person who happens to have the luck of pointing the telescope.

It would be a huge cultural moment. It shouldn't just be some guy saying, hey, I found it, so we're going to name it George. Which, by the way, is what William Herschel tried to do when he discovered Uranus. Unfortunately for him, we didn't stick with George.

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