
 
 Mike Brown stands for a portrait in front of an artist portrayal of Planet Nine. (Photo by Patrick T. Fallon for The Washington Post) 
 
 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. 
 
 
 
 Mike Brown points to the gold ring showing the orbital path of Planet Nine. (Photo by Patrick T. Fallon for The Washington Post) 
 
 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. Here's what they say they know about it. (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. 
 
 
 
 An artist's impression of Planet Nine, which could sit at the edge of our solar system. (Caltech/R. Hurt) 
 
 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. 
 
 Read More: 
 
 New evidence suggests a ninth planet lurking at the edge of the solar system 
 
 ‘Have you SEEN Pluto?’ — New Horizons scientist still rallying for planetary status 
 
 Why NASA’s top scientist is sure that we’ll find signs of alien life in the next decade 
 
 This broken space telescope keeps spotting new planets 
 
 This suspected supernova is 570 billion times as bright as our sun 
 
 Why is this famous physicist tweeting rumors about gravitational waves? ||||| Astronomers say they have evidence of a ninth planet in our solar system. Here's what they say they know about it. (Joel Achenbach,Gillian Brockell/The Washington Post) 
 
 Astronomers at the California Institute of Technology announced Wednesday that they have found new evidence of a giant icy planet lurking in the darkness of our solar system far beyond the orbit of Pluto. They are calling it "Planet Nine." 
 
 Their paper, published in the Astronomical Journal, estimates the planet's mass as five to 10 times that of the Earth. But the authors, astronomers Michael Brown and Konstantin Batygin, have not observed the planet directly. 
 
 Instead, they have inferred its existence from the motion of recently discovered dwarf planets and other small objects in the outer solar system. Those smaller bodies have orbits that appear to be influenced by the gravity of a hidden planet – a "massive perturber." The astronomers suggest it might have been flung into deep space long ago by the gravitational force of Jupiter or Saturn. 
 
 
 
 Telescopes on at least two continents are searching for the object, which on average is 20 times farther away than the eighth planet, Neptune. If "Planet Nine" exists, it's big – about two to four times the diameter of the Earth, which would make it the fifth-largest planet after Jupiter, Saturn, Uranus and Neptune. But at such extreme distances, it would reflect so little sunlight that it could evade even the most powerful telescopes. 
 
 [Q&A: The ‘Pluto Killer’ who thinks he’s found the true ninth planet] 
 
 Confirmation of its existence would reconfigure the models of the solar system. Pluto, discovered in 1930, spent three-quarters of a century as the iconic ninth planet. Then, a decade ago, Pluto received a controversial demotion, in large part because of Brown. 
 
 His observations of the outer solar system identified many small worlds there – some close to the size of Pluto – and prompted the International Astronomical Union to reconsider the definition of a planet. The IAU voted to change Pluto's classification to "dwarf planet," a decision mocked repeatedly last summer when NASA's New Horizons probe flew past Pluto and revealed a world with an atmosphere, weather and a volatile and dynamically reworked surface. 
 
 Brown, who tweets under the handle @plutokiller and who wrote the book "How I Killed Pluto and Why It Had It Coming," said now may be the time to rewrite the textbooks yet again. 
 
 "My daughter, she's still kind of mad about Pluto being demoted, even though she was barely born at that time," Brown said. "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." 
 
 [Is there a Planet X, a ‘massive perturber,’ hidden beyond Pluto?] 
 
 NASA's director of planetary science, Jim Green, cautioned Wednesday that there could be other explanations for the observed motion of the small bodies in the outer solar system. He referenced the famous dictum from Carl Sagan that "extraordinary claims require extraordinary evidence." 
 
 “The Sagan Rule applies. If it's there, find it. I challenge you. Somebody out there oughta find it," Green said. 
 
 But he said he was personally excited about the new research: "What an era we're in, where we’re discovering new things about our solar system that we never thought possible even a handful of years ago." 
 
 Brown and Batygin initially set out to prove that Planet Nine didn't exist. Their paper builds on earlier research by two other astronomers that revealed a peculiar clustering of the small, icy objects discovered in the past decade or so in the remote regions of the solar system. 
 
 [Nah, this possible new ninth planet isn’t Pluto] 
 
 In 2014, Scott Sheppard of the Washington-based Carnegie Institution of Science and Chad Trujillo of the Gemini Observatory in Hawaii published a paper in the journal Nature that discussed the potential existence of a giant planet affecting the orbits of those dwarf worlds. Sheppard and Trujillo noted a similarity in the motion of those bodies when they are closest to the sun. 
 
 "We thought their idea was crazy," Brown said, explaining that extra planets are always the "go-to suggestion" when astronomers find orbital behavior they can't explain. But he and Batygin struggled to debunk that hypothetical ninth planet. They used mathematical equations and then computer models, ultimately concluding that the best explanation for the smaller objects' clustering was the gravitational effects of something far bigger. 
 
 Such clustering is similar to what's seen in some asteroids that are about as close to the sun as the Earth. They wind up in stable orbits that keep them far from Earth and free from any significant disturbance by the Earth's gravity. 
 
 
 
 Mike Brown and Konstantin Batygin stand for a portrait in front of an artist portrayal of Planet Nine at Caltech (Photo by Patrick T. Fallon for The Washington Post) 
 
 "Until then, we didn't really believe our results ourselves. It just didn't make sense to us," Brown said. But their modeling showed that a planet with 10 times the mass of Earth would exert an influence over the orbits of the smaller bodies and keep them from coming as close to the sun as they should. It would also slowly twist these orbits by 90 degrees, making them periodically perpendicular to the plane of the solar system. 
 
 "In the back of my head, I had this nagging memory that someone had found some of these modulating objects and not known what to make of them," Brown said. "And sure enough, these objects do exist. And they were exactly where our theory predicts they should be." 
 
 That's when the Caltech researchers started to take Planet Nine seriously. "That was the real jaw-dropping moment, when it went from a cute little idea to something that might be for real," he said. 
 
 [NASA estimates 1 billion ‘Earths’ in our galaxy alone] 
 
 Sheppard, who co-wrote the paper that Brown and Batygin set out to disprove, says the existence of a hidden planet is still a big unknown. "Until we actually see it for real, it will always be questionable as to whether it exists," he said, cautioning that the latest calculations are based on a relatively small number of known objects and that further observations and detections of perturbed bodies would bolster the hypothesis. 
 
 Still, Sheppard significantly upped the odds of discovery – from 40 percent before to 60 percent now. “Some people took it seriously, but a lot of people didn’t," he said of his own study's findings. "With this new work, it’s much more rigorous, and people will take it more seriously now.” 
 
 Brown said he puts the odds of Planet Nine being real as "maybe 90 percent." 
 
 From the Côte d'Azur Observatory in Nice, France, planetary scientist Alessandro Morbidelli agreed that the evidence was stronger this time. "I immediately felt that this paper, for the first time, was providing convincing evidence for a new planet in the solar system," said Morbidelli, an expert in these kinds of orbital movements who was not involved in either study. "I don't see any alternative explanation to that offered by Batygin and Brown." 
 
 "We will find it one day," he added. "The question is when." 
 
 The past two decades have seen a burst of discoveries as astronomers have scrutinized the light of distant stars and looked for signs of orbiting planets. More than a thousand such planets have been detected through analysis of starlight that has traveled across the vast interstellar distances. Brown and Batygin, however, have been searching closer to home, looking for objects that orbit the sun and remain unseen only because the outer regions of the solar system are exceedingly dark. 
 
 [Scientists caught a new planet forming for the first time ever] 
 
 The thought of a hidden planet larger than Earth is intriguing, but for now it's difficult to say too much about the hypothetical conditions there. Brown believes it's probably an icy, rocky world with a small envelope of gas – a planet that could have been the core of a gas giant had it not been ejected into a wonky, highly elliptical orbit. It might not make its closest approach of the sun more than once every 10,000 years, and even then it would remain far beyond the known planets. 
 
 The situation mimics what happened in the 19th century when careful observation of the seventh planet, Uranus, indicated that there must be another body in far-distant space influencing its orbit. That work led eventually to the discovery of Neptune. 
 
 It would be difficult to see the ninth planet if it's not at or near its closest approach to the sun. Brown doesn't believe the object is at that point, saying it would have been spotted by now. But he does think that the most powerful telescopes on the planet, if pointed in precisely the right direction, might be able to detect it even when it is most distant from the sun. 
 
 "We've been looking for it for a while now, but the sky is pretty big," Brown said. "We know its path, but not where it is on that path." 
 
 He and Batygin hope their paper's publication will infuse the search with new energy. "If other people – better astronomers – get excited about the idea of finding Planet Nine, we could hopefully see it within a couple of years," he said. 
 
 The two know they may not get credit for that discovery. Until the planet is spotted directly with a telescope, any work surrounding it is theoretical. Brown, Batygin and other scientists who have made the case for Planet Nine's existence are providing treasure maps and clues – but someone else could very well strike gold before they do. 
 
 If and when it's spotted, Planet Nine would be evaluated by the same criteria that got Pluto demoted. Brown isn't concerned about that. 
 
 "That's not even a question -- it's definitely a planet," he said. One of the trickiest criteria for planet status, based on the standards set by the International Astronomical Union, is that a planet must "clear the neighborhood" around its orbital zone. It needs to have the gravitational prowess to change the orbits of other objects. 
 
 "Planet Nine is forcing any objects that cross its orbit to push into these misaligned positions. It fits that concept perfectly," Brown said. 
 
 The "Pluto killer" added: "Not to mention the fact that it's 5,000 times the mass of Pluto." 
 
 1 of 22 Full Screen Autoplay Close Skip Ad × 22 stunning photos of our solar system and beyond in 2016 View Photos A look at the best photos of space. Caption A look at the best photos of space. This undated photo shows the icy moon Mimas dwarfed by Saturn’s enormous rings. NASA/JPL-CalTech/Space Science Institute via AP Buy Photo Wait 1 second to continue. 
 
 Read More: 
 
 Nah, this possible new ninth planet isn’t Pluto 
 
 Q&A: The ‘Pluto Killer’ who thinks he’s found the true ninth planet 
 
 Flashback to 2004: Mike Brown discovers "Sedna" 
 
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 This broken space telescope keeps spotting new planets 
 
 Why NASA’s top scientist is sure that we’ll find signs of alien life in the next decade 
 
 For more science news, you can sign up for our weekly newsletter here. ||||| CAPE CANAVERAL, Fla. (AP) — Scientists say they finally have "solid evidence" for Planet X, a true ninth planet on the fringes of our solar system. 
 
 The gas giant is thought to be almost as big as Neptune and orbiting billions of miles beyond Neptune's path — distant enough to take 10,000 to 20,000 years to circle the sun. This Planet 9, as the two Caltech researchers call it, hasn't been spotted yet. They base their findings on mathematical and computer modeling, and anticipate its discovery via telescope within five years. 
 
 The two reported on their research Wednesday in the Astronomical Journal. 
 
 Once it's detected, the researchers insist there will be no Pluto-style planetary debate. They ought to know; one of them, Mike Brown, is the so-called Pluto killer. ||||| Image copyright SPL Image caption Pluto killer: Prof Mike Brown says he has no regrets about his role in Pluto's demotion 
 
 The New Horizons flyby has renewed calls for Pluto to be reinstated into the club of planets from which it was unceremoniously dumped in 2006. 
 
 But Dr Mike Brown of Caltech in Pasadena, who rejoices in the sobriquet of "the man who killed Pluto" told BBC News that those arguing for its reinstatement should stop living in the past. 
 
 "The people you hear most talking about reinstatement are those who are involved in the (New Horizons) mission. It is emotionally difficult for them," he said. 
 
 Those involved in the New Horizons mission want Pluto to be a planet because they want to be flying to a planet. They would be far better off embracing the reality that it is not a planet Prof Mike Brown, Caltech 
 
 "They want Pluto to be a planet because they want to be flying to a planet. They would be far better off embracing the reality that it is not a planet and being excited about the fact that they are going to a new type of object in the outer Solar System." 
 
 Coup de grace 
 
 Calls for Pluto to be downgraded intensified after a candidate Kuiper Belt object was discovered in 1992. Some argued that Pluto was simply the first resident of this outer Solar System region to be discovered. 
 
 But the coup de grace was delivered by Prof Brown with his discovery of the dwarf planet Eris in January 2005. It was like Pluto but more massive. 
 
 This was one of the finds that eventually led the International Astronomical Union to set up a committee to examine how planets should be defined. 
 
 Image copyright FRIEDRICH SAURER/SCIENCE PHOTO LIBRARY Image caption An artist's impression of Eris; its discovery did much to force the issue over Pluto's planet status 
 
 The IAU was forced to decide in 2006 whether it would admit Eris and other small worlds like Ceres, or boot out Pluto. It was a choice of one or the other - the status quo was not an option. 
 
 Prof Brown argues that had the IAU had decided to keep Pluto as a planet and enrol Eris, it would eventually find itself having to consider the candidacies of hundreds, possibly thousands of wannabe planets. 
 
 If outrageously calling myself 'Pluto killer' helps people understand what the solar system is really like then I'm happy with it Dr Mike Brown, Caltech 
 
 "There is no other way of categorising our Solar System than to describe it as having eight dominant objects in it which are the planets we know. It is really no good keeping Pluto and classifying it as one of the major objects because it is just not," he told BBC News. 
 
 So how did Prof Brown feel the day he heard that Pluto had been demoted? Was it a moment of joy, or was he wracked with guilt? He told me he felt it was more like a cold-blooded mercy killing necessary for the good of science. 
 
 "It had been clear to me for some years that Pluto had been wrongly classified. So I was quite happy with the idea (of demoting Pluto) that we could go back and fix those mistakes," Prof Brown explained. 
 
 No regrets 
 
 Pluto's demotion continues to be acrimonious. Many scientists say that it should remain a planet, arguing that it looks like a planet, behaves like a planet and has been thought of as a planet for three quarters of a century. 
 
 I asked Mike Brown whether he had any regrets. 
 
 "No, no regrets. But I am sad about the way the last decade has gone since Pluto's demotion. I wish that people had embraced Pluto's status as an interesting member of the Kuiper Belt rather than the discussion of whether Pluto was a planet or not," he explained. 
 
 When I met Mike Brown for the first time at a scientific meeting, there was a gleam in his eye - more mischievous than murderous - when I greeted him by saying: "You're the guy who killed Pluto." 
 
 When I spoke to him this week, I put it to him that he seems to relish in the title, indeed he uses it himself on his website, and his book is called "How I killed Pluto and why it had it coming". 
 
 He told me that it was intended as a clever joke but no one really understood it at the time. 
 
 Image copyright SCIENCE PHOTO LIBRARY Image caption And then there were eight. In 2006 Pluto was expelled from the solar system's exclusive club of planets 
 
 "I thought it was funny to talk about 'killing Pluto' because Pluto was god of the Underworld, but no one really picked up on that," he explained. 
 
 "It is a bold claim that grabs attention. And that is important educationally. I want people to understand what the Solar System is really like and if outrageously calling myself 'Pluto killer' helps then I'm happy with it." 
 
 Hate mail 
 
 Dr Brown still gets hate mail on Twitter, mostly - he says - from people who had learned that Pluto was a planet at school. But he says that children who have grown up with the idea of Pluto not being a planet are comfortable with the idea. He believes that the controversy will gradually die down. 
 
 "The Sun and the Moon used to be (thought of as) planets too but then we got over that a long time ago. I think it is far more interesting to be going to this new type of object rather than an odd ball planet on the edge of the Solar System. 
 
 "I'm hoping that after New Horizons, those discussions will die down and we can begin talking about Pluto and what we have learned about the rest of the Kuiper Belt." 
 
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– Pluto is gonna be PO'd. While the dwarf planet tries to fight its way back into the good graces of Those Who Deem What Counts as a Planet, another icy orb even further out may snatch that designation first. Astronomers at the California Institute of Technology say a giant celestial body "lurking at the end of our solar system," as the Washington Post puts it, may actually be a planet, and they're even already calling it "Planet Nine." In their research published in the Astronomical Journal, the sky-watching scientists—one of whom is known as the "Pluto killer" for his role in getting Pluto demoted—think their find is five to 10 times as massive as Earth, and per the AP, almost as big as Neptune and orbiting billions of miles past that planet's orbit. Michael "Pluto Killer" Brown and Konstantin Batygin haven't seen the supposed planet directly, but say they can infer the "massive perturber" exists by how the orbits of smaller bodies nearby are affected by its gravitational pull, the Post notes. What's interesting is that Brown and Batygin originally set out to disprove the existence of Planet Nine. "We thought their idea was crazy," Brown says of the scientists who originally floated the idea of a large, hidden planet. But as they did their own research, they soon came to their own conclusion in what Brown calls a "jaw-dropping moment" that Planet Nine could be the real deal. Now they're simply hoping more astronomers join in to actually try to spot the alleged planet—and they're not concerned it will face Pluto's fate. "That's not even a question—it's definitely a planet," Brown says. Not everyone's convinced. "I have seen many, many such claims in my career," a planetary scientist at Colorado's Southwest Research Institute tells Nature. "And all of them have been wrong." (The Washington Post caught up with Brown for a Q&A on the latest Pluto-killing endeavor.)