‘Pluto killer’ calls on other astronomers to help find a ninth planet


The scientists who brought you “Planet Nine” want you — and your giant fancy telescope — to help catch the mysterious world in action.

This week, two Caltech scientists announced that an unseen giant planet some 10 times the mass of Earth may lie beyond the Kuiper belt, the icy ring of debris where dwarf planet Pluto resides. If true, it would become the first planet found in more than 150 years, since Neptune was discovered in 1846.

“If this planet is out there, it’s really a big deal,” said Gregory Laughlin, an astronomer at UC Santa Cruz who was not involved in the paper. “It’s a major, major discovery if it turns out to be true.”


So if this planet exists, why haven’t we seen it yet? Let’s put this in perspective: The Kuiper belt is way, way out there — it stretches from about Neptune’s orbit at 30 astronomical units (or Earth-sun distances) out to 50 astronomical units. But this potential ninth planet lies far beyond that. The closest it would come to the sun is a whopping 200 astronomical units — that’s about 18.6 billion miles. And its orbit could extend from 600 to 1200 astronomical units from the solar system’s center.

Finding Planet Nine may not the easiest task. After all, it’s unclear how large its orbit is — it could take anywhere between 10,000 and 20,000 years to complete one revolution around the sun. They also don’t know where the planet would currently lie in this orbit. That’s part of why astrophysicist Konstantin Batygin and self-professed “Pluto killer” Mike Brown published their paper this week in the Astronomical Journal, instead of keeping their cards close to the vest.

“We could have kept quiet and not told anybody what we know and where it is, and we could have just been quietly searching for the next five years to try to find it,” Brown said. “But we thought, this is important enough that we really want people to find it as fast as possible.”

To that effect, Brown and his colleague Konstantin Batygin have set up, where they offer helpful pointers to interested parties about where Planet Nine might be. For example, Planet Nine’s closest approach would probably be directly overhead in late May; its orbit is tilted 30 degrees with respect to the eight main planets; and if it’s in an orbit that passes in front of the bright Milky Way, it will be very difficult to see.

“I will predict that we will not be the people who find it,” Brown said. “There are a lot of people with a lot of telescopes out there and the sky is a big place.”

In the meantime, other astronomers are weighing the odds that Planet Nine really is out there. Scott Sheppard, who co-wrote a previous paper that tipped Batygin and Brown off to the possibility of a ninth planet, said his certainty went from 50% to 60%.


Laughlin was far more specific, putting the odds at 68.3%. He’s started a prediction market where users can weigh in on whether this mysterious, massive world will soon be found.

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