This TED Talk features planetary astronomer Mike Brown recorded live at TED at NAS 2019.

I'm going to tell you a story from 200 years ago. In 1820, French astronomer Alexis Bouvard almost became the second person in human history to discover a planet.

He had been tracking the position of Uranus across the night sky using old star catalogs, and it didn't

quite go around the sun the way that his predictions said it should. Sometimes it was a little too

fast, sometimes it's a little too slow. Bouvard knew that his predictions were perfect.

So it had to be that those old star catalogs were bad.

He told astronomers of the day, do better measurements.

So they did.

Astronomers spent the next two decades meticulously tracking the position of Uranus across the sky,

but it still didn't fit Beauvard's predictions. By 1840, it had become obvious. The problem was

not with those old star catalogs. The problem was with the predictions. And astronomers knew

why. They realized that there must be a distant giant planet

just beyond the orbit of Uranus that was tugging along at that orbit, sometimes pulling it

along a little bit too fast, sometimes holding it back. It must have been frustrating back in 1840

to see these gravitational effects of this distant giant planet,

but not yet know how to actually find it. Trust me, it's really frustrating. But in 1846,

another French astronomer, Urbane LaVarie, worked through the math and figured out how to predict

the location of the planet. He sent his prediction to the Berlin

Observatory. They opened up their telescope and in the very first night, they found this faint

point of light slowly moving across the sky and discovered Neptune. It was this close on the sky

to Laverier's predicted location. The story of prediction and discrepancy and new theory and triumphant discovery is so

classic, and Laveria became so famous from it that people tried to get in on the act right

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