Discovering Planet Nine

Very briefly some years ago, Mike Brown discovered the tenth planet in the solar system. This was in 2005; Brown, an astronomer at Caltech, had spotted an object that officially became known as Eris (he preferred the nickname Xena). Eris was about as big as Pluto, which was still a planet back then, and it orbited the sun at a distance nearly three times greater. But the existence of Eris raised troubling questions, such as: What’s a planet, exactly? And if Eris is a planet, why not also various other small spheres that orbit the sun? In the end, the An Exoplanet Too Far
The End of Darkness
Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them