[Lewis Thomson (OP)]

Brody has been puzzled by this idea and would like some insights.

"As NASA studies have shown, the smallest known black hole is XTE J1650-500. This black hole is only about 3.8 times the mass of the sun. XTE J1650-500 is in The Milky Way Galaxy. Could this be the 9th planet you mentioned in your latest Naked Scientists Q&A episode?"

Discuss in the comments below...

[Origin]

No, this is not the 9th planet of sol.  That black hole is about 10,000 ly away and is in orbit with another star.

[evan_au]

There is no clear-cut edge to the Solar System, so it's not clear that we have discovered all the planets that are circling the Sun.

There was a great competition to be the one to predict and/or find the "next" planet.
- In fact Herschel (who discovered the planet Uranus) campaigned for the name asteroids (star-like) for the objects between Mars and Jupiter - supposedly so that no one else could claim to discover a planet! (Until Neptune was discovered...)
- I am sure that many astronomers are keen to be the next one to discover another planet in our Solar System...