On March 13, 1781, astronomer William Herschel pointed his telescope into the night sky and discovered the planet Uranus, the first such discovery in historical times. Over the next 45 years, as astronomers observed Uranus looking for possible moons, they realized its orbit didn’t follow the known laws of gravity. Some astronomers surmised that perhaps Newton’s gravitational laws didn’t work so far from the sun.
But two mathematicians decided
that the discrepancy occurred due to the gravitational attraction of an eighth
planet even farther out. Unbeknownst to each other, both French astronomer Rubin Le Perrier
and British mathematician John Couch Adams began calculating the likely
position of such a planet based on the aberrations of the orbit of Uranus. On
September 23, 1846, French astronomer Johanne Galle found Neptune within one
degree of the predicted position. The effect of Neptune’s gravitational pull on the
orbit of Uranus led to its discovery.
In January 2015, Caltech
astronomers Konstantin Batygin and Mike Brown noted some rather odd orbits of five
asteroids in the far outer regions of our solar system. Using computer
modeling, they predicted the presence of a ninth planet ten times larger than
Earth whose gravity they theorized affected the asteroids. They dubbed it
“Planet Nine.”
That planet has never been
discovered even with today’s sophisticated telescopes, yet according to Brown
and Batygin, new evidence continues to support their hypothesis. This negative
result has led some astronomers to propose a radical idea: maybe the cause of
the odd orbits isn’t a large planet but rather a small black hole.
Typically, black holes result from the death of massive stars and are typically five to ten times the mass of our sun. But the extreme conditions at the earliest stages of the formation of the universe could have created smaller, planet-sized black holes, called primordial black holes, although we’ve yet to find any.
A planet ten times the size of
Earth should have been found, many astronomers believe, but a black hole of the
same mass would be tiny and impossible to see. The only way to discover one is to use the same method astronomers use to find any stellar-sized black hole, by way of its massive gravity. A
planet-sized black hole might be invisible, but the gravity of one could
certainly cause the observed effect.
I think the idea of a
primordial black hole at the edge of our solar system is exciting. As long as
it stays out there.
