Mysterious Planet Nine “just has to be out there” as the net closes in

We all remember learning about the planets in the solar system in elementary school. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, in order of their closeness to the Sun. Pluto was long considered the ninth planet before being reclassified as a dwarf planet in 2006, but Planet Nine “just has to be out there” nevertheless.

“One-in-a-million chance” of Planet Nine not existing

That’s the view of US astronomer Michael Brown, who, along with a colleague at Caltech, theorised in 2016 that a huge planet weighing up to 10 times more than Earth was lurking on the edges of our solar system.

Eight years later, Brown and Konstantin Batygin have published a new study (which hasn’t yet been peer reviewed) showing the chances of “Planet Nine” not existing are around one in a million.

“I don’t see how we can have a Solar System without Planet Nine,” claims Brown. The issue, however, is that no astronomer has been able to find it, contributing to its somewhat unimaginative name.

What is the evidence for Planet Nine?

Initially, the scientists’ evidence was the clustering of six trans-Neptunian objects (TNOs), icy bodies which cross the orbit of Neptune and take long, steep paths around the sun, which they considered abnormal. The reason, they believed, was that Planet Nine’s gravity was pulling the TNOs away from what “should” have been their more natural orbit.

Brown and Konstantin’s study concluded there was a 99.993% probability the objects’ unexpected paths weren’t down to chance and they hypothesised that a planet five to 10 times heavier than Earth, and 20 times the distance of Neptune to the Sun was the reason the TNOs weren’t following the expected circular loops.

Since then, the duo have conducted a new study looking at another TNOs with less extreme objects, with results showing that “all the things in the outer part of the Solar System are also being pushed inward,” as Brown explains.

That has led to the belief that there is just a one-in-a-million chance that Planet Nine - or something like it - is not out there.

What other theories are there for the TNO’s behaviour?

Conclusive proof, however, is still lacking. According to more research published by Brown and colleagues, 78% of the sky in which Planet Nine could potentially be located has already been searched, with no trace of it. Time, and particularly Space, is running out.

Other theories for the TNOs’ bizarre behaviour are related to small black holes, telescopes not being advanced enough (that could change when the new Vera Rubin Observatory to come online in Chile early next year) and our understanding of gravity being wrong.

It does, though, seem as if something is going on in the outer reaches of our solar system, and the race is on to be the first to definitively discover that there is actually a ninth planet out there, nearly two decades after Pluto was demoted.