Has No One Seen it?

 

We thought ‘this is quite interesting – how can this be?

In 2006, the International Astronomical Union voted to demote Pluto’s status to a “dwarf planet”, along with the newcomers. Mike Brown, a professor of planetary astronomy at the California Institute of Technology – Caltech – who led the team that identified Eris, is self-styled as the “man who killed Pluto” to this day. The ninth planet was no more.

A ghostly signature

At the same time, the discovery of these objects uncovered a major new lead in the search for a hidden planet.

It turns out that Sedna is not moving in the way everyone expected – tracing elliptical rings around the Sun, from within the Kuiper Belt. Instead, this dwarf planet takes a bizarre and unexpected path, swinging from just 76 Earth-Sun distances (roughly 11 billion k/7 billion miles) from the centre of our solar system to more than 900 (roughly 135 billion km/84 billion miles). Its orbit is so meandering, it takes 11,000 years to complete – the last time Sedna was at its current position, humans had only just invented farming.

t’s as though something is tugging at Sedna and dragging it away.

Enter a hypothetical new addition to our solar system – but not as it was thought of before. In 2016, the same Mike Brown who had slain Pluto, together with his colleague Konstantin Batygin – also a professor of planetary science at Caltech – co-authored a paper proposing a massive planet, between five and 10 times the size of Earth.

Their idea came from the observation that Sedna was not the only object out of place. It was joined by six others, and all of them are being pulled in the same direction. There are also other clues, such as the fact that each is tilted on its axis in exactly the same direction. The pair calculated that the probability of all six objects being pulled in the exact same direction, with the same tilt by chance was just 0.007%.

“We thought ‘this is quite interesting – how can this be?'” says Batygin. “It was quite remarkable because such a clustering, if left alone for a sufficiently long period of time, would disperse, just due to interaction with the gravity of the planets.”

Instead, they proposed that Planet Nine had left its ghostly imprint in the outer reaches of our solar system, distorting the orbits of the objects around it with its gravitational pull. Several years on, and the number of objects that fit the eccentric orbital pattern and tilt has continued to increase, “We now have around 19 overall,” says Batygin.

Though no one has yet seen the hypothetical planet, it’s possible to infer a surprising amount about it. As with the other objects beyond the Kuiper Belt, the orbit of the new Planet Nine would be so distorted that its farthest reach is expected to be twice as far away as its nearest – around 600 times the distance from the Sun to Earth (90 billion km/56 billion miles), vs 300 (45 billion km/28 billion miles). Scientists have also hazarded a guess at its aesthetic – icy, with a solid core, like Uranus or Neptune.

Then there’s the slippery question of where Planet Nine might have come from in the first place. So far, there are three main ideas. One is that it formed where it currently hides, which Batygin dismisses as relatively unlikely because this would require the early solar system to have stretched out as far as its distant refuge.

There’s also the intriguing suggestion that the ninth planet is actually an alien imposter, an object that was stolen from another star long ago when the Sun was still in the stellar cluster in which it was born. “The problem with such a story is that you’re just as likely to then lose the planet upon the next encounter,” says Batygin. “So, statistically, that model runs into trouble.”

Then there’s Batygin’s personal favourite, which he admits is also “not a complete slam dunk”. In this scenario, the planet formed much closer to the Sun, at a time when the solar system was in its early stages and the planets were just beginning to coalesce out of the surrounding gas and dust. “It kind of hung around the giant planet formation region, before being scattered out by Jupiter or Saturn, and subsequently had its orbit modified by passing stars,” he says.