Good news, everyone: Our solar system likely has nine planets again, so there’s no reason to forget that number you learned in grade school. The bad news is is that the ninth planet still isn’t Pluto. Instead it’s an entirely new planet, according to a team of astronomers from Caltech who found evidence of its existence and are calling it Planet Nine.
Planet Nine, the astronomers say, is likely the fifth-largest planet in our solar system, having a mass 10 times that of Earth and a diameter four times the size. That puts Planet Nine just after Uranus and Neptune in size. As for the former planet Pluto? Planet Nine is 5,000 times the size of that tiny snowball. But if Planet Nine is that big, how come we haven’t seen it before?
The reason, astronomers say, is because Planet Nine has a unique feature: It has a highly elliptical orbit that faces the opposite direction of the orbits of other planets. The orbit is so long it takes Planet Nine between 10,000 and 20,000 years to make one trip around the sun.
The astronomers that say Planet Nine exists haven’t seen the planet itself. Instead they hypothesize its existence from the way its gravity affects the orbit of over a dozen objects in the Kuiper Belt, a region in our solar system past all the visible planets that is made of rings of icy objects.
In 2014, other astronomers had hypothesized that a potentially unknown planet could be nudging the Kuiper Belt’s objects out of place and the Caltech astronomers actually set out to disprove that theory, thinking that no planet on an aligned orbit could do that. But when they entered an "anti-aligned" orbit into their simulation, it fit perfectly.
"Your natural response is, 'This orbital geometry can't be right. This can't be stable over the long term because, after all, this would cause the planet and these objects to meet and eventually collide,'" Konstantin Batygin, one of the Caltech astronomers said. However, he and fellow astronomer Mike Brown found that the orbits aligned just right so that no collisions ever occurred.
It should be noted that, in a bit of astronomy irony, Mike Brown is also the astronomer known as the "Pluto Killer" who found evidence that stripped Pluto of its planetary status.
The Caltech astronomers now say the next step is to visualize Planet Nine so that its existence can be confirmed without a doubt. Though that may mean pointing some of Earth’s biggest telescopes toward its anti-aligned orbital path, the astronomers also note that it's possible someone has already photographed the planet before, hiding among a backdrop of stars.
As for Brown, in a tweet he wrote, "OK, OK, I am now willing to admit: I DO believe that the solar system has nine planets."