In early January, the news that an immense ninth planet likely exists beyond Pluto set the scientific community ablaze. We still have a lot to learn about this potential new solar sibling, but we do know that it’s huge—at least 10 times as massive as the Earth. The astronomers who discovered it even nicknamed it “Fatty.” And the fact that such a huge body has gone undetected just goes to show how little we truly know about our own solar system and how much science has left to teach us.

10 It Was Discovered By The Guy Who Killed Pluto

Even if you haven’t heard of Mike Brown, you are indirectly familiar with his work. Back in 2005, he discovered a Kuiper Belt object dubbed Eris, which briefly seemed a candidate for planet status. The discovery touched off an How I Killed Pluto and... Mike Brown Best Price: $1.75 Buy New $7.00 (as of 11:35 UTC - Details) debate over the definition of a planet that ended with Pluto and Eris being bumped down to the status of dwarf planet. This earned Brown a measure of notoriety—he even wrote a book entitled How I Killed Pluto (And Why It Had It Coming.

But in a curious twist of fate, the man who robbed the solar system of a planet could now be giving it a new one. Working with his fellow astronomer Konstantin Batygin, Brown announced in the Astronomical Journal that the unusual orbital behavior of 13 trans-Neptunian objects (i.e. objects outside the orbit of Neptune) was strong evidence for the existence of a massive, distant planet: “We realized that the only way we could get the [trans-Neptunian objects] to all swing in one direction is if there is a massive planet keeping them in place.”

Pluto’s absence from the roster of planets has long been a sore point for many space enthusiasts. Here’s hoping that Planet Nine can finally bring them some closure.

9 It’s An Ice Giant

Celestron - AstroMaste... Best Price: $187.88 Buy New $227.99 (as of 02:30 UTC - Details) Unlike Pluto and Eris, Brown and Batygin think that Planet Nine is a full planet. Brown even told the New Yorker that the proposed body would be “more of a planet than anything else in the solar system.” After all, “what we now call planets are objects that can gravitationally dominate their neighborhood. Pluto is a slave to the gravitational influence of Neptune. By area, Planet Nine dominates more of the solar system than any other known planet—it’s only because of this that we can infer its existence. And because of this we’re pretty sure it’s not a small object: it’s, at least, ten times more massive than Earth and five thousand times more massive than Pluto.”

And its predicted size tells us something important about its physical makeup. The bigger a planet gets, the thicker its atmosphere becomes, as it accumulates more and more gaseous elements in a process known as core accretion. This is thought to be why rocky planets like Earth and Mars can only reach a certain size before they become a gas giant, like Jupiter or Saturn. Ice giants seem to be a kind of in-between: their atmospheres are thick and made mostly of the same stuff as gas giants, but they’re nowhere near as big.

Planet Nine’s size, larger than any rocky planet, yet smaller than any gas giant, suggests that it may fall into this odd category. Scientists are divided as to how ice giants form, with most models causing problems for more accepted The Velikovsky Heresie... Laird Scranton Best Price: $7.23 Buy New $2.99 (as of 10:15 UTC - Details) models of how gas giants form. As a result, ice giants remain the subject of hot debate in the scientific community, so a confirmed ninth plant could provide a wealth of new information on these planetary oddballs.

8 It‘s An Unbelievably Long Way Away

Even in astronomical terms, Planet Nine is staggeringly far away—its average distance from the Sun is 56 billion miles, over 20 times the distance from the Sun to Neptune, the most distant of the current planets. To put it another way, NASA’s New Horizons probe, which recently reached Pluto after a journey of nine years, would have taken at least 54 years to reach Planet Nine. And that’s a best-case scenario, calculated for Nine’s closest approach to the Sun. Reaching Nine during the most distant phase of its orbit could take up to 350 years. Of course, those are both hypothetical scenarios, since New Horizons can’t carry enough fuel to reach Nine anyway.

This incredible distance could help explain why nobody has noticed Planet Nine until now. Based on their calculations, Mike Brown and Konstantin Batygin think that their hypothetical planet would actually be visible through most backyard telescopes—but only when its orbit brings it relatively close to Earth. Since nobody has noticed Planet Nine in their backyard yet, we can presume that it’s currently at a more distant point in its orbit, although Brown and Batygin still think it should be possible to detect with extremely powerful observatory telescopes.

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