How Astronomers Will Actually Image the Solar System's Ninth Planet

Astronomer Mike Brown axed Pluto as the ninth planet. Now, he may provide a bonafide replacement.

The minds of space nerds around the world collectively exploded yesterday, after astronomers announced that there is likely an undiscovered Neptune-sized planet in the outer reaches of the Solar System.

After centuries of fruitless theories about Planet X, Nibiru, or Vulcan, it's natural to side-eye claims of random planets in our cosmic backyard. Hell, even Mike Brown and Konstantin Batygin, the two Caltech astronomers who published tantalizing clues that point toward a so-called "Planet Nine," second-guessed their results.

"[W]e were initially quite skeptical that this planet could exist," said Batygin in Wednesday's announcement. Brown even called the prospect of a ninth planet "crazy" at first.

But as the pair continued to work through the problem, the odds just kept stacking up in favor of Planet Nine. The real smoking gun was at the edge of the Kuiper belt, the disk of debris that circles the Solar System and is home to dwarf planets like Pluto and Eris. Brown and Batygin observed that some of the most distant KBOs (Kuiper belt objects) seemed to be tugged out in one direction, toward the outer reaches of the Solar System, by some phantom force lurking there.

This gravitational anomaly is best explained by the existence of a planet ten times more massive than Earth (for comparison, Neptune is 17 times more massive than Earth). It seems to be sweeping along an extraordinarily elongated path around the Solar System and would have to be very distant, about 20 times farther out than Neptune. Based on what we know so far, Planet Nine orbits the Sun once every 10,000 to 20,000 years.

Brown and Batygin explain the evidence for Planet Nine. Video: caltech/YouTube

Bolstering the theory are many observations of similar planets in other solar systems, suggesting this is a fairly common planetary configuration in the universe.

"We believe that planets such as Planet Nine form together with more 'conventional' planets such as Uranus and Neptune, out of the same material," Batygin told me over email. "The expansive, highly elongated orbit is a consequence of chaotic gravitational stirring that occurred during the infancy of the Solar System."

"There are multiple theories," Brown noted, in the same email interview. "[These planets] could have been ejected from the inner parts of the planetary systems, or they could even have been captured from passing stars."

So as surreal as it seems, Brown and Batygin have provided extremely compelling evidence of a ninth planet.

This is especially fitting for Brown, because he is the astronomer who notoriously demoted Pluto to a dwarf planet, thus slimming the Solar System to eight known planets. Brown has humorously embraced his reputation—his Twitter handle is @plutokiller—but now, it seems, he has the opportunity to offer up a bonafide replacement for the planet he axed.

But before we start getting too caught up in the frenzy over this hypothetical new world, direct evidence of its existence has to be rooted out.

"The astronomical hunt for Planet Nine is on," Batygin told me.

"It's going to take the biggest telescopes on Earth to do it," Brown said, "but it is definitely in the range of these telescopes. We're also looking for more of the objects at the very edge of the Solar System that are being pushed around by Planet Nine, as those will help us better pinpoint where to look."

Diagram of Planet Nine's proposed orbit. Image: Caltech/R. Hurt (IPAC); [Diagram created using WorldWide Telescope]

World class telescopes like the W. M. Keck Observatory and the Subaru Telescope, which are located atop the Hawaiian volcano Mauna Kea, are the most likely facilities to get a visual on Planet Nine first. But if and when this world is directly imaged, it will take much larger and more sophisticated facilities—such as the controversial Thirty Meter Telescope—to learn more about its size, composition, and other properties.

"The best exploration right now is happening from the big telescopes on the ground," Brown told me. "We can scan so much sky, see what's there, learn incredible things. We then use all of the other telescopes around to study the objects in more details."

"There is much excitement to come!"