Both images: Caltech/R. Hurt (IPAC)

Super-Earths, also known as mini-Neptunes since they may be gassy or rocky, are among the most common planets in the Milky Way. Hundreds have been spotted in other solar systems over the past two decades, but even so there appeared to be none back home.

But now Konstantin Batygin and Michael Brown, astrophysicists at the California Institute of Technology, say they can make a convincing case for a super-Earth here in our solar system.

Only a giant planet will do

“We didn’t want to invoke a distant giant planet to explain anything about the outer solar system,” says Batygin. “We tried every trick in the book, and some that aren’t in the book. Simply nothing else worked.”


“We usually talk about how the most common type of planet, two to three times the size of Earth, is not found in our solar system, yet now we might have one,” says Sara Seager at the Massachusetts Institute for Technology. “That’s cool.”

But even if there is a super-Earth out on the edges of our solar system, we’re still pretty weird. “Even with Planet Nine bringing us a bit closer to the galactic planetary catalogue, the solar system does still appear to be an overall outlier,” says Batygin, as super-Earths in other star systems are much closer to their stars.

Jupiter is the culprit

Simulations of the young solar system suggest that the early formation of Jupiter probably prevented this from happening here. But those simulations also suggest that a super-Earth couldn’t have formed as far away as Planet Nine would have to be, at least, not then – there simply wasn’t enough material out there to make a planet that big.

So how did Planet Nine, if it exists, get all the way out there?

The answer must lie in the very beginnings of the solar system, when our stellar neighbourhood was a very different – and much more dangerous – place. “We know the process of planet formation is not placid,” says Batygin. “There was a very chaotic sequence of events.” Planets were thrown around by their gravitational pulls on each other, occasionally colliding, as in the process that is thought to have formed Earth’s moon.

Planet Nine was the “core of a giant planet, which was forming with the other planets we know and love, that was thrown out of the solar system,” says Batygin.

Violent births

“We know that Uranus and Neptune probably had a very violent final phase of formation,” says Sean Raymond of Bordeaux Observatory in France – they essentially came together through a series of collisions. Last year he and colleagues simulated this process and found that planets around five times the mass of Earth were ejected from the solar system during the pileup.

At the time the team didn’t think too much about their eventual fate, but hints of Planet Nine suggest one could have settled down into a lengthy orbit. “This extra planet could be representative of the building blocks of the ice giants,” says Raymond.

That gives us a new mystery: what prevented the planet’s escape? Brown believes that gas and dust in the very early solar system may have slowed down the planet, but Batygin has his eye on a different culprit: the other stars nearby in the early days of our solar system. He thinks that gravitational tugs from those stars, are the best candidates for slowing down Planet Nine enough to keep it in a huge, elongated orbit around the sun.

Random pattern or real object?

But Planet Nine might not even be there. While Batygin and Brown have pointed out compelling evidence for the existence of Planet Nine by studying the unusual orbits of six distant rocks called Kuiper belt objects (KBOs), that is not the same as a discovery, says Brown. “People are good at seeing patterns in places, and maybe we have just seen a random pattern and convinced ourselves it’s statistically significant. That’s my biggest worry.”

Future discoveries of KBOs will provide the true test. “If you find more of these objects and they really are randomly distributed around the sky, then it’s time for us to hang our heads in shame and retreat from the national media,” says Brown. But if the anomalies in the outer Kuiper belt remain, then Planet Nine will remain a likely explanation for them.

“Further work is needed to explain the origin of Planet Nine,” says Lucie Jílková of Leiden Observatory in The Netherlands, who last year suggested the strange orbits of these KBOs could be explained if they were stolen from a passing star. “Observations of more outer solar system bodies will help to further constrain and distinguish the theories.”

Lightbulb on the moon

If Planet Nine really is out there, with any luck astronomers will be able to spot it soon, despite the scant light it reflects from the sun. “Finding this object is like finding a lightbulb on the moon,” says Batygin. “But our calculations have provided a roadmap for where to look for this very dim object in the sky. Hopefully, that will trigger a hunt.”

We might even send a probe there one day. “It’s not as crazy as it sounds,” says Brown. “The way you do it is head straight for the sun, and as you swing around the sun and you’re getting this gravitational slingshot, you fire a whole bank of rocket engines.” Depending on the distance to Planet Nine, this could take anywhere from a few years to 20 years.

Other schemes like this have been proposed in the past, some with nuclear-powered rockets, but they never got past the planning stages. But a target like Planet Nine could make all the difference. “The societal impact of having that destination would be very exciting,” says Greg Laughlin at the University of California, Santa Cruz. “The stars are just too far away, we’re not going there. But this is a place where we could get to.”