Our solar system might not be the weird kid on the block after all. Astronomers announced this week that they鈥檝e seen strong evidence for a planet around 10 times Earth鈥檚 mass lurking unseen in the outer solar system. If it bears out, this world would fill a hole in our solar system鈥檚 family tree: a missing super-Earth.
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 and , astrophysicists at the California Institute of Technology, say they can make a convincing case for a super-Earth here in our solar system.
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Only a giant planet will do
鈥淲e didn鈥檛 want to invoke a distant giant planet to explain anything about the outer solar system,鈥� says Batygin. 鈥淲e tried every trick in the book, and some that aren鈥檛 in the book. Simply nothing else worked.鈥�
鈥淲e 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,鈥� says at the Massachusetts Institute for Technology. 鈥淭hat鈥檚 cool.鈥�
But even if there is a super-Earth out on the edges of our solar system, we鈥檙e still pretty weird. 鈥淓ven with Planet Nine bringing us a bit closer to the galactic planetary catalogue, the solar system does still appear to be an overall outlier,鈥� 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鈥檛 have formed as far away as Planet Nine would have to be, at least, not then 鈥� there simply wasn鈥檛 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 鈥� and much more dangerous 鈥� place. 鈥淲e know the process of planet formation is not placid,鈥� says Batygin. 鈥淭here was a very chaotic sequence of events.鈥� Planets were thrown around by their gravitational pulls on each other, occasionally colliding, as in the process that is thought to have formed Earth鈥檚 moon.
Planet Nine was the 鈥渃ore of a giant planet, which was forming with the other planets we know and love, that was thrown out of the solar system,鈥� says Batygin.
Violent births
鈥淲e know that Uranus and Neptune probably had a very violent final phase of formation,鈥� says of Bordeaux Observatory in France 鈥� 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鈥檛 think too much about their eventual fate, but hints of Planet Nine suggest one could have settled down into a lengthy orbit. 鈥淭his extra planet could be representative of the building blocks of the ice giants,鈥� says Raymond.
That gives us a new mystery: what prevented the planet鈥檚 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. 鈥淧eople are good at seeing patterns in places, and maybe we have just seen a random pattern and convinced ourselves it鈥檚 statistically significant. That鈥檚 my biggest worry.鈥�
Future discoveries of KBOs will provide the true test. 鈥淚f you find more of these objects and they really are randomly distributed around the sky, then it鈥檚 time for us to hang our heads in shame and retreat from the national media,鈥� says Brown. But if the anomalies in the outer Kuiper belt remain, then Planet Nine will remain a likely explanation for them.
鈥淔urther work is needed to explain the origin of Planet Nine,鈥� says Lucie J铆lkov谩 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. 鈥淥bservations of more outer solar system bodies will help to further constrain and distinguish the theories.鈥�
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. 鈥淔inding this object is like finding a lightbulb on the moon,鈥� says Batygin. 鈥淏ut our calculations have provided a roadmap for where to look for this very dim object in the sky. Hopefully, that will trigger a hunt.鈥�
We might even send a probe there one day. 鈥淚t鈥檚 not as crazy as it sounds,鈥� says Brown. 鈥淭he way you do it is head straight for the sun, and as you swing around the sun and you鈥檙e getting this gravitational slingshot, you fire a whole bank of rocket engines.鈥� 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. 鈥淭he societal impact of having that destination would be very exciting,鈥� says at the University of California, Santa Cruz. 鈥淭he stars are just too far away, we鈥檙e not going there. But this is a place where we could get to.鈥�
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