Forget Planet X; a hidden planet the size of Mars seems just a little more plausible.

Numerous planets have been discovered outside our solar system in recent years. But perhaps it is time that we also take a critical look at our own system, because perhaps there is also a planet hidden in it. That is what researchers in the Annual Review of Astronomy and Astrophysics. It would be a planet that is about the same size as Mars or can barely measure up to the Earth in size and that is still beyond Neptune’s orbit.

Review

That conclusion is not based on new research or new insights, researcher Kathryn Volk emphasizes in conversation with Scientias.nl. “The research article is a review, which means we summarize and respond to published work. So we are not presenting any new evidence for new planets in the outer solar system.”

Ninth Planet

It doesn’t make the scientists’ conclusion any less interesting. Because based on all those publications by colleagues, Volk and colleagues still state that it seems plausible that our solar system has a ninth planet. Or at least hosted a ninth planet in the past. And there are even several indications for this, says Volk. “Our models describing the history of the outer reaches of our solar system suggest that, in addition to the gas giants known to us, there may have been at least 20 to 30 Earth masses of material, from which the current trans-Neptunian population (objects beyond Neptune’s orbit, ed. ) originated. Objects of a wide range of sizes – up to dwarf planets – would have formed in that disk of material and it seems unlikely that we have already observed the largest objects born from this disk.” In addition, there are a number of trans-Neptunian objects (tnos) with a perihelion (the point in their orbit where they are closest to the sun) that are far from our parent star. “Its existence is difficult to explain with the current architecture of the solar system.” Things will change if we equip the young solar system with an extra planet the size of Mars or Earth that has been pushed deeper into space from that disk of material or even left the solar system at some point. Along the way, such a planet may have radically changed the orbits of some TNOs. “These are all factors that support the idea that objects much larger than the dwarf planets known to us could be found in that original disc of material in the outer reaches of our solar system.”

Planet X

The story may sound familiar to you. And that may be true. Years ago, researchers also stated that our solar system is home to nine planets. That ninth planet — also referred to as Planet X — is also said to be in the outer reaches of the solar system. But where Volk and colleagues now hint at a planet the size of Mars or Earth, Planet X would be significantly larger than Earth. “The literature evidence for a large ‘Planet X’ is inconclusive at this point,” Volk says. She believes it is more likely that searches for a ninth planet will result in the discovery of an object the size of Mars.

Ninth planet or not?

If those searches yield anything, of course. Because what the review also makes clear is that it is far from certain that our solar system contains nine planets. “There is currently no conclusive evidence for an additional planet in the solar system,” Volk emphasizes. “The evidence for such a planet is intriguing, but the suggested clues to the existence of undiscovered planets become weaker rather than stronger as more new trans-Neptunian objects are discovered.” And even if our solar system ever spawned a ninth planet, it has not yet been proven to still reside in our solar system. “The TNOs with a remote perihelion remain the most compelling evidence for the idea that there are additional, large celestial bodies that influenced these TNOs, but those objects need not be present in the solar system anymore. Moreover, there are also other possible explanations for these TNOs – such as interactions with neighboring stars in the Sun’s birth cluster) – that cannot yet be ruled out.”

Gone for good

If our solar system ever counted a ninth planet, but at some point it skidded into interstellar space, it’s unlikely we’ll ever find that planet again. “If one or more planets were ejected from the solar system, it probably happened almost 4 billion years ago,” Volk said. So no, we would never be able to find those planets again.”

LSST

If the ninth planet is still in the solar system, it’s a different story. “If there are even larger planets in the outer reaches of our solar system right now, Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST) gives us the best chance of finding these planets. During this research, a large part of the sky is searched for relatively faint objects. We expect that this will expand the number of trans-Neptunian objects known to us from a few thousand to several tens of thousands.” But we still don’t know everything after the Legacy Survey of Space and Time, Volk emphasizes. “Our picture of the outer reaches of the solar system remains incredibly incomplete beyond that, because orbiting objects spend most of their time at distances where they don’t reflect enough sunlight and aren’t bright enough to be detected. I definitely expect LSST to discover some major trans-Neptunian objects. But even very large objects can simply be too far away to be detected.”

And so it remains unclear whether our solar system houses a ninth planet and whether we could also observe it. But all these uncertainties actually encourage researchers to keep (re)searching. If that search is successful, many books will have to be rewritten. And scientists can certainly not sit back just yet; the find would only be the prelude to follow-up studies. “Each additional large object we discover will help us better understand conditions in the young solar system.”