Planet Nine, do you copy?

Today, Pluto is no longer considered a true planet, and there are only eight such objects in the solar system. Really? What if there is an undiscovered ninth planet lurking out there, beyond the Kuiper Belt? Many astronomers believe this is the case, including Konstantin Batygin of the California Institute of Technology (Caltech), who in 2016, together with his colleague Michael Brown, announced that they had found evidence that Planet Nine had a gravitational influence on the orbits of trans-Neptunian objects (That is, its orbit intersects with Neptune’s orbit or is outside its orbit, editor’s note)However, these findings have generated considerable controversy: some say the number of objects is too small and that selection effects in astronomical surveys may not have been properly accounted for. The researchers and their collaborators therefore turned their attention to a class of objects that were a priori less susceptible to observational biases. They came to the same conclusion: their orbits could only be explained if they were influenced by as-yet-undiscovered planets.

Non-random distribution

Extreme trans-Neptunian objects are Solar System objects whose perihelion (the point in their orbit closest to the Sun) is at least 50 times the distance from the Earth to the Sun, and whose orbits are highly elliptical. As a result, their perihelion is so far away that they never come close to Neptune at all, and are therefore barely affected by the giant planet’s gravity. Surprisingly, decades of astronomical surveys conducted in the 20th and early 21st centuries have shown that extreme trans-Neptunian objects are extremely distant. Over the past century, astronomers have observed the orbits of about 10 objects and found that the orbits of about 10 of them seem to be aligned. “They all point in the same direction across the sky, so their distribution is not random.” Sean Raymond of LAB explains.

However, the laws of celestial mechanics suggest that this is not the case. As an object orbits the Sun, its orbital axis also wobbles, carving out a path around the star, a phenomenon known as precession.”So even if many objects were flung in the same direction 4 billion years ago by disturbances in the early solar system, their orbits should have diverged since then because of these precessions.” Alessandro Morbidelli, an astronomer and planetary scientist at Lagrange Laboratory, said: and a professor at the Collège de France. The fact that the orbits of these planets always remain aligned suggests that there is “something” forcing them to behave differently than expected. “Because these objects are so far from Neptune, its gravitational influence cannot force their orbits to remain clustered together. The only explanation for this anomaly is the presence of another planet.” Astronomers explained.

Eliminating bias

This is how the hypothesis of an undiscovered planet in the solar system first emerged, and its main features are outlined in a paper A new study published in 2016 found this: its mass is thought to be between five and seven times that of Earth, and it orbits in a distant, elliptical, tilted orbit. However, this conclusion soon sparked heated debate and considerable skepticism. How can we be sure that the orbits are really aligned? Then, because these orbital alignments only apply to objects in highly elliptical orbits far from Neptune, there are a limited number of them: just ten or so may not be enough to have statistical reliability. Furthermore, these objects were discovered by many different people using a wide range of astronomical surveys. How can we be sure that all the deviations behind these observations are modeled correctly?

“In certain regions of the sky, like the plane of the Milky Way, it’s very difficult to detect distant objects because there are so many stars there. If you don’t see any TNOs passing through it and you only draw conclusions from the ones you actually observed, this can introduce biases.” Raymond explained. “Batygin’s team was convinced that the alignment they saw was intrinsic to the population of objects they were observing, but this was not generally accepted by the scientific community.”

Jean-Marc Petit, astronomer at the UTINAM Institute He has spent some time looking at the observational biases of the objects mentioned in the 2016 paper and how to deal with them. While he agrees that orbital clustering is possible, he also thinks the probability of this happening is much lower than Batygin and his colleagues suggest. “We are not saying that Planet Nine doesn’t exist,” He stressed,It’s that the arguments they put forward were not strong enough.” Morbidelli said:That’s a perfectly reasonable point of view.”

Impossible absence

In a new paper, Batygin, Morbidelli, Brown and David Nesvony, a planetary scientist at the Southwest Research Institute in Texas, tried a new approach. Rather than focusing on distant objects, which are difficult to observe because their orbits are highly elliptical and never come close to Neptune, they turned their attention to a class of trans-Neptunian objects that cross the orbits of blue giant stars. “These objects are relatively close to us and bright, which makes them easier to study.” Morbidelli pointed out. “Some of these we already understand, and their observational biases are easier to model.”

In addition, these objects are extremely unstable. When they cross the orbits of the giant planets, they disperse and their trajectories change. Therefore, their lifespan is only tens of millions of years. Therefore, they are constantly replenished by true trans-Neptunian objects. “We compared systems with and without Planet Nine to find out at what rate these Neptune-transiting objects can be renewed,” Morbidelli added.We found that without Planet Nine, this rate is too low because there are not enough objects crossing Neptune’s orbit. On the other hand, with Planet Nine, our model is able to reproduce the observations much better.” In other words, the new study is a priori less biased than previous studies and also explicitly concludes that there is an as-yet-undiscovered planet. “It’s a very good idea, and the results are very clear.” Raymond said. “It’s a really well done piece of work and I think they achieved something.” Petit agreed.

So, does this confirm the existence of the ninth planet? “It’s not that simple.” Raymond pointed out. “In principle, there are fewer observational biases in this study, but it is really difficult to eliminate all biases. It is still possible that there are some hidden biases.” Petit lists some possible biases: “For example, the survey they used was not specific for TNOs. Also, they did not take into account the size of the objects, which can be an important factor.” Any observation is subject to errors and estimates that are difficult to account for. “That’s why, even if there are good reasons to believe Planet Nine does exist, you shouldn’t believe it exists until you actually find it.” Raymond pointed out. “You have to go look for it with an open mind.”

In their search for Planet Nine, astronomers will soon be assisted by a large new telescope, the Vera C. Rubin Observatory, which is currently under construction in Chile and is expected to be operational in early 2025. “The advantage of the Vera Rubin Telescope is that it can observe more than half of the sky every two or three days, and its depth is certainly as good as the Hubble Space Observatory,” Raymond said. “It’s designed specifically to look for dark objects moving across the sky, like Planet Nine. Even if it does exist, you can’t be 100% sure the telescope will detect it, but if it doesn’t find it, it’s hard to continue to believe the planet is there.”

What if it’s not a planet?

The alignment of certain objects’ orbits has prompted some researchers to propose alternative ideas, including that the anomaly could be caused by a giant disk of objects in the outer reaches of the solar system or by a microscopic black hole. “These are pretty crazy ideas.” Morbidelli smiled.I would love for someone to tell me how a 5 Earth mass black hole is formed.” The same is true for a disk with excessive eccentricity: if it consisted of a large number of objects, some of them should have been discovered because stellar occultations would occur when one of them passed in front of it. But no such event has ever been seen. “On the other hand, the presence of a planet in the orbit of what is thought to be Planet Nine is quite normal, given the formation process of the giant planets: as they grow, they eject several Earth-mass planets outwards, which can remain in distant, eccentric orbits.” Morbidelli noted. So if Planet Nine does exist, it is likely a planet.

Further reading on our website

Approaching Planet Nine
The ninth planet (Column by Denis Gutleben, historian of science at the French National Institute of Scientific Research)