Doubts about the Ninth Planet.
Astronomers doubt that there is an undiscovered 9th planet in our solar system.
Planet Nine is a theoretical undetected giant planet in the mysterious remote corners of our solar system.
The presence of Planet 9 is thought to explain everything from the tilt of the Sun’s rotation axis to the apparent cluster of small, icy asteroids orbiting beyond Neptune. But does planet Nine really exist?
Discoveries at the edge of our solar system.
The Kuiper Belt is a collection of small icy bodies that orbit the Sun outside of Neptune at distances greater than 30 au (one astronomical unit or AE is the distance between the Earth and the Sun). These Kuiper Belt objects range in size from large boulders up to 2,000 km across. The Kuiper Belt contains remnants of planetary material that have never been incorporated into any planet, like the asteroid belt.
The most successful Kuiper Belt study to date, the OSSOS program, offers a trickier explanation for the orbits we see. Many of these objects have been found to have very elliptical and inclined orbits, such as Pluto.
Mathematical calculations and detailed computer simulations have shown that the orbits we see in the Kuiper Belt could only have been created if Neptune originally formed a few AU closer to the Sun, and then migrated outward to its current orbit. Neptune migration explains the prevalence of highly elliptical orbits in the Kuiper Belt, and can explain all the orbits of Kuiper Belt objects we observe, with the exception of a few objects in extreme orbits that always remain at least 10 au outside of Neptune.
According to the theory, planet nine is five to ten times more massive than Earth, and its orbit ranges from 300 to 700 AU. Several predictions have been published regarding its location in the Solar System, but none of the search teams have yet found it. After more than four years of searching, there is still only circumstantial evidence in favor of the ninth planet.
Search for objects in the Kuiper Belt.
The search requires careful planning, precise calculations, and careful monitoring. The OSSOS mission is a collaboration of 40 astronomers from eight countries. They used the Canada-France-Hawaii telescope for five years to discover and track more than 800 new objects in the Kuiper Belt, nearly doubling the number of known objects with well-measured orbits. Objects detected by OSSOS range in size from a few kilometers to more than 100 km, and the detection distance is from a few AU to more than 100 AU, with most of them located in 40-42 AU in the main Kuiper belt.
Objects in this belt do not emit their own light: these small ice bodies only reflect the light of the sun. So the biases against detection at long distances are extreme: if you move an object 10 times further, it becomes 10,000 times weaker. And because of the laws of physics, objects in the Kuiper Belt in elliptical orbits will spend most of their time in the most distant parts of their orbits. Thus, it is easy to find objects in elliptical orbits when they are close to the Sun and bright, but these objects spend most of their time being much fainter and harder to detect.
This means that objects in elliptical orbits are particularly difficult to detect when they are relatively far from the sun. To date, only a few of them have been found, and with the help of modern telescopes, we can only detect them when they are near the pericenter – the closest point to the Sun in its orbit.
This leads to a bias in observations that has historically been ignored by many Kuiper Belt studies: objects in each part of the Solar System can only be detected at certain times of the year. Ground-based telescopes are further limited by seasonal weather, and discoveries are less likely to occur during periods when cloudy, rainy, or windy conditions dominate the planet. Discoveries are also much less likely near the plane of the Milky Way galaxy, where countless stars make it difficult to find faint, icy wanderers in the images.
OSSOS has discovered several new extreme objects in the Kuiper Belt, half of which are located outside the restricted area and are statistically consistent with a uniform distribution.
All the extreme objects discovered before the advent of OSSOS were found as a result of studies that did not fully describe their orientation. We conducted additional simulations that showed that if observations are made only in one season from one telescope, then extreme Kuiper Belt objects will naturally be detected only in one quadrant of the Solar System.
Further testing the planet nine theory, we examined in detail the orbits of all known “extreme” objects and found that all but two objects with a pericenter can be explained by known physical effects. These two Kuiper Belt objects are extreme, but our previous detailed computer simulations of the Kuiper Belt, which included the gravitational effects from planet nine, produced a set of similarly “extreme” objects with pericenters varying smoothly from 40 to over 100 AU.
This simulation predicts that there should be a set of belt objects with pericenters equal to the two found, but also a set of objects with smaller pericenters that should be much easier to detect. Why do the discoveries of the orbit not correspond to the forecasts? The answer may be that the planet nine theory is not supported by our observations.
Our observations with careful investigation have found objects that are not related to planet Nine, and our simulations show that the Kuiper Belt must have orbits of objects different from those we observe – if planet nine exists. Other theories should be used to explain extreme objects with a high pericenter, but there is no shortage of proposed theories in the scientific literature.
Many beautiful and amazing objects have yet to be discovered in the mysterious outer solar system, but the study does not confirm that planet nine is one of these objects.