The presence of Planet Nine has been hypothesized to explain everything from the tilt of the Sun’s spin axis to the apparent clustering in the orbits of small icy asteroids beyond Neptune. But does Planet Nine really exist?
The Kuiper Belt is a collection of small icy bodies that orbit the sun beyond Neptune, at distances greater than 30 AU (an astronomical unit or AU is the distance between the Earth and the sun). These Kuiper Belt Objects (KBO) range in size from large rocks up to 2000 km in diameter. KBOs are remnants of small fragments of planetary material that were never incorporated into planets, similar to the asteroid belt.
Discoveries at the edge of our solar system
Findings from the most successful Kuiper Belt survey to date, the Outer Solar System Origins Study (OSSOS) project, suggest a more astute explanation for the orbits we see. Many of these KBOs have been found to have highly elliptical and inclined orbits, like Pluto.
Mathematical calculations and detailed computer simulations have shown that the orbits we see in the Kuiper Belt can only have been created if Neptune originally formed a few AUs (Astronomical Unit) closer to the sun and migrated outward to its current orbit. Neptune’s migration explains the omnipresence of highly elliptical orbits in the Kuiper Belt, and it may explain all the KBO orbits we’ve observed except for a handful of extreme orbiting KBOs that always remain at least 10 AU beyond Neptune.
A test of Planet Nine?
These extreme orbits have provided the strongest evidence for Planet Nine. The first discoveries were limited to one quadrant of the solar system. Astronomers hope to observe orbits in all different orientations, unless there is an external force confining them. Finding several extreme KBOs in orbits pointed in the same direction was a clue that something was happening.
Two separate groups of researchers calculated that only a large and very distant planet could keep all orbits confined to one part of the solar system , and thus the Planet Nine theory was born.
Planet Nine is believed to be five to ten times more massive than Earth, with an orbit ranging from 300-700 AU. Several predictions have been published for its location in the solar system, but none of the search teams have discovered it yet. After more than four years of searching, there is still evidence, not even indirect, in favor of Planet Nine.
The search for KBO (Kuiper Belt objects)
Searching for KBO requires careful planning, accurate calculations, and meticulous follow-up. I am part of OSSOS, a collaboration of 40 astronomers from eight countries. We used the Canada-France-Hawaii Telescope for five years to discover and track more than 800 new KBOs, almost doubling the number of known KBOs with well-measured orbits. The KBOs discovered by OSSOS range in size from a few kilometers to over 100 km, and vary in discovery distance from a few AU to over 100 AU, with the majority at 40-42 AU in the main Kuiper belt.
KBOs don’t emit their own light – these small, icy bodies only reflect sunlight. Therefore, the biases against detection at long distances are extreme – if you move a KBO 10 times farther, it will become 10,000 times weaker. . And due to the laws of physics, KBOs in elliptical orbits will spend most of their time in the most distant parts of their orbits. So while it is easy to find KBOs in elliptical orbits when they are close to the Sun and bright, these KBOs spend most of their time being much fainter and difficult to detect.
This means that KBOs in elliptical orbits are particularly difficult to discover, especially the extremes that always stay relatively far from the sun. Only a few of these have been found to date, and with current telescopes we can only discover them when they are near perihelion, the closest point to the Sun in its orbit.
This leads to another observational bias that has historically been ignored by many KBO surveys: KBOs in each part of the solar system can only be discovered at certain times of the year. Spotting scopes are also limited by seasonal weather, and discoveries are less likely to occur when cloudy, rainy, or windy conditions are more prevalent. The KBO discoveries are also much less likely near the plane of the Milky Way galaxy, where countless stars make it difficult to find the faint and icy vagabonds in telescopic images.
What makes OSSOS unique is that we are very honest about these biases in the findings. And because we understand our biases so well, we can use computer simulations to reconstruct the true shape of the Kuiper Belt after removing these biases.
Adjustment for biases
OSSOS discovered a handful of new extreme KBOs, half of which are outside the confined region, and are statistically consistent with a uniform distribution. A new study (currently under review) corroborates the OSSOS unclustered findings. A team of astronomers using data from the Dark Energy Survey (DES) found more than 300 new KBOs without orbital clustering. So now two independent projects, which carefully tracked and reported their observing biases by discovering independent sets of extreme KBOs, have found no evidence of clustered orbits.
All of the extreme KBOs that had been discovered prior to OSSOS and DES were from projects that did not fully report their directional biases. So we don’t know if all of these KBOs were discovered in the same quadrant of the solar system because they are actually confined, or because the other quadrants were not searched deep enough. We ran additional simulations that showed that if observations are made in just one season from a telescope, extreme KBOs will only be discovered naturally in one quadrant of the solar system.
After testing the Planet Nine theory, we looked in detail at the orbits of all known “extreme” KBOs and found that all but the two highest perceiver KBOs can be explained by known physical effects. These two KBOs are atypical, but our previous detailed computer simulations of the Kuiper Belt, which included gravitational effects from Planet Nine, produced a set of “extreme” KBOs with perihelion ranging from 40 to more than 100 AU.
These simulations predict that there should be many KBOs with pericenters as large as the two outliers, but also many KBOs with smaller pericenters, which should be much easier to detect. Why don’t the orbit discoveries match the predictions? The answer may be that the Planet Nine theory is not supported by detailed observations.
Our observations have discovered KBOs that are not limited by Planet Nine, and our simulations show that the Kuiper Belt should contain different orbits than we observe if Planet Nine exists. Other theories are necessary to explain the extreme KBOs of high perihelion, but the hypotheses proposed in the scientific literature are not lacking.
Many beautiful and amazing objects remain to be discovered in the mysterious outer solar system, but I don’t think Planet Nine is one of them.
Samantha Lawler, Assistant professor of astronomy, University of Regina
This article was originally published on The Conversation. Read the original.
