As the search for “Planet Nine“In the outer solar system continues, new research suggests there may be an Earth twin buried deep in the icy waste of the Kuiper Belt beyond Neptune.
Astronomers struggle to explain all the properties of trans-Neptunian objects (TNOs), which are generally small icy bodies that orbit the Sun beyond the orbit of Neptune, generally with orbits larger than 30 astronomical units (AU). (An AU is the average distance between the Earth and the sun: approximately 93 million miles, or 150 million kilometers.)
TNOs represent the remaining bits of the file formation of the solar system that lashed out at the system’s outer fringes during the chaotic early years of planetary creation. Due to the extreme difficulty of discovering and mapping such small, distant and faint objects, known TNOs (which represent a very small sample of all objects orbiting at that distance) have some strange properties that currently defy coherent and consistent explanation .
First, about 10% of all TNOs are “detached,” meaning they have no orbital relationship with Neptune. Although Neptune is the smallest of the giant planets, it is the dominant gravitational force in the outer solar system. That so many TNOs do not have motion controlled or influenced by Neptune is strange, since solar system formation models generally predict that far fewer detached TNOs should remain until the present day.
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Second, there is a large population of TNOs – about a small percentage – that have very high orbital inclinations, meaning they orbit the planet. Sun at an angle greater than 45 degrees. Once again, models of solar system formation have difficulty dispersing so many objects into such extremely angled orbits.
Next, there are some TNOs with very large orbits that are simply known as “extreme.” The most famous example of these is Sedna, a minor planet whose closest approach to the Sun is about 76 AU and over 900 AU at its farthest point. Extreme orbits like this require a lot of adjustments and gravitational pulls, and it’s hard to get the severity of the known planets to explain these extreme objects.
Finally, some TNOs are locked in orbital resonances with Neptune, meaning that for every integer number of Neptune’s orbits, they complete a single orbit. Some of these orbits have been stable for billions of years, despite the gravitational pull that other members of the TNO population have received.
A coherent theory of the outer solar system must be able to explain all these discordant observations, and that’s where a pair of researchers, reporting their findings in a paper in The Astronomical Journal, have found an interesting opportunity.
In the past, researchers have speculated that a massive planet could be lurking in extremely distant places solar system (like hundreds of AUs away). However, such arguments mainly focused on the behavior of a handful of extreme TNOs. The problem is that the presence of a huge planet – something that rivals Neptune or even more – confuses other TNOs so much that it violates our understanding of their inclinations and relationships to Neptune.
In the new study, the researchers ran hundreds of simulations of the formation of the solar system. The first system probably produced 10 to 100 planets with at least the mass of the Earth beyond the orbit of Neptune. Most of these planets were expelled from the solar system altogether by the complex gravitational machinations of the remaining planets. But a handful may have managed to survive and may even survive to the present day.
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The researchers found that a single planet is the size of Earth and weighs between 1.5 and three times Earthof mass, could be hiding in the outer solar system, living in an orbit between 250 and 500 AU. This would place the planet firmly within the Kuiper beltwhich otherwise is composed only of minor planets, such as Pluto and Sedna, and countless smaller icy bodies.
This hypothetical large planet, if placed on an orbit with a 45-degree inclination, could explain the well-known collective behavior of TNOs: as some of them are detached from Neptune while others are locked in resonances, the large angles of some of their orbits, and the extreme nature of a small TNO population.
However, there is no evidence for the existence of this Earth-sized planet or other proposed new planets in the Kuiper Belt. So far, all we have to do is the strange orbital dance of the dim and distant TNOs. But it’s definitely an intriguing sign and one of the best ways to explain all the available evidence.
The next step for researchers is to develop more detailed models so they can start predicting where this Earth-mass planet might be in its orbit and make plans for an observation campaign to hunt it down.