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Jonti Horner, University of South Queensland and Jake Clark, University of South Queensland
In the depths of the solar system, astronomers have recently discovered a small frozen object, named 2015 TG387.
Observed for the first time in October 2015, he was nicknamed "The Goblin" by his discoverers. It is currently about 12 billion kilometers from the Sun – about 80 times the distance between the Earth and the Sun (or 80au, where 1au is the distance between the Earth and the Sun).
"The Goblin" would have a diameter of about 300 km and would move in a very elongated orbit, well beyond the realm of the eight planets (Mercury to Neptune). It is so far apart that it takes more than 34,000 years to orbit the Sun.
Read more: Discovery: a huge lake of liquid water under the south pole of Mars
The most exciting thing about this new discovery is that it may be able to help astronomers discover an invisible planet that some say hides in the icy depths of the solar system.
The history of the planet X
In 1984, research has suggested that in the past 250 million years, massive extinctions have occurred on Earth every 26 million years. But what could cause such periodic extinctions?
A hypothetical red dwarf star, called Nemesis, has been proposed to orbit the Sun at a great distance. Every 26 million years, passing at its closest approach to the Sun, the star would disperse a deluge of comets towards the inner solar system. The result? One or more of these comets would collide with the Earth, triggering mass extinction.
But over the years, the evidence has faded and no star has been found. Nemesis has gone down in history.
At the turn of the millennium, a new "Planet X" was proposed – nicknamed Tyche. Where Nemesis brought death, Tyche's influence was more subtle, resulting in a slight increase in the number of incoming comets from certain regions of the sky and explaining the peculiarities observed in the distribution of these comets.
Once again, the observations quickly weakened Tyche's case. The latest highlight came from NASA's Wide-field Infrared Survey Explorer (WISE), which examined the sky as a whole with infrared wavelengths. If Tyche had existed, WISE would certainly have found it.
Yet the idea of an invisible planet beyond the orbit of Neptune is emerging every few years. In 2008, an invisible and far-flung land mass planet was proposed to explain the distribution of small icy bodies beyond Neptune.
Other researchers have pointed out that planetary mass objects could be formed with the outer planets of the solar system before being dispersed outward without ever being ejected.
All this brings us to the last incarnation of Planet X – known as the "new planet".
Planet X is reborn: Planet Nine
The story of Planet Nine begins with Sedna, a dwarf planet discovered beyond Pluto in November 2003. Sedna has an extremely unusual orbit, with an orbital distance varying between 76 and 936 degrees from the Sun. This poses an obvious question: how was Sedna captured in this orbit?
Artist view of Sedna's view, looking back to the sun. NASA, ESA and Adolf Schaller
Sedna is too far away for the planets to disturb – they can not be responsible for it. At the farthest distance, Sedna is still only 1 / 400th of the nearest star – so close to the Sun that it is very unlikely that a passing star can do the act.
Several theorists have suggested that Sedna could have been placed in its orbit while the solar system was still young. At that time, the Sun would have been integrated into a star group and close encounters with other stars would have been more frequent.
Now, if Sedna's orbit was result of the capture during the youth of the solar system, one would expect that other objects share the same fate. The theory predicted that Sedna could be a population of Sednoids, all moving in similar orbits.
During the last decade, several other Sednoids have been found. Curiously, all their orbits seem to align approximately in space. In other words, the long axes of their orbits point roughly in the same direction. This is not what you expect from a population born from the Sun's birth cluster. So what could be the cause?
To explain the unusual orbit of Sedna, several researchers have evoked various X planets. As new objects were discovered, these theories were constantly reviewed, until American researchers Konstantin Batygin and Mike Brown offer the current version, nicknamed "Planet Nine".
Batygin and Brown estimate that Planet Nine's mass is comparable to Neptune in terms of mass, as it moves into an extremely eccentric orbit, with a period of about 15,000 years.
This theory suggests a prediction: as we find more Sednoids, the evidence of the planet will multiply, because these objects will also have been carved on their current orbit by the influence of the hidden planet.
But astronomers are not universally convinced that an invisible planet is to blame for the alignment of unusual objects. Extraordinary claims require extraordinary evidence and the question of whether the theory withstands scrutiny has been the subject of much debate.
As with Tyche, proposed at the turn of the millennium, the apparent clustering of these distant objects could rather be the result of observation bias.
In simple terms, we are more likely to find objects as pale in some parts of the night sky than in others. Since we find these objects perihelion or nearby – when they are the brightest – it would naturally create such a cluster of orbits from the first discoveries.
How to get out of the dead end? More discoveries are needed. Which brings us to our newly discovered friend, "The Goblin".
Discover our macabre friend
2015 TG387 was first observed on 13 October 2015 and follow-up observations have been made in the last three years. Extensive observations show that the orbit of TG387 is even more extreme than that of Sedna.
The orbit of 2015 TG387 is even more extreme than that of Sedna – but it is oriented in much the same direction, which adds weight to Planet Nine's hypothesis. Robert Molar Candanosa and Scott Sheppard, Carnegie Research Institution
The orbit of TG387 places it at around 65au, but extends up to 2,027au and a turn takes more than 34,000 years.
As with other objects suggesting the presence of Planet Nine, the TG387 orbit is perfectly geared to add weight to this hypothesis.
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Does this mean that Planet Nine is real?
Although this may be additional evidence of the existence of the planet, it is far from definitive. It may also be that TG387 is additional evidence that observational bias leads to clustering.
We have to find more objects before we can be safe, anyway. Fortunately, there will probably be millions of similar objects like TG387 – indeed, its discoverers predict that it's only one of millions of bodies, all moving on similar orbits in the icy depths of the solar system.
Jonti Horner, Professor (Astrophysics), University of South Queensland and Jake Clark, doctoral candidate, University of South Queensland
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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