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Mystery orbits do not require a Planet 9, researchers say. | Space



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 Artist's concept of distant trans-Neptunian objects, moving in far-flung orbits around our sun. Some of their orbits are strange, and that strangeness has caused astronomers to search for another undiscovered, large planet in our solar system. Image </p>
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<p> In early 2016, astronomers at Caltech announced evidence for another large planet in the outer solar system. They call this a distant, unknown planet Planet 9 (although some call it Planet X, in spite of those still smarting over the IAU's 2006 decision to downgrade Pluto to dwarf planet status). The evidence for Planet 9 has always been indirect. It came from the strangely aligned orbits of small objects in the outer solar system. Since 201<div class=
6, astronomers have searched for a Planet 9 (or Planet X) in the outer solar system, but so far they have not found one. Now another group of astronomers says the strange orbits of the outer solar system can be explained without a Planet 9.

What's going on here? Nothing more than the process of science. Science is, after all, primarily a process and a search for truth.

Researchers at the University of Cambridge and the American University of Beirut have put forth the alternative explanation for the 2016 Planet 9 hypothesis. They agree with the observations of some small outer bodies – called trans-Neptunian objects or TNOs, moving more than 30 times Earth's distance from the sun – have strangely aligned orbits. But the new work suggests the combined gravitational force of many small, distant objects – not a large, single Planet 9 – could have created those weird orbits. According to their statement, the team:

… proposes a disk made of small icy bodies with a combined mass as much as 10 times that of Earth. (19659010) The new results are reported in the peer-to-peer model of the solar system, and the gravitational forces of the hypothesized disk can account for the unusual orbital architecture exhibited by some objects at the outer reaches of the solar system. reviewed Astronomical Journal .

 Multiple small body orbits, all grouped on one side of the sun, with a single orbit depicting a 9th planet pulling on them

Illustration showing the hypothesized orbit of Planet 9 along with the known orbits of several trans-Neptunian objects (TNOs). Image via R. Hurt / JPL-Caltech

Generally speaking, the trans-Neptunian objects – or TNOs – circle the sun on almost-circular paths oriented in all directions. However, said these astronomers:

Since 2003, around 30 TNOs have been spotted: they stand out from the rest of the TNOs by sharing, on average, the same spatial orientation …

The Planet 9 hypothesis suggesting that to account for the unusual orbits of these TNOs, there would be another planet … 'shepherding' the TNOs in the same direction through the combined effect of its gravity and that of the rest of the solar system

Antranik Sephilis of University of Cambridge explained how they came to an alternative viewpoint:

We wanted to see if there could be another, less dramatic and perhaps more natural cause of the unusual orbits we see in some TNOs. We thought, rather than allowing for a 9th planet, and then worrying about its formation and an unusual orbit, why not just account for the gravity of small objects constituting a disk beyond the orbit of Neptune and see what it does for us

Sefilian is a former student of Jihad Touma of the American University of Beirut. Together, they created a new computer model to explain the unusual orbits without a Planet 9. Sefilian said:

If you remove Planet 9 from the model and instead allow a lot of small objects scattered over a wide area, Objects could just as easily account for the excentric orbits we see in some TNOs

Earlier attempts to do this same work fell short, these astronomers said, because in earlier models the total mass of objects beyond Neptune has only added up to around one-tenth of the mass of the Earth. That's too small to account for the strange orbits of some TNOs. For the TNOs to have their observed orbits – without needing a Planet 9 – the model put forward by Sefilian and Touma required a combined mass of external solar bodies to be between a few to ten times the mass of Earth. Sefilian explained why he and Touma think it's fair to explore the possibility that mass might be out there. He said:

When observing other [solar] systems, we often study the disk surrounding the host star to infer the properties of any planets in orbit around it. The problem is when you're watching the disk from inside the system, it's almost impossible to see the whole thing at once. While we do not have direct observational evidence for the disk, we do not have it for Planet 9, which is why we're investigating other possibilities

He added:

It's also possible that both things could be true – there could be a massive disk and a 9th planet. With the discovery of each new TNO, we gather more evidence to help explain their behavior

Bottom line: New work suggests the combined gravitational force of many small, remote objects in the outer solar system – not a large, single planet 9 – may have created the weird orbits of some external solar bodies. In other words, there may be no reason for a Planet 9 to exist.

Source: Shepherding in a Self-gravitating Disk of Trans-Neptunian Objects

Via University of Cambridge

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