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Thursday 22 September 2016

Is there a ninth planet in our Solar System?

By now, you’ve probably heard the hubbub in the news about the hypothetical “Ninth Planet” in our Solar System, and, unfortunately for those of us who studied astronomy before 2006, no, it’s not Pluto. There’s a new Planet Nine on the block, although no-one has ever seen it and we don’t actually know if it exists.

Yes, some scientists think there may be another, unseen planet in our Solar System. How can they think that?

Why haven’t scientists seen Planet Nine yet?

In our solar system, planets are generally considered to be visible things - get yourself a 12-inch telescope and you can see Pluto, and that’s not even a planet any more! So yes, it sounds daft to say that there’s a whole other planet in our Solar System that we haven’t seen yet, but, in our defence, it’s very far away.

Planet Nine is thought to be a trans-Neptunian object - a minor planet that orbits the Sun at a distance further out than Neptune. Both the Kuiper belt and the Oort cloud are included in this region, and Planet Nine is thought to orbit somewhere between the two.

An artist’s rendering of the relationship between the Kuiper belt and the Oort cloud
The Oort cloud is a spherical region of icy debris thought to surround the Solar System. Although it was thought to extend from 5,000 to 100,000 Earth-Sun distances, it may start much closer to the Sun. Image credit: NASA and A. Feild (Space Telescope Science Institute)

Why do scientists think Planet Nine exists?

This is a good question, and I know you’ll be expecting a very good answer - it’s all well and good saying there’s an entire planet out there, but we do have to back up our suspicions with some facts.

Reason 1: Funny business in the Kuiper Belt.

There are strange goings-on in the Kuiper belt, and these have been known about for a while. In 2014, American researchers Chadwick Trujillo and Scott Sheppard tried to create a simulation of objects in the Oort cloud based on their observations of the distant solar system, and found that they were unable to explain the clustering of the orbits of Kuiper Belt objects (KBOs) in that region[1]. Two Spanish scientists found what they refer to as “unexpected patterns” in the parameters of the orbits of so called “extreme trans-Neptunian objects[2], and they consider their results to confirm the findings of Trujillo and Sheppard. These results were further confirmed by Caltech planetary scientists Konstantin Batygin and Michael “Pluto Killer” Brown, who found that objects in the Kuiper belt were clustering together, despite this being a less favourable configuration in terms of the configuration of their orbits and the space they take up[3].

The Caltech scientists worked out the probability of this configuration happening by chance was just 0.007% - which led them to conclude that something must have caused the Kuiper belt objects to group together as they have[3]. They suggest that such celestial nudging could be caused by a planet whose mass is at least ten times that of the Earth, and whose longest orbital radius is 700 AU - the fabled Planet Nine. In fact, when the Outer Solar System Origin Survey first found the KBO with an eccentric orbit (called uo3L91), Mike Brown tweeted that it was exactly where Planet Nine said it should be.

Orbit diagram of planet nine and other Kuiper Belt Objects created using WorldWide Telescope.
The pink ellipses depict the orbits of some of the most distant objects found in the solar system. The orange is a possible orbit of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

Reason 2: The Sun is Skewiff.

All of the (known) planets in our Solar System orbit the Sun in the same plane - that is, you could draw all of their orbits on a flat a piece of card (though it would have to be a very large piece of card).

The Sun doesn’t spin in this plane - the plane it spins in is about six degrees off[4]. Now, this might seem quite inconsequential to us, but it’s significant enough that scientists are using it to suggest that there’s another planet in our Solar System that we’ve never even seen. One that orbits it in another plane, just to be different[4].

Graphic showing the planets of the Solar System, the Kuiper belt, and also the Hale-Bopp comet
Although the planets all have similarly oriented orbits, this is not true for all of the objects in the Solar System. Image credit: Public domain (via NASA)

Two separate studies[4],[5] have examined whether Planet Nine could have caused the six degree deviation. One international group of researchers worked together to show it would be possible for the tilt to be caused by Planet Nine. They suggest Planet Nine tugged on the orbits of the planets, tilting them by a smidge[4]. Others agreed that a planet whose mass was between five and twenty times heavier than Earth, and whose closest point to the Sun was approximately 250 AU, might explain the six degree tilt[5].

Where did Planet Nine come from?

A European team of physicists suggest that Planet Nine was captured from another star. Their idea is formed from the hypothesis that the Sun wasn’t created on its own; rather, it formed in a group of thousands of stars, known as its birth cluster[6].They propose that Planet Nine was pinched by the Sun from another star in it’s birth cluster. They state three conditions which must be satisfied by any potential planet-theft scenario: it has to happen far enough away that it won’t set the Kuiper belt off-kilter; the “victim” star must have a planet with a wide orbit (easy to steal); and, once stolen, the planet must end up in an orbit that explains all the phenomena that made us suspect it existed in the first place (including the Kuiper belt clustering and the Sun’s tilt)[6]. For Planet Nine, all these conditions could be satisfied simultaneously[7].

What do we think Planet Nine is made of?

We don’t know, but our best guesses are that Planet Nine is similar to Uranus and Neptune[8]; that is, an ice giant (a cold, gaseous planet different to “gas giants” like Saturn and Jupiter). This means it (probably) has an iron core, surrounded by a mantle, then ice, and then a thick layer of hydrogen and helium gas. You can think of it as a second Neptune, as Planet Nine is likely similar in mass.

Image showing the likely structure of Planet Nine
Astrophysicists at the University of Bern estimated size and temperature of planet 9. Image credit: University of Bern

How can we prove whether Planet Nine exists or not?

Scientists across the world have been studying ways of searching for a Neptune-like planet, and think they may have found the answer. As Planet Nine is thought to be very far from the Sun, any heat given off from Planet Nine is to do with changes in its internal temperature, rather than it emitting heat it received from the Sun. The heat it releases in the form of radiation could be used to detect it[9]. We could also use telescopes with sufficient resolution and sensitivity to search for Planet Nine[10].

The South Pole Telescope, in fact, might be able to survey 20% of the sky in very good detail within a year - which would help narrow down where Planet Nine is hiding[10]. According to experts, the best search strategy is to survey as much as possible in, for example, the three months it would take the South Pole Telescope to survey the sky of the Southern Hemisphere (in less detail than discussed previously) [10]. This would allow scientists to figure out how Planet Nine would appear to move, relative to the motion of Earth[10].

For these experiments to work, Planet Nine must be roughly Neptune-sized (already considered likely), with an effective temperature of 40 K (-233°C), and the farthest point in its orbit at 700 AU from the Sun[10].

A brief summary of Planet Nine.

In order to explain the clustering of objects in the Kuiper belt, and the tilt of the Sun’s spin compared to the plane in which the planets orbit, scientists have proposed that there is a new planet (referred to as Planet Nine) somewhere in the distant solar system.

There are a few things we can assume about Planet Nine (if it exists) based on those effects: it’s mass is likely somewhere in the Uranus-Neptune range, and it has a similar structure. In terms of its orbit, it is probably always at least 250 times further away from the Sun than the Earth is, but unlikely to be further away than 700 Earth-Sun distances. We might be able to detect Planet Nine by the radiation it emits when it cools, or by repeatedly scanning the sky, but either way, we don’t know it exists yet.

So, that’s the story of Planet Nine, a gaseous planet that might be playing hide-and-seek in the sky.

This article was written by TWDK's 2016 physics writing intern Alice Wayne, from Royal Holloway University of London.

why don't all references have links?

[1] C. A. Trujillo, and S. S. Sheppard. “A Sedna-like body with a perihelion of 80 astronomical units.” Nature, vol.507(7493), p.471-4, 2014. doi:10.1038/nature13156

[2] C. de la Fuente Marcos, and R. de la Fuente Marcos. “Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets.” Monthly Notices of the Royal Astronomical Society: Letters, vol.443(1), p.L59-L63, 2014. doi:10.1093/mnrasl/slu084

[3] K. Batygin, and M. E. Brown. “Evidence for a distant giant planet in the Solar System.” The Astronomical Journal, vol.151(2), 2016. doi:10.3847/0004-6256/151/2/22

[4] R. Gomes, R. Deienno, and A. Morbidelli. “The inclination of the planetary system relative to the solar equator may be explained by the presence of Planet 9.” Submitted to American Astronomical Society, 2016.

[5] E. Bailey, K. Batygin, and M. Brown. “Solar Obliquity Induced by Planet Nine.” Submitted to AJ, 2016.

[6] A. Mustill, S. Raymond, and M. Davies. “Is there an exoplanet in the Solar System?” Monthly Notices of the Royal Astronomical Society: Letters, vol.460(1), p.L109-L113, 2016. doi:10.1093/mnrasl/slw075

[7] G. Li, and F. C. Adams. “Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine.” The Astrophysical Journal: Letters, vol.823(1), p.L3, 2016. doi:10.3847/2041-8205/823/1/L3

[8] E. F. Linder, and C. Mordasini. “Evolution and Magnitudes of Candidate Planet Nine.” Astronomy and Astrophysics, vol.589, A134, 2016. doi:10.1051/0004-6361/201628350

[9] S. Ginzburg, R. Sari, A. Loeb. “Blackbody radiation from isolated Neptunes.” The Astrophysical Journal Letters, vol.822(1), 2016. doi:10.3847/2041-8205/822/1/L11

[10] N. B. Cowan, G. Holder, and N. A. Kaib. “Cosmologists in Search of Planet Nine: the Case for CMB Experiments.” The Astrophysical Journal: Letters, vol.822(1), p.L2, 2016. doi:10.3847/2041-8205/822/1/L2

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