Kuiper Belt Objects Point The Way To Planet 9

Artist's impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign

On January 20th, 2016, researchers Konstantin Batygin and Michael E. Brown of Caltech announced that they had found evidence that hinted at the existence of a massive planet at the edge of the Solar System. Based on mathematical modeling and computer simulations, they predicted that this planet would be a super-Earth, two to four times Earth’s size and 10 times as massive. They also estimated that, given its distance and highly elliptical orbit, it would take 10,000 – 20,000 years to orbit the Sun.

Since that time, many researchers have responded with their own studies about the possible existence of this mysterious “Planet 9”. One of the latest comes from the University of Arizona, where a research team from the Lunar and Planetary Laboratory have indicated that the extreme eccentricity of distant Kuiper Belt Objects (KBOs) might indicate that they crossed paths with a massive planet in the past.

For some time now, it has been understood that there are a few known KBOs who’s dynamics are different than those of other belt objects. Whereas most are significantly controlled by the gravity of the gas giants planets in their current orbits (particularly Neptune), certain members of the scattered disk population of the Kuiper Belt have unusually closely-spaced orbits.

The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta), including Sedna (dark magenta), all mysteriously line up in a single direction. Also, when viewed in three dimensions, they tilt nearly identically away from the plane of the solar system. Another population of Kuiper belt objects (cyan) are forced into orbits that are perpendicular to the plane of the solar system and clustered in orientation. Batygin and Brown show that a planet with 10 times the mass of the earth in a distant eccentric orbit (orange) anti-aligned with the magenta orbits and perpendicular to the cyan orbits is required to maintain this configuration. Credit: Caltech/R. Hurt (IPAC)
The orbits of Neptune (magenta), Sedna (dark magenta), a series of Kuiper belt objects (cyan), and the hypothetical Planet 9 (orange). Credit: Caltech/R. Hurt (IPAC)

When Batygin and Brown first announced their findings back in January, they indicated that these objects instead appeared to be highly clustered with respect to their perihelion positions and orbital planes. What’s more, their calculation showed that the odds of this being a chance occurrence were extremely low (they calculated a probability of 0.007%).

Instead, they theorized that it was a distant eccentric planet that was responsible for maintaining the orbits of these KBOs. In order to do this, the planet in question would have to be over ten times as massive as Earth, and have an orbit that lay roughly on the same plane (but with a perihelion oriented 180° away from those of the KBOs).

Such a planet not only offered an explanation for the presence of high-perihelion Sedna-like objects – i.e. planetoids that have extremely eccentric orbits around the Sun. It would also help to explain where distant and highly inclined objects in the outer Solar System come from, since their origins have been unclear up until this point.

In a paper titled “Coralling a distant planet with extreme resonant Kuiper belt objects“, the University of Arizona research team – which included Professor Renu Malhotra, Dr. Kathryn Volk, and Xianyu Wang – looked at things from another angle. If in fact Planet 9 were crossing paths with certain high-eccentricity KBOs, they reasoned, it was a good bet that its orbit was in resonance with these objects.

Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA
Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA

To break it down, small bodies are ejected  from the Solar System all the time due to encounters with larger objects that perturb their orbits. In order to avoid being ejected, smaller bodies need to be protected by orbital resonances. While the smaller and larger objects may pass within each others’ orbital path, they are never close enough that they would able to exert a significant influence on each other.

This is how Pluto has remained a part of the Solar System, despite having an eccentric orbit that periodically cross Neptune’s path. Though Neptune and Pluto cross each others orbit, they are never close enough to each other that Neptune’s influence would force Pluto out of our Solar System. Using this same reasoning, they hypothesized that the KBOs examined by Batygin and Brown might be in an orbital resonance with the Planet 9.

As Dr.  Malhotra, Volk and Wang told Universe Today via email:

“The extreme Kuiper belt objects we investigate in our paper are distinct from the others because they all have very distant, very elliptical orbits, but their closest approach to the Sun isn’t really close enough for them to meaningfully interact with Neptune. So we have these six observed objects whose orbits are currently fairly unaffected by the known planets in our Solar System. But if there’s another, as yet unobserved planet located a few hundred AU from the Sun, these six objects would be affected by that planet.”

After examining the orbital periods of these six KBOs – Sedna, 2010 GB174, 2004 VN112, 2012 VP113, and 2013 GP136 – they concluded that a hypothetical planet with an orbital period of about 17,117 years (or a semimajor axis of about 665 AU), would have the necessary period ratios with these four objects. This would fall within the parameters estimated by Batygin and Brown for the planet’s orbital period (10,000 – 20,000 years).

Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign

Their analysis also offered suggestions as to what kind of resonance the planet has with the KBOs in question. Whereas Sedna’s orbital period would have a 3:2 resonance with the planet, 2010 GB174 would be in a 5:2 resonance, 2994 VN112 in a 3:1, 2004 VP113 in 4:1, and 2013 GP136 in 9:1. These sort of resonances are simply not likely without the presence of a larger planet.

“For a resonance to be dynamically meaningful in the outer Solar System, you need one of the objects to have enough mass to have a reasonably strong gravitational effect on the other,” said the research team. “The extreme Kuiper belt objects aren’t really massive enough to be in resonances with each other, but the fact that their orbital periods fall along simple ratios might mean that they each are in resonance with a massive, unseen object.”

But what is perhaps most exciting is that their findings could help to narrow the range of Planet 9’s possible location. Since each orbital resonance provides a geometric relationship between the bodies involved, the resonant configurations of these KBOs can help point astronomers to the right spot in our Solar System to find it.

But of course, Malhotra and her colleagues freely admit that several unknowns remain, and further observation and study is necessary before Planet 9 can be confirmed:

“There are a lot of uncertainties here. The orbits of these extreme Kuiper belt objects are not very well known because they move very slowly on the sky and we’ve only observed very small portions of their orbital motion. So their orbital periods might differ from the current estimates, which could make some of them not resonant with the hypothetical planet. It could also just be chance that the orbital periods of the objects are related; we haven’t observed very many of these types of objects, so we have a limited set of data to work with.”

Based on a careful study of Saturn's orbit and using mathematical models, French scientists were able to whittle down the search region for Planet Nine to "possible" and "probable" zones. Source: CNRS, Cote d'Azur and Paris observatories. Credit:
Estimates of Planet Nine’s “possible” and “probable” zones. by French scientists based on a careful study of Saturn’s orbit and using mathematical models. Source: CNRS, Cote d’Azur and Paris observatories. Credit: Bob King

Ultimately, astronomers and the rest of us will simply have to wait on further observations and calculations. But in the meantime, I think we can all agree that the possibility of a 9th Planet is certainly an intriguing one! For those who grew up thinking that the Solar System had nine planets, these past few years (where Pluto was demoted and that number fell to eight) have been hard to swallow.

But with the possible confirmation of this Super-Earth at the outer edge of the Solar System, that number could be pushed back up to nine soon enough!

Further Reading: arXiv.org

Uranus’ “Frankenstein Moon” Miranda

Ever since the Voyager space probes ventured into the outer Solar System, scientists and astronomers have come to understand a great deal of this region of space. In addition to the four massive gas giants that call the outer Solar System home, a great deal has been learned about the many moons that circle them. And thanks to photographs and data obtained, human beings as a whole have come to understand just how strange and awe-inspiring our Solar System really is.

This is especially true of Miranda, the smallest and innermost of Uranus’ large moons – and some would say, the oddest-looking! Like the other major Uranian moons, its orbits close to its planet’s equator, is perpendicular to the Solar System’s ecliptic, and therefore has an extreme seasonal cycle. Combined with one of the most extreme and varied topographies in the Solar System, this makes Miranda an understandable source of interest!

Discovery and Naming:

Miranda was discovered on February 16th, 1948, by Gerard Kuiper using the McDonald Observatory‘s Otto Struve Telescope at the University of Texas in Austin. Its motion around Uranus was confirmed on March 1st of the same year, making it the first satellite of Uranus to be discovered in almost a century (the previous ones being Ariel and Umbriel, which were both discovered in 1851 by William Lassell).

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s moons. Image credit: NASA/JPL

Consistent with the names of the other moons, Kuiper decided to the name the object “Miranda” after the character in Shakespeare’s The Tempest. This continued the tradition set down by John Herschel, who suggested that all the large moons of Uranus – Ariel, Umbriel, Titania and Oberon – be named after characters from either The Tempest or Alexander Pope’s The Rape of the Lock.

Size, Mass and Orbit:

With a mean radius of 235.8 ± 0.7 km and a mass of 6.59 ± 0.75 ×1019 kg, Miranda is 0.03697 Earths times the size of Earth and roughly 0.000011 as massive. Its modest size also makes it one of the smallest object in the Solar System to have achieved hydrostatic equilibrium, with only Saturn’s moon of Mimas being smaller.

Of Uranus’ five larger moons, Miranda is the closest, orbiting at an average distance (semi-major axis) of 129,390 km. It has a very minor eccentricity of 0.0013 and an inclination of 4.232° to Uranus’ equator. This is unusually high for a body so close to its parent planet – roughly ten times that of the other Uranian satellites.

Since there are no mean-motion resonances to explain this, it has been hypothesized that the moons occasionally pass through secondary resonances. At some point, this would have led Miranda into being locked in a temporary 3:1 resonance with Umbriel, and perhaps a 5:3 resonance with Ariel as well. This resonance would have altered the moon’s inclination, and also led to tidal heating in its interior (see below).

Size comparison of all the Solar Systems moons. Credit: The Planetary Society
Size comparison of all the Solar Systems moons. Credit: NASA/The Planetary Society

With an average orbital speed of 6.66 km/s, Miranda takes 1.4 days to complete a single orbit of Uranus. Its orbital period (also 34 hours) is synchronous with its rotational period, meaning that it is tidally-locked with Uranus and maintains one face towards it at all times. Given that it orbits around Uranus’ equator, which means its orbit is perpendicular to the Sun’s ecliptic, Uranus goes through an extreme seasonal cycle where the northern and southern hemispheres experience 42 years of lightness and darkness at a time.

Composition and Surface Structure:

Miranda’s mean density (1.2 g/cm3) makes it the least dense of the Uranian moons. It also suggests that Miranda is largely composed of water ice (at least 60%), with the remainder likely consisting of silicate rock and organic compounds in the interior. The surface of Miranda is also the most diverse and extreme of all moons in the Solar System, with features that appear to be jumbled together in a haphazard fashion.

This consists of huge fault canyons as deep as 20 km (12 mi), terraced layers, and the juxtaposition of old and young surfaces seemingly at random. This patchwork of broken terrain indicates that intense geological activity took place in Miranda’s past, which is believed to have been driven by tidal heating during the time when it was in orbital resonance with Umbriel (and perhaps Ariel).

This resonance would have increased orbital eccentricity, and along with varying tidal forces from Uranus, would have caused warming in Miranda’s interior and led to resurfacing. In addition, the incomplete differentiation of the moon, whereby rock and ice were distributed more uniformly, could have led to an upwelling of lighter material in some areas, thus leading to young and older regions existing side by side.

Miranda
Uranus’ moon Miranda, imaged by the Voyager 2 space probe on January 24th, 1986. Credit: NASA/JPL-Caltech

Another theory is that Miranda was shattered by a massive impact, the fragments of which reassembled to produce a fractured core. In this scenario – which some scientists believe could have happened as many as five times – the denser fragments would have sunk deep into the interior, with water ice and volatiles setting on top of them and mirroring their fractured shape.

Overall, scientists recognize five types of geological features on Miranda, which includes craters, coronae (large grooved features), regiones (geological regions), rupes (scarps or canyons) and sulci (parallel grooves).

Miranda’s cratered regions are differentiated between younger, lightly-cratered regions and older, more-heavily cratered ones. The lightly cratered regions include ridges and valleys, which are separated from the more heavily-cratered areas by sharp boundaries of mismatched features. The largest known craters are about 30 km (20 mi) in diameter, with others lying in the range of 5 to 10 km (3 to 6 mi).

Miranda has the largest known cliff in the Solar System, which is known as Verona Rupes (named after the setting of Shakespeare’s Romeo and Juliet). This rupes has a drop-off of over 5 km (3.1 mi) – making it 12 times as deep as the Grand Canyon. Scientists suspect that Miranda’s ridges and canyons represent extensional tilt blocks – a tectonic event where tectonic plates stretch apart, forming patterns of jagged terrain with steep drops.

. Credit: NASA/JPL
Image taken by the Voyager 2 probe during its close approach on January 24th, 1986, with a resolution of about 700 m (2300 ft). Credit: NASA/JPL

The most well known coronae exist in the southern hemisphere, with three giant ‘racetrack’-like grooved structures that measure at least 200 km (120 mi) wide and up to 20 km (12 mi) deep. These features, named Arden, Elsinore and Inverness – all locations in Shakespeare’s plays – may have formed via extensional processes at the tops of diapirs (aka. upwellings of warm ice).

Other features may be due to cryovolcanic eruptions of icy magma, which would have been driven by tidal flexing and heating in the past. With an albedo of 0.32, Miranda’s surface is nearly as bright as that of Ariel, the brightest of the larger Uranian moons. It’s slightly darker appearance is likely due to the presence of carbonaceous material within its surface ice.

Exploration:

Miranda’s apparent magnitude makes it invisible to many amateur telescopes. As a result, virtually all known information regarding its geology and geography was obtained during the only flyby of the Uranian system, which was made by Voyager 2 in 1986. During the flyby, Miranda’s southern hemisphere pointed towards the Sun (while the northern was shrouded in darkness), so only the southern hemisphere could be studied.

At this time, no future missions have been planned or are under consideration. But given Miranda’s “Frankenstein”-like appearance and the mysteries that still surround its history and geology, any future missions to study Uranus and its system of moons would be well-advised.

We have many interesting articles on Miranda and Uranus’ moons here at Universe Today. Here’s one about about why they call it the “Frankenstein Moon“, and one about Voyager 2‘s historic flyby. And here’s one that answers the question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Miranda.

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