New Work on Planet Nine

Considering how long we’ve been thinking about a massive planet in the outer Solar System — and I’m going all the way back to Percival Lowell’s Planet X here — the idea that we might find the hypothetical Planet Nine in just three years or so is a bit startling. But Caltech’s Mike Brown and colleague Konstantin Batygin, who predicted the existence of the planet last January based on its effects on Kuiper Belt objects, are continuing to search the putative planet’s likely orbital path, hoping for a hit within the next few years, a welcome discovery if it happens.

The duo are working with graduate student Elizabeth Bailey, lead author of a new study being discussed at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, which is occurring in conjunction with the European Planetary Science Congress. The new paper is all about angles and alignments, focusing on the fact that the relatively flat orbital plane of the planets is tilted about six degrees with respect to the Sun. That’s an oddity, and Planet Nine, hypothesized to be about ten times the mass of the Earth and in an orbit averaging 20 times Neptune’s distance from the Sun, just may be the cause.

The calculations on display in the new paper depict a planet some 30 degrees out of alignment with the orbital plane of the other planets. That can help to explain orbital observations of Kuiper Belt objects, but also the unusual system-wide tilt, which stands out because of the assumed formation of the planets through the collapse of a spinning cloud into a disk and, eventually, a collection of planets orbiting the Sun. We would expect the angular momentum of the planets to maintain a rough alignment with the Sun along the orbital plane.

Unless, of course, something is disrupting the system. Throw in the angular momentum of Planet Nine, based on its assumed mass and distance from the Sun, and profound effects on the system’s spin become evident, creating a long-term wobble that shows up in the system’s tilt. As Bailey puts it, “Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment.”

And this from the paper:

… a solar obliquity of order several degrees is an expected observable effect of Planet Nine. Moreover, for a range of masses and orbits of Planet Nine that are broadly consistent with those predicted by Batygin & Brown (2016); Brown & Batygin (2016), Planet Nine is capable of reproducing the observed solar obliquity of 6 degrees, from a nearly coplanar configuration. The existence of Planet Nine therefore provides a tangible explanation for the spin orbit misalignment of the solar system.

Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC).

The six-degree tilt we see between planetary disk and Sun thus fits into the team’s calculations regarding Planet Nine’s size and distance from the central star. And if this does indeed turn out to be the explanation, speculation will then center on how Planet Nine came to be so far out of line with the other planets. We know that gravitational interactions in young planetary systems can sometimes result in disruption, causing some planets to be thrown out of their systems, and others to be moved into distant orbits. Such gravitational byplay may well be the reason for Planet Nine’s unusual position. Now we just need to discover the planet.

I also want to mention that Renu Malhotra (University of Arizona) and team have continued their analysis of a possible Planet Nine, likewise presenting their results at the AAS/EPSC meeting in Pasadena. Through analysis of what they call ‘extreme Kuiper Belt Objects’ —on eccentric orbits with aphelia hundreds of AU out — the team finds a clustering of orbital parameters that may point to the existence of a planet of 10 Earth masses with an aphelion of more than 660 AU. Two orbital planes seem possible, one at 18 degrees offset from the mean plane, the other inclined at 48 degrees.

Dr. Malhotra confirmed in an email this morning that her own constraints on the current position of this possible planet line up with Mike Brown and team at Caltech. But her team continues to point out that we have no detection at this point, and much to learn about the orbits of the Kuiper Belt objects under study. From her paper:

…we note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, depending on the planet mass; if so, this would weaken the argument for a resonant planet orbit. In future work it would be useful to investigate scattering efficiency as a function of the planet mass, as well as dynamical lifetimes of non-resonant planet-crossing orbits in this region of the outer system. Nevertheless, the possibility that resonant orbital relations could be a useful aid to prediction and discovery of additional high mass planets in the distant solar system makes a stimulating case for renewed study of aspects of solar system dynamics, such as resonant dynamics in the high eccentricity regime, which have hitherto garnered insufficient attention.

The Bailey, Batygin & Brown paper is “Solar Obliquity Induced by Planet Nine,” accepted for publication in the Astrophysical Journal (preprint). The Malhotra paper is Malhotra, Volk & Wang, “Coralling a Distant Planet with Extreme Resonant Kuiper Belt Objects,” Astrophysical Journal Letters Vol. 824, No. 2 (20 June 2016). Abstract / preprint.