Haystack Latest: The Hunt for Planet Nine Goes On…

Academic papers aimed at further constraining the parameters of the purported ‘Planet Nine’ body continue to emerge from various quarters, many from researchers with long-term interests in outer solar system anomalies. Fairly quickly after Brown and Batygin’s announcement about Planet Nine (1), a paper was published by A. Fienga et al examining the astrometry of Saturn through Cassini’s radio ranging data (2). This work served to constrain the possible locations of Planet Nine, which were wide ranging to say the least. This is because if one can establish the very precise positioning of outer planets over time, then this can provide clues to any slight gravitational effect, or perturbation, the planet might be experiencing from an undiscovered distant substantial Planet X body (3). However, given that Planet Nine is thought to have a highly elliptical orbit, then if it is located at the further end of that ellipse, its effect upon the outer planets gravitationally becomes vanishingly small. It turns out then, as one might predict, that we can rule out its current location being in the nearer half of its elliptical path, according to the Cassini data about Saturn. Which is more or less common sense, anyway.

An Artist's impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

An Artist’s impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

Now, new work has emerged on the astrometry of Pluto (4) and other Trans-Neptunian Objects. These objects are, evidently, located much further away from us than Saturn, and their more loosely bound orbits may provide fertile hunting ground for gravitational perturbation effects caused by an unknown body beyond. In this case, astrophysicists from the Harvard-Smithsonian Center for Astrophysics have come to an altogether different conclusion: this Planet Nine body is likely either larger than Brown’s ‘Super-Earth’ object of 5-15 Earth-masses; or instead closer than he, or Fienga’s team, concluded. In other words. this thing packs a bigger perturbing punch than first thought:

“Our best fits suggest a planet that is either more massive or closer than argued for by Batygin and Brown (2016) based on the orbital distribution of distant trans-neptunian objects (or by Fienga et al. (2016) [2] based on range measured to the Cassini spacecraft). The trend to favor larger and closer perturbing planets is driven by the residuals to the astrometry of Pluto, remeasured from photographic plates using modern stellar catalogs (Buie and Folkner 2015) [4], which show a clear trend in declination, over the course of two decades, that drive a preference for large perturbations. Although this trend may be the result of systematic errors of unknown origin in the observations, a possible resolution is that the declination trend may be due to perturbations from a body, additional to Planet Nine , that is closer to Pluto, but less massive than, Planet Nine.” (5)

Relative Positions of the KBO Cluster Pointing to Planet Nine. Image credit: Caltech

Relative Positions of the KBO Cluster Pointing to Planet Nine. Image credit: Caltech

To be fair to Dr Brown, he had already built into his paper an allowance for a larger object (by “a factor of a few”) (1), which I highlighted when his paper was first published online (6). For anyone familiar with my own writing down the years will know, it would not surprise me at all if this planet turned out to be very substantial in mass indeed, although this would then require it to move in an orbit far longer than even Dr Brown’s 20,000+ years.
Another paper to emerge this month uses super-computer ‘Monte Carlo’ calculations to provide modelling of the effect upon the solar system from a series of Planet Nine positional and mass parameters (7). Results from these theoretical tests provide the Spanish astronomers with the most likely zones in the sky to search for the proposed planet; although the authors of the paper strike a note of caution about some of the data Drs Brown and Batygin have based their work upon. At the moment, it is not clear whether there is any significant cross-over between all these studies in terms of a common focus for this search. Here’s a summary of the four areas of the sky favoured by the authors of this paper for the location of Planet Nine, including how they compare these results to those of Drs Brown and Batygin’s work (1) and that of Dr Fienga et al (2):
1) Most probably Hydra, away from the main bulk of the Milky Way,
2) Equally probably between Scorpius and Lupus, towards the Galactic bulge, but relatively far from the galactic centre. These first two are the most probable, say the authors, but have values inconsistent with the ‘pseudo resonant scenario’ described by Brown and Batygin (1)
3) Less probable, they say, is Taurus, a calculation which uses values favoured by Brown and Batygin, and also Fienga et al, and is also consistent with apsidal anti-alignment. (A similar Monte Carlo approach to Fienga’s values additionally provides the constellation of Cetus)
4) Least likely, between Microscopium and Sagittarius (7)
Once again, I seem to be backing the outsider, with Microscopium and Sagittarius at long-odds! I suspect we have some way to go before these dark horses enter their final furlong.
On top of all of the frenzied work being carried out to narrow the search down, hypotheses are also being considered to explain how Planet Nine ended up where it is (or at least, where it’s purported to be) (8). Ideas include migrating gas giant remnants (1), a captured exoplanet from a passing star (9), or gradual accretion of diffuse material in the outer solar system (10).

Additionally, exoplanet experts from the University of Bern in Switzerland, astrophysics professor Cristoph Mordasini and Ph.D. student Esther Linder, have modelled Planet Nine to provide some insight into what they think it might be composed of, as well as its various properties

“Their model reflects a very cold gas giant. It would be, as previously ascertained by the Caltech team, around 10 times Earth’s mass, with a radius 3.7 times that of Earth’s. Its temperature would be minus 226 degrees Celsius, or 47 Kelvin. Any radiation it emits would be dominated by its cooling core, which means it would be much brighter in infrared than visible light. This could help provide some clues to finding Planet Nine.


“As to why it hasn’t been found yet, previous sky surveys only have a very small chance of spotting an object so far away if it is less than 20 Earth masses. NASA’s Wide-field Infrared Survey Explorer could pick up such a planet, but only if it was 50 Earth masses or larger. “This puts an interesting upper mass limit for the planet,” Linder said.” (11)

Given the paucity of data, this all seems highly speculative, but does fairly reasonably follow the planetary models of ‘ice giants’ Uranus and Neptune, to provide us with an outer solar system planet which would be the runt of this ice giant litter. Using their planet evolution model, Mordasani and Linder calculated how parameters like the planetary radius or the brightness evolved over time since the Solar System formed 4.6 billion years ago (12), and how its relative magnitude might vary with size and distance from the Sun (13). How such an object might have evaded detection by an infra-red sky survey as powerful as WISE seems to be at the forefront of these planetary scientists’ minds, and one can’t help but think that the variable parameters have been cherry-picked to provide the scientific community with a best-fit scenario which hopes to get around this conundrum.

Written by

5th -12th April 2016
1) K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” 20th January 2016, The Astronomical Journal, Volume 151, Number 2, http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22
2) A. Fienga et al “Constraints on the location of a possible 9th planet derived from the Cassini data” 19th Feb 2016, Astronomy and Astrophysics, arXiv:1602.06116
3) Jonathan O’Callaghan “Where Is Planet Nine?” 25th February 2016, http://www.iflscience.com/space/search-planet-nine-continues-astronomers-hone-possible-location
4) M. W. Buie and W. M. Folkner. Astrometry of Pluto from 1930-1951 Observations: the Lampland Plate 17 Collection. AJ, 149:22, Jan. 2015. doi: 10.1088/0004-6256/149/1/22; cited in (5) below
5) Matthew Holman and Matthew Payne “Observational Constraints on Planet Nine: Astrometry of Pluto and Other Trans-Neptunian Objects” 30th Mar 2016, http://arxiv.org/pdf/1603.09008v1.pdf, thanks to Eitan
6) Andy Lloyd “Massive Planet X Now Urgently Sought by Top Planet-Hunters” 20th – 23rd January 2016 http://www.andylloyd.org/darkstarblog34.htm
7) C. de la Fuente Marcos & R. de la Fuente Marcos “Finding Planet Nine: a Monte Carlo Approach” 4th April 2016, MNRAS pre-print via ResearchGate, arXiv:1603.06520, thanks to Eitan
8) Shannon Hall “Planet Nine might be an exoplanet stolen by the Sun” 4th April 2016 https://www.newscientist.com/article/2082970-planet-nine-might-be-an-exoplanet-stolen-by-the-sun/ with thanks to Lee
9) Alexander Mustill et al “Is there an exoplanet in the Solar System?” 23rd March 2016 http://arxiv.org/abs/1603.07247
10) Scott Kenyon et al “Making Planet Nine: Pebble Accretion at 250–750 AU in a Gravitationally Unstable Ring” 25th March 2016 http://arxiv.org/abs/1603.08008

11) Michelle Starr “Is this what Planet Nine looks like?” 8th April 2016 http://www.cnet.com/uk/news/is-this-what-planet-nine-looks-like/ with thanks to Mark
12) Paul Rincon “Planet Nine’s profile fleshed out” 10th April 2016 http://www.bbc.co.uk/news/science-environment-35996813
13) Esther Linder & Christoph Mordasini “Evolution and Magnitudes of Candidate Planet Nine” 6th April 2016, http://arxiv.org/abs/1602.07465

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