February 22, 2026
consider an upper limit to the mass of Planet Nine, the most distant acceptable orbit has a9 = 800
AU, e9 = 0.45, and an aphelion of 1160 AU. Such an object would have an aphelion magnitude between 23.0 and 24.0 (perihelion magnitude between 19.9 and 20.8).
Counterintuitively, a lower mass Planet Nine is brighter owing to its requirement for a smaller heliocentric distance to have the same dynamical effect. To this end, a 5 M⊕ Planet Nine, for example, is bright enough to be detectable by wide-field surveys such as Pan-STARRS throughout most of its orbit. It is not yet known how complete the moving object search for Pan-STARRS is, but with the survey still ongoing a discovery
could in principle happen at any time. An important complication, however, lies in that the inferred aphelion of Planet Nine’s orbit is close to the intersection between P9’s orbital path and the galactic plane, where higher source density can impede detection (see Figure 25).
A higher mass and thus more distant Planet Nine will require a dedicated survey along the predicted orbital path (Brown and Batygin, 2016), but with a lower limit to the brightness of 24th magnitude, such an object is readily observable by the cur- rent generation of telescopes with wide field cameras such as the Dark Energy Camera on the Blanco 4m telescope in Chile and the Hyper-Suprime Camera on the Subaru telescope in Hawaii. Finally, all but the very faintest possible Planet Nine will be ob- servable with the Large Scale Synoptic Telescope (LSST), currently under construction in Chile and scheduled for operations in 2022. Therefore, Planet Nine – if it exists as described here – is likely to be discovered within a decade.
While detecting sunlight reflected at optical wavelengths might seem like the natu- ral method for searching for Planet Nine, the 1/r4 dependency of the flux of reflected sunlight makes the brightness of any object drop precipitously with distance. At long wavelengths, thermal emission, on the other hand, drops as only T /r2, where T could be approximately constant with distance (or drop as 1/r2 if the planet is in thermal equilibrium with the sun). For sufficiently distant planets, thermal emission constitutes a potentially preferable avenue towards direct detection. While current cosmological experiments at millimeter wavelengths have sufficient sensitivity to detect Planet Nine (Cowan et al., 2016), a systematic search would require millimeter telescopes with both high sensitivity and high angular resolution to robustly detect moving sources. The proposed CMB S-4, a next-generation cosmic microwave background experiment (Abazajian et al., 2016) could fulfill these requirements. Such an observatory would be sensitive not only to Planet Nine, but to putative even more distant bodies that might be present in the solar system.
It is worth noting that Planet Nine could emit more strongly than a blackbody at some wavelengths. Fortney et al. (2016) found that in extreme cases the atmosphere of Planet Nine could be depleted of methane and have order of magnitude more emis- sion in the 3-4 ยตm range. Meisner et al. (2017b) exploited this possibility to search for Planet Nine in data from the Wide-field Infrared Survey Explorer (WISE) dataset. In particular, they examined 3u steradians and placed a strongly atmospheric-model- dependent constraint on the presence of a high-mass Planet Nine at high galactic lati- tudes.
Gravitational Detection
A separate approach towards indirect detection of Planet Nine was explored by Fienga et al. (2016). Employing ranging data from the Cassini spacecraft, these au- thors sought to detect Planet Nine’s direct gravitational signature in the solar system ephemeris (somewhat akin to the technique Le Verrier 1846a,b used to discover Nep- tune; section 1.1). Adopting the P9 parameters from (Batygin and Brown, 2016a), they were able to immediately constrain P9 to the outer ~ 50% of the orbit (in agreementwith observational constraints; Brown and Batygin 2016). Moreover, the calculations of Fienga et al. (2016) point to a small reduction in the residuals of the ranging data if the true anomaly of P9 is taken to be u9 118 deg. This line of reasoning was further explored by Holman and Payne (2016a,b), who additionally considered the long base- line ephemerides of Pluto to place additional constraints on the sky-location of Planet Nine.
The reanalysis of the ephemeris carried out by Folkner et al. (2016), however, high- lights the sensitivity of Fienga et al.’s results to the specifics of the underlying dynam- ical model, suggesting that the gravitational determinations of Planet Nine’s on-sky location are in reality considerably less precise than advocated by Fienga et al. (2016); Holman and Payne (2016a,b). An additional complication pertinent to this approach was recently pointed out by Pitjeva & Pitjev (2018), who caution that failure to prop- erly account for the mass contained within the resonant and classical Kuiper belt (which
they determine to be on the order of 2 10–2 M⊕) can further obscure P9’s gravitational signal in the solar system’s ephemeris. Particularly, Pitjeva & Pitjev (2018) find that the anomalous acceleration due to a 0.02 M⊕ Kuiper belt is essentially equivalent to that arising from a m9 = 10 M⊕ planet at a heliocentric distance of r = 540 AU (for orbits around Saturn), further discouraging the promise of teasing out Planet Nine’s
gravitational signal from spacecraft data.
In terms of both physical and orbital characteristics, the inferred properties of Planet Nine are certainly unlike those of any other planet of the solar system. Re- cent photometric and spectroscopic surveys of planets around other stars (Borucki et al., 2010; Batalha et al., 2013), however, have conclusively demonstrated that m
5 10 M planets are exceedingly common around solar-type stars, and likely repre- sent one of the dominant outcomes of the planet conglomeration process20. A moder- ately excited orbital state (and in particular, a high eccentricity) is also not uncommon among long-period extrasolar planets, and is a relatively well-established byproduct of post-nebular dynamical relaxation of planetary systems (Juric´ & Tremaine, 2008). Nevertheless, the formation of Planet Nine represents a formidable problem, primarily due to its large distance from the sun.
To attack this speculative issue, several different origin scenarios have been pro- posed and are discussed in this section. The first option is for the planet to form in situ, via analogous formation mechanism(s) responsible for the known giant planets (section 7.1). Another option is for Planet Nine to form in the same annular region as the other giant planets, and then be scattered outward into its present orbit (section 7.2). Yet another possibility is for the planet to originate from another planetary sys- tem within the solar birth cluster, and then be captured during the early evolution of the solar system (section 7.3). While all of these scenarios remain in play (Figure 26), each is characterized by non-trivial shortcomings, as discussed below.
20Jovian-class objects like Jupiter and Saturn, on the other hand, are comparatively rare and are believed to reside within 20 AU in only ~ 20% of sun-like stars (Cumming et al., 2008).
In Situ Formation
Perhaps the most straightforward model for the origin of Planet Nine is for it to form in situ, at its present orbital location. An attractive feature of this scenario is the fact that it does not require any physical processes beyond conglomeration itself. The advantages, however, stop there. Generally speaking, the timescale over which plan- etary building blocks (pebbles, planetesimals) amass into multi-Earth mass objects21 is set by the orbital period at the location of the forming planet (Johansen & Lam- brechts, 2017). With a 10, 000 year orbital period (corresponding to a 500 AU), a forming Planet Nine would only complete 300 revolutions around the sun within the typical lifetime of a protoplanetary disk (Hernandez et al., 2007). The corresponding impedance of growth by the slowness of the orbital clock is illuminated by the calcula- tions of Kenyon and Bromley (2016), who find that even under exceptionally favorable conditions, formation of super-Earths at hundreds of AU requires billions of years.
Another shortcoming of in situ formation concerns the availability of planet-forming material at large heliocentric distances. Various lines of evidence indicate the solar sys- tem did not originate in isolation, and instead formed within a cluster of 103 104 stars (Adams and Laughlin 2001; Portegies Zwart 2009; Adams 2010; Pfalzner et al. 2015). Such a cluster environment can be highly disruptive to the outer regions of circum- stellar disks and hence to planet formation. At minimum, a number of authors have shown that over a timescale of 10 Myr, passing stars are expected to truncate the disk down to a radius of 300 AU, about one third of the minimum impact parameter (Heller, 1995; Ostriker, 1994). More importantly, these clusters also produce intense FUV radiation fields that evaporate circumstellar disks, removing all of the material beyond 30 40 AU over a time scale of 10 Myr (Adams et al., 2004). Observational evidence supports this picture and indicates that disks in cluster environments expe- rience some radiation-driven truncation (e.g., Anderson et al. 2013). Moreover, even in regions of distributed star formation, where external photoevaporation is unlikely to play a defining role, observations find that typical disk radii are only of order 100 AU (Haisch et al., 2001; Andrews et al., 2009, 2010). As a result, both observational and theoretical considerations suggest that the early Solar Nebula was unlikely to have
extended much farther than the current orbit of Neptune at a ~ 30 AU (see also Kretke et al. 2012). Forming Planet Nine at a radius of ~ 500 AU is thus strongly disfavored.
A somewhat more natural origin scenario is for Planet Nine to form within the region that produces the known giant planets (i.e., the annulus defined roughly by
5 AU ~< a ~< 30 AU), and to be scattered out later. The physics of the planet forma-
tion process is notoriously stochastic, and the number of planets produced in a given planet-forming region cannot be calculated in a deterministic manner (Morbidelli & Raymond, 2016), meaning that additional ice giants other than Uranus and Neptune
21While gravitational instability of the early solar nebula provides an alternate means of forming planets, it is irrelevant to the problem at hand, since the mass scale for objects generated through this channel is of order 10 MJ (or higher, e.g., Rafikov 2005)
Figure 26: Above, we show not-to-scale schematics for the three possible mechanisms by which Planet Nine could have been formed and placed in its current orbit in the solar system. (top panel) In in situ formation, Planet Nine forms in its current distant orbit while the protoplanetary disk is still present, and resides there throughout the history of the solar system. (middle panel) If Planet Nine forms among the outer planets in the solar system, it could subsequently be scattered outwards onto a high-eccentricity orbit through interactions with the other solar system planets. Then, its orbit could be circularized through interactions with passing stars. (bottom panel) If Planet Nine originally formed around a host star other than the sun, a subsequent close encounter between this other star and the sun could result in Planet Nine being captured into its current- day long-period orbit around the sun.
may have occupied the outer solar system during its infancy. To this end, analytic ar- guments put forth by Goldreich et al. (2004) suggest that the outer solar system could have started out with as many as five ice giants. Along similar lines, numerical models for the agglomeration of Neptune and Uranus through collisions among large plane- tary embryos find that 5 M⊕ objects routinely scatter away from the primary planet
forming region (Izidoro et al., 2015). Calculations of ancillary ice giant ejection during
the outer solar system’s transient epoch of dynamical instability are also presented in Nesvorny´ (2011); Batygin et al. (2012); Nesvorny´ & Morbidelli (2012).
It is important to recognize that this model of Planet Nine formation must neces- sarily involve a two-step process. This is because outward scattering of Planet Nine facilitated by the giant planets places it onto a temporary, high-eccentricity (q 5 AU) orbit, which must subsequently be circularized (thus lifting its perihelion out of the planet-forming region) by additional gravitational perturbations arising from the clus- ter. The difficulty with this scenario, however, is that the likelihood of producing the required orbit for Planet Nine is low. Li and Adams (2016) estimated the scattering probability for this process by initializing Planet Nine on orbits with zero eccentricity and semi-major axis a 100 200 AU, and found that stellar fly-by encounters pro- duce final states with orbital elements a = 400 1500 AU, e = 0.4 0.9, and i < 60 deg only a few percent of the time. We note however, that the calculations of Brasser et al. (2006, 2012) obtain considerably more favorable odds of decoupling a scattered plan- etary embryo from the canonical giant planets and trapping it in the outer solar system, with reported probability of success as high as 15%, depending on the specifics of the adopted cluster model.
As an alternative to invoking cluster dynamics, Ericksson et al. (2018) considered the circularization of a freshly scattered Planet Nine through dynamical friction arising from a massive disk of planetesimals, extending far beyond the orbit of Neptune. Un- like the aforementioned cluster calculations, in this scenario the chances of producing a favorable Planet Nine orbit can be as high as 30%. An important drawback of this model, however, is that it suffers from the same issues of disk truncation outlined in the previous sub-section. Moreover, simulations of the Nice model (Tsiganis et al., 2005) require the massive component of the solar system’s primordial planetesimal disk to end at a 30 AU, to prevent Neptune from radially migrating beyond its current orbit. Therefore, within the framework of Ericksson et al. (2018), some additional physical process would be required to create an immense gap ranging from 30 AU to 100 AU in the solar system’s primordial planetesimal disk.
Although gravitational perturbations arising from passing stars can act to alter the orbital properties of Planet Nine, as discussed above, the possibilities do not end there – the birth environment of the solar system can also lead to the disruption of Planet Nine from its wide orbit. Several groups have worked to quantify the effects of scattering encounters in young stellar clusters using N-body methods over the last decade (e.g., Portegies Zwart 2009; Malmberg et al. 2011; Pfalzner 2013; Pfalzner et al. 2015, 2018). Another way to study this class of disruption events is to separately calculate the cross sections for fly-by encounters to ionize the solar system,
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