#  A search for Planet 9 in the IRAS data

Michael Rowan-Robinson1
1Astrophysics Group, Imperial College London, Blackett Laboratory, Prince
Consort Road, London SW7 2AZ, UK

###### Abstract

I have carried out a search for Planet 9 in the IRAS data. At the distance
range proposed for Planet 9, the signature would be a 60 $\mu$m unidentified
IRAS point source with an associated nearby source from the IRAS Reject File
of sources which received only a single hours-confirmed (HCON) detection. The
confirmed source should be detected on the first two HCON passes, but not on
the third, while the single HCON should be detected only on the third HCON. I
have examined the unidentified sources in three IRAS 60$\mu$m catalogues: some
can be identified with 2MASS galaxies, Galactic sources or as cirrus. The
remaining unidentified sources have been examined with the IRSA Scanpi tool to
check for the signature missing HCONs, and for association with IRAS Reject
File single HCONs. No matches of interest survive. For a lower mass planet
($\leq 5M_{E}$) in the distance range 200-400 AU, we expect a pair or triplet
of single HCONs with separations 2-35 arcmin. Several hundred candidate
associations are found and have been examined with Scanpi. A single candidate
for Planet 9 survives which satisfies the requirements for detected and non-
detected HCON passes. A fitted orbit suggest a distance of 225$\pm$15 AU and a
mass of 3-5 $M_{E}$. Dynamical simulations are needed to explore whether the
candidate is consistent with existing planet ephemerides. If so, a search in
an annulus of radius $2.5-4^{o}$ centred on the 1983 position at visible and
near infrared wavelengths would be worthwhile.

###### keywords:

Planets and satellites: terrestrial planets; minor planets, asteroids,
general; infrared: general.

##  1 Introduction

In the 1980s there had been a long history of interest in what would at that
time have been a tenth planet, Planet X. There appeared to be unexplained
residuals in the orbit of Neptune. Though these were much smaller than the
Uranus residuals by which Le Verrier and Adams discovered Neptune, they had
motivated Tombaugh‘s search for a new planet, which resulted in the discovery
in 1930 of what we now know as the dwarf planet Pluto. It quickly became clear
that Pluto was too small to explain the Neptune residuals and so the
possibility of a tenth planet remained (see Batygin et al (2019) for a full
historical account and references).

In 1983, while working on the preparation of the IRAS Point Source Catalog, I
undertook a systematic search for Planet X in the IRAS data. The search was
unsuccessful though it did yield a detection of Comet Bowell (Walker and
Rowan-Robinson 1984). Amusingly a misunderstanding of an earlier IRAS science
team briefing by senior NASA personnel resulted in a press story in 1983 that
IRAS had discovered Planet X (see Rowan-Robinson 2013 for full details of how
this misunderstanding arose).

Interest in Planet X flared up again in the late 1980s (Harrington 1988,
Seidelmann and Harrington 1988, Jackson and Killen 1988, Neuhauser and
Feitzinger 1991) and the Royal Astronomical Society staged a discussion
meeting in 1991 on ‘Solar System Dynamics and Planet X‘. I gave a report on my
IRAS searches and concluded that I was 70$\%$ certain that Planet X did not
exist. The figure of 70$\%$ referred to the area of sky in which I was able to
carry out my IRAS search. Reports of the meeting were given by Morrison (1992)
and Crosswell (1991).

Subsequently a remeasurement of the mass of Neptune eliminated the Neptune
residuals (Standish 1992). The absence of deviations from the orbits of the
Pioneer and Voyager spacecraft shows that no unknown massive solar system
planet resides in the plane of the ecliptic. Luhman (2014) has used WISE data
to set severe limits on Saturn or Jupiter mass objects in the solar system out
to 28,000 and 82,000 AU, respectively.

The discovery of dozens of new dwarf planets during the past twenty years has
resulted both in the redefinition of Pluto as a dwarf planet and in their
potentiality as probes of possible distant massive planets in highly inclined
orbits. Batygin and Brown (2016) and Brown and Batyin (2016), developing an
idea of Trujillo and Sheppard (2014), proposed that a planet of a few tens of
earth-masses in an inclined and eccentric orbit at 280-1000 AU could explain
alignments in the orbits of the Kuiper belt dwarf planets. As this was
significantly more distant than the Planet X I was looking for in 1983, I
thought it worthwhile to redo my IRAS search and set out quantitatively what
are the constraints on such an object. Fienga et al (2016), Holman and Payne
(2016), Iorio (2017), Millholland and Laughton (2017), Medvedev et al (2017),
Caceres and Gomes (2018), Brown and Batygin (2019), Batygin et al (2019) and
Fienga et al (2020), have given additional dynamical constraints on Planet 9‘s
orbit. Specifically Fienga et al (2016) argue against d $<$ 370AU for a 10
$M_{E}$ planet, using Cassini radio ranging data, which they revise to d $<$
650AU in Fienga et al (2020).

Sections 2 discusses the IRAS capability to detect moving objects, section 3
focuses on different categories of cool ($<$1000 K) movers, sections 4 reviews
unidentified sources in the IRAS PSCz and RIIFSCz surveys, section 5 discusses
the IRAS 60 $\mu$m single HCON database. Section 6 discusses a new search for
warm, slow movers, relevant to the search for Planet 9. Section 7 discusses
how a Planet 9 candidate might be recovered and section 8 gives my discussion
and conclusions.

##  2 IRAS capability to detect movers

IRAS surveyed 98 $\%$ of the sky at 12, 25, 60 and 100 $\mu$m. The focal plane
array consisted of two strips of detectors for each of the four bands. The
resolution of IRAS at 60 $\mu$m was $\sim$1 arcmin.

The confirmation strategy for the IRAS Point Source Catalog (PSC: IRAS
Explanatory Supplement, 1984) was at four levels: (i) seconds confirmation, as
sources crossed the two array strips for each waveband, (ii) hours
confirmation (HCON) as the same area was observed on two or sometimes three
successive orbits (iii) weeks confirmation, as the same process was repeated
roughly one to two weeks later (1$\%$ of the sky received only a single HCON
coverage) (iv) months confirmation, when the process was repeated six months
later. At the termination of the mission 75$\%$ of the sky had achieved a
third HCON coverage. 2 $\%$ of the sky had no coverage at all. The IRAS
coverage gaps, two ribbons of sky centred at ecliptic longitudes 160o and
240o, are edged by narrow strips in which only a single HCON was achieved.

This confirmation strategy allowed the identification of different categories
of moving object:

(a) fast movers: Sources which passed seconds-confirmation but failed hours-
confirmation were examined in real time at the IRAS ground station at
Rutherford Appleton Laboratory. Several comets and Apollo asteroids were found
and published (Davies et al 1984).

(b) medium movers: sources which passed hours-confirmation, and so were
recorded as HCONs, but failed weeks-confirmation were retained in a ‘PSC
Reject File’. These would include main-belt asteroids, which were published as
the IRAS Asteroid and Comet Catalog (1986).

For areas covered by 3 hours-confirming coverages we would expect to see 3
separate HCONs.

(c) slow movers: sources which passed weeks confirmation but failed months
confirmation would be present in the PSC but could have a separate single-HCON
companion from the third coverage. These are the main cases of interest for a
Planet 9 candidate.

(d) very slow movers: sources which pass months confirmation will be present
in the PSC. Separate analysis of time-line data would be needed to reconstruct
motions of moving sources which moved less than $\sim$ 1 arcmin in 6 months.
However such objects would have to be at distances $>$ 7000 AU and so are not
of interest as Planet 9 candidates.

What positional disagreement results in non-confirmation of HCONs ? This
varies with direction on the sky but the 60 $\mu$m (2-$\sigma$) error ellipses
for confirmed sources have semi-axes typically 30 arcsec (cross-scan) by 10
arcsec (in-scan). I adopt 1 arcmin as the characteristic motion which could
cause confirmation failure at 60 $\mu$m.

Figure 1:  $log_{10}$ planetary radius (in earth radii, blue), and $log_{10}$
predicted temperature (oC, red) due to internal heating only, versus
$log_{10}$ planetary mass (in earth masses). Filled red hexagrams are
temperature predictions from Reynold et al (1980), while filled blue hexagrams
are data for solar system planets. Crosses are for recent Planet 9 models
explored by Fortney et al (2016, their Table 1). Blue dotted curves indicate
range of predicted radii for exoplanet models of Mordasini et al 2012 (their
Fig 10). Solid lines indicate relations assumed for predictions of Fig 2.

##  3 IRAS sensitivity to cool movers

The capability of IRAS to detect cool ($<$ 1000 K) nearby objects was
discussed by Reynolds, Tarter and Walker (1980), who compared the expected
sensitivity of IRAS to existing (partial) ground-based surveys. They gave the
expected surface temperature of the known planets (and hypothetical objects of
mass greater than Jupiter) if they were located at great distances from the
Sun, and therefore radiating internal heat only. Figure 1 (upper part) shows
their estimates of surface temperature for different masses and also the
temperatures calculated by Fortney et al (2016) in their recent detailed study
of possible Planet 9 models. My adopted mean relation is shown as a solid
line. The lower part of Fig 1 shows the radii of the solar system planets, the
model estimates of Fortney et al (2016), and also the range of model radii
considered by Mordasini et al (2012) in their discussion of possible exoplanet
models, again with my adopted mean line. We can then construct a mass-distance
sensitivity diagram (Fig 2) which shows the distance to which different mass
planets would be detectable in the four IRAS bands. The prime search-area for
a Batygin and Brown object would be 5-20 $M_{E}$, with the distance range
being 280-1100 AU. At 60$\mu$m IRAS would in principle have been able to
detect a 5 $M_{E}$ planet to 220 AU, a 10 $M_{E}$ planet to 380 AU, and a 20
$M_{E}$ planet to 690 AU. But if we allow for the maximum radii considered by
Mordasini et al (2012), a 5 $M_{E}$ planet could be detected to 350 AU, a 10
$M_{E}$ planet to 600 AU, and a 20 $M_{E}$ planet to 1000 AU. These are
worthwhile limits.

Figure 2:  Planetary mass (in earth units) versus distance (in AU). Green,
blue and red dashed loci show limit of IRAS 25, 60 and 100 $\mu$m
detectability (assumed flux-limits 0.25, 0.45 and 1.0 Jy). The right-hand blue
dotted locus corresponds to the extended planetary models of Mordasani et al
(2012). Vertical dotted lines divide distances into those in which three
separate single HCONs would be seen (1+1+1), those at which sources would
weeks-confirm but fail 6-months confirmation (2+1), and those beyond which all
three HCONs should be combined together (3). The black locus contains the
region in which a planet of 1-26 earth-masses, at 180-1000 AU could be found:
the left-hand boundary is the limit set by Cassini ephemerides, the right-hand
boundary is the limit for non-disruption during planet formation, objects
below the lower right boundary would be dwarf planets (Batygin et al 2019). U,
N indicate the locations of Uranus and Neptune. The cyan locus is the limit
for the WISE search at 3.4$\mu$m (Meisner et al 2018),which covers 70$\%$ of
the sky to a depth of m(3.4$\mu$m)=16.7 (Vega), for an assumed albedo 0.75.
Red horizontal bars denote the preferred mass-distance ranges of Holman (2016)
and Brown and Batygin (2019). The Planet 9 candidate discussed in sections 6.5
and 7 is shown as a black hexagon with error bars.

The main motion of distant solar system movers is an ellipse due to parallax,
with major diameter 2$\alpha(arcmin)$, where $\alpha=3438/d(AU)$. I have shown
a vertical dashed line in Fig 2 corresponding to $\alpha=0.5arcmin$,
essentially the most distant solar object for which IRAS could detect motion
(1arcmin in 6 months). Two weeks-confirming HCONs would have a separation of 1
arcmin when $\pi\alpha(arcmin)=26$, or d$\sim$ 415AU and this is also shown
(thicker dashed line). For the proposed Planet 9 at 280-1100 AU, the expected
parallactic motion in 6 months would be 6-24 arcmin, easily detectable by
IRAS. Superposed on the elliptical motion on the sky due to parallax there
would be a drift along a great circle due to the orbital motion, typically
0.3-1.3 arcmin in 6 months for d=1100-400AU.

Fienga et al (2016) have set a dynamic constraint of d$>$370 AU for a 10
$M_{E}$ planet on a particular hypothetical Planet 9 orbit, based on the
ephemeris of Saturn as observed by Cassini. Batygin et al (2019) have scaled
this as $M/r^{3}$ to give the left-hand edge of the permitted region in Fig 2.

Fortney et al (2016) and Meisner et al (2018) have discussed the detectability
of Planet 9 in the WISE W1 (3.4$\mu$m) channel. We have shown the
corresponding locus for an assumed WISE 3.4$\mu$m sensitivity of 16.7 m(Vega),
which Meisner et al (2018) have achieved for 70$\%$ of the sky. At 3.4$\mu$m
the emission from Planet 9 will be through a combination of reflected light
from the Sun, which depends on the albedo at 3.4$\mu$m, and emission. The net
flux is highly dependent on assumptions about the Planet 9 atmosphere and
abundances, especially of methane (Fortney et al 2016). We have adopted the
Rayleigh scattering limit albedo of 0.75, though for most Planet 9 models, and
for Neptune, the albedo would be orders of magnitude smaller than this. Even
with this optimistic assumption and the adopted mean radius-mass line of Fig
1, the predicted 3.4 $\mu$m flux from reflected light for Planet 9 in the
expected mass-distance range is not really detectable by WISE. However the
most extreme model of Fortney et al (2016) does give such strong molecular
emission that the effective albedo becomes $>$ 1 and such model planets would
be detectable by WISE at 3.4$\mu$m. The range of radius predictions at a given
planet mass makes the limiting distance estimates uncertain by $\pm 50\%$.

Unfortunately there are a number of alibis for Planet 9: (i) IRAS has no
coverage at all in about 2$\%$ of the sky and only a single coverage in a
further 1$\%$, (ii) some IRAS galaxy catalogues (e.g. PSCz, QIGC) impose
additional constraints, excluding regions of the sky with high source-density
flag at 12, 25, or 60 $\mu$m (PSCz, QIGC), with high-source-density flag at
100 $\mu$m (QIGC), or with high extinction (PSCz).These constraints, which
have a similar effect, exclude a further $\sim 13\%$ of the sky. (iii) a
quarter of the sky received only 2 hours-confirmed coverages, which makes
detection of slow movers difficult, unless they are detected in 3 bands so
that colour information can be used.

##  4 Unidentified sources in the IRAS surveys

Our starting point in any search for Planet 9 in the IRAS data is the
unidentified sources in the IRAS Point source Catalog (PSC) and Faint Source
Catalog (FSC) surveys. In this section I discuss the unidentified sources in
the PSCz catalogue (Saunders et al 2000), as the most complete and well-
studied compilation from the IRAS PSC survey, and in the RIIFSCz galaxy
catalogue (Wang et al 2014), as the best studied compilation from the IRAS FSC
survey.

###  4.1 Unidentified sources in PSCz

For slow movers we are interested in PSCz sources which are not identified
with galaxies or Galactic objects. The IRAS PSCz survey sought to identify and
get optical spectra of every IRAS PSC 60$\mu$m sources brighter than 0.6Jy,
after exclusion of Galactic sources and cirrus. The full catalogue consists of
15459 sources, of which 15200 have $|b|>5^{o}$. Of the latter all but 791
(5.2$\%$) have measured redshifts (these numbers differ slightly from those
given by Saunders et al 2000). $[b|<5^{o}$ sources are discussed below. The
PSCz completeness limit is given as 98.5$\%$ outside the masked area (see Fig
5), ie for 85$\%$ of the sky.

I have examined these 791 sources in the NASA$/$IPAC Extragalactic Database
(NED). 493 were either identified with catalogued galaxies (especially with
2MASS galaxies), were identified galaxies in the RIFSCz catalogue (Wang et al
2014), were classified as cirrus in the literature, were classified as known
Galactic sources, or were within 1.5 arcmin of a 2MASS galaxy. A plot of
60$\mu$m flux versus K-band magnitude for IRAS Faint Source Catalogue galaxies
shows that most, but not quite all, IRAS galaxies with 60 $\mu$m flux density
$>$ 0.6 Jy are likely to be detected as 2MASS sources.

The distribution of distances of the nearest 2MASS galaxy for sources not
associated with 2MASS galaxies in NED is consistent with being the tail of a
30 arcsec Gaussian, which is a plausible value for the rms error in IRAS-2MASS
associations. Most sources with separations less than 1.5 arc min are
therefore likely to be real associations and few of those with larger
separations are likely to be true associations. However because the size of
the 60 $\mu$m detectors (4.5x0.5 arc min), errors in processing can
occasionally result in separations between an IRAS source and its associated
galaxy of more than 1.5 arc min.

304 (2$\%$) sources remain unidentified in NED. Many of these are at
$|b|=5-20^{o}$, where identification is harder because of strong extinction in
the optical and because of the high surface-density of stars, and where the
probability of spurious cirrus sources is higher. Reviews of targeted searches
for low latitude galaxies have been given by Kraan-Korteweg and Lahav (2000)
and Staveley-Smith et al (2016). Examination of the Schlegel et al extinction
maps (Schlegel et al 1998) suggests that a further 72 of these 304
unidentified sources are cirrus. 164 are classified as g or gf (‘faint
galaxy’) in the NASA-HEASARC PSCz archive, although without confirmation of
the source position by radio observations (available for only 27 of the
sources) such an identification is ambiguous, since several faint objects lie
within the IRAS error ellipse. For a further three, a radio association
confirms the source as a galaxy. HEASARC identify a further 2 objects as
stellar and 8 as cirrus. 3 are sources recovered by Saunders et al (2000) from
the IRAS Reject File, but without a confirming galaxy or IRAS FSC association
the reality of these sources is doubtful. The final number of unidentified
IRAS PSCz sources is then 52, if ‘gf’ designation is accepted as an
identification, and IRAS Reject File sources are rejected. However the
resulting constraint on very slow movers brighter than 0.6 Jy at 60 $\mu$m is
still rather limited, that the surface-density is no greater than 0.001 per sq
deg. But from Fig 2 it is clear that no very slow movers of interest
(parallactic angle $>3arcsec$, $M<100M_{E}$) would have 60 $\mu$m flux-
densities greater than 0.6 Jy. Unidentified PSCz sources could still be of
interest as candidates for Planet 9 if they lay within the areas that received
only 2 HCON coverages.

###  4.2 Unidentified sources in the RIIFSCz catalogue

The revised Imperial IRAS FSC redshift (RIIFSCz) catalogue (Wang et al 2014)
is also of interest to look for unidentified 60 $\mu$m sources to fainter
fluxes than the PSCz catalogue. The IRAS Faint Source Catalog was processed in
a different way to the PSC, essentially coadding all the IRAS data before
searching for point sources. Moshir et al (2019) give the 90$\%$ completeness
limit for the IRAS Faint Source Catalog at 60 $\mu$m as 0.28 Jy and this is
confirmed by 60 $\mu$m source-counts. There are 2727 sources in the RIIFSCz
brighter than 0.3 Jy which do not have a galaxy association. I have examined
all of these using the National Extragalactic Database (NED) and find that
1720 have plausible associations with 2MASS galaxies, which I define as
follows: for ‘bright’ 2MASS galaxies (K$<$13.5) an association within 1.5
arcmin; for 2MASS galaxies with 13.5$<$K$<$14.5 an association within 1.0
arcmin; and for ‘faint’ 2MASS galaxies (14.5$<$K$<$15.5) an association within
30 arcsec. The redshifts of the associated galaxies, where available, are
generally in the range 0.01-0.15. A few are associated with stars (68) or
other Galactic sources (2). The remaining 937 sources, 3.1$\%$ of sources
brighter than 0.3Jy, are unidentified and may be higher redshift(z$>$0.1)
galaxies or spurious sources. I investigate whether any of these can be
associated with a single HCON source below (section 6.2).

The distribution on the sky of PSCz and RIIFSCz unidentified sources is shown
in Fig 5.

##  5 The IRAS 60 $\mu$m single HCON database

To find a Planet 9 candidate in the IRAS data we need to look for an
unidentified point source associated with a separated single HCON source. Here
I characterise in detail for the first time the IRAS single HCON database
(Point Source Reject File). This consists of 21045 single HCONs with
detections at 60 $\mu$m. For one reason or another these have failed to be
associated with IRAS PSC sources. These can include main-belt asteroids,
planets, distant comets, cirrus, galaxies whose different HCONs did not get
properly processed together, and point sources in areas of the sky which
achieved only a single HCON coverage. Counts of 60$\mu$m single HCONs suggest
the 90$\%$ completeness limit is 0.45 Jy.

12760 of these have detections also at 100 $\mu$m and for these Fig 3R shows
S100 versus S60. For comparison a similar plot is shown (Fig 3L) for the QMW
IRAS Galaxy Catalogue (Rowan-Robinson et al 1991). This contains 17706
galaxies and differs from PSCz in including sources with 60 $\mu$m fluxes in
the range 0.5-0.6 Jy). The straight lines which bound most IRAS galaxies
correspond to $log_{10}S100/S60$ = 0.7 and -0.1. Strong concentrations of
single HCON sources lie outside the region occupied by galaxies. I divide the
single HCONs into (i) hot sources ( $log_{10}S100/S60<-0.1$), (ii) warm
sources ($-0.1<log_{10}S100/S60<0.7$), (iii) cold sources (
$log_{10}S100/S60>0.7$. Figure 4 shows the sky distribution, RA versus dec,
for the three classes, with the loci of the ecliptic and Galactic planes
indicated.

It is apparent that most of the hot single HCONs are main-belt asteroids and
that most of the cold single HCONs are Galactic cirrus. The warm sources show
concentrations along the edges of the IRAS coverage gaps, with a strong
concentrations where these gaps cross the Galactic plane. Otherwise the
distribution of warm single HCONs is broadly isotropic, consistent with most
being incorrectly processed bits of galaxies.

Figure 3:  L: S100 versus S60 (Jy) for sources from the QMW IRAS Galaxy
Catalogue (Rowan-Robinson et al 1991). R: S100 versus S60 (Jy) for isolated
single HCON sources from IRAS Reject File. The blue lines demarcate ‘hot
movers’, S100$/$S60$<$0.8 (T$>$82K), ‘warm movers’ $0.8<S100/S60<5$
(30$<$T$<$82 K), and ‘cold movers’, $S100/S60>5$ (T$<$30K).

Figure 4:  L: RA versus dec for hot single HCONs. Most are confined to a band
around the ecliptic plane and are main belt asteroids. centre: RA versus dec
for cold single HCONs. Most lie close to the Galactic Plane and are due to
cirrus. R: RA versus dec for warm single HCONs. Apart from heavy
concentrations where the IRAS coverage gaps meet the Galactic Plane, and a
weaker concentration along the IRAS coverage gap, the distribution is
relatively isotropic, indicating that most are probably galaxies.

##  6 New search for warm slow movers (Planet 9 candidates)

We now look for Planet 9 candidates. Firstly I look for associations of
unidentified sources from the different IRAS catalogues with single HCONs,
which would be candidates for Planet 9 at distances $>$ 400 AU. Then I look
for cross-associations of single HCONs, which would be candidates for a lower
distance Planet 9.

###  6.1 Association of PSCz unidentified sources with single HCONs

To see whether there any candidates in the IRAS database for the hypothetical
Planet 9, an object of a few tens of earth-masses at a distance of 280-1100
AU, I have first searched for 60 $\mu$m-detected warm single HCONs (including
60 $\mu$m only objects) at distances of 2-25 arcmin from the 52 unidentified
PSCz sources at $|b|>5^{o}$. 8 sources found 1HCON matches. There were two
cases where multiple single HCONs were found associated with the same PSCz
source. These are presumed to be either main-belt asteroids or due to cirrus
or other Galactic structures. Each of the remaining associated single HCONs
was examined for associations in NED. Associations of 1HCON sources with 2MASS
galaxies were accepted up to 2 arcmin, allowing for the poorer error ellipse
for single HCONs. A number of single HCONs were identified as due to cirrus
from the Schlegel et al extinction maps.

One further constraint is that the PSCz source should be missing the third
HCON. I have examined all 52 unidentified PSCz sources using the IRSA Scanpi
tool, which allows the individual IRAS scans crossing any specified position
to be examined. The signature of a Planet 9 candidate would be a source seen
on the scans associated with the first two HCONs but missing on the third
HCON. One source, 88.313751+45.864166, showed this signature, and has a single
HCON nearby (87.50875+45.64139, at 36.3 arcmin), but the latter is detected on
all three HCONs and is not a mover. Also the flux agreement is poor (0.8Jy for
PSCz source, 0.4Jy for 1HCON), so this source is not a convincing candidate
for Planet 9.

Of the 259 PSCz sources at $|b|<5^{o}$, 105 did not have redshifts determined
by Saunders et al (2000). I examined these for associations in NED and found
45 did not have associations. These 45 were then run with Scanpi. 2 were
spurious, 4 were cirrus, 2 probably Galactic, and there were just two with the
characteristic missing HCON profile: 74.024582+47.587219 and
85.358749+21.475834. The first does not have a nearby single HCON of interest
(the nearest is at 36 arcmin and is not unidentified). The second has a nearby
HCON (see Table 1) but this does not show any convincing detections on the
Scanpi scans.

The conclusion of the search in PSCz is that no plausible candidate survives.

Figure 5:  RA versus dec for remaining unidentified PSCz sources (blue
crosses), RIIFSCz unidentified sources (black filled hexagrams), and QIGC
unidentified sources at $|b|<20^{o}$ (red filled hexagrams). Areas which did
not receive a third HCON coverage are delineated in blue. The IRAS PSCz mask
(zone of avoidance) is indicated in yellow. The areas in which it has been
proposed that Planet 9 may lie are indicated as black rectangles (H: Holman
and Payne 2016, Me: Medvedev et al 2017, Mi: Millholland and Laughton 2017).
Galactic latitudes 0 and $\pm 20^{0}$ are shown as black loci and the red
locus is the preferred Planet 9 orbit of Batygin et al (2019). My Table 1
sources of interest are indicated as cyan open hexagrams.

###  6.2 Association of RIIFSCz unidentified sources with single HCONs

I looked for associations of single HCONs with the 937 unidentified RIIFSCz
sources, at separations between 2 and 25 arcmin. A total of 126 associations
were found. The associated RIIFSCz sources were checked with Scanpi, looking
for the pattern of detections on the first two HCONs and non-detections on the
third. The single HCONs were examined with NED, so that identified sources
could be rejected, and with Scanpi to check the reality of the sources.

There were three cases cases of interest ( listed in Table 1). However for
two, the single HCON is not unidentified. The third RIIFSCz source is non
really undetected on the third HCON and the flux match with the single HCON is
poor.

The conclusion is that no Planet 9 candidate survives in the RIIFSCz
catalogue.

###  6.3 Association of QMW IRAS galaxy catalogue unidentified sources with
single HCONs

While the RIIFSCz takes our search down to 0.3 Jy, it does so only for
$|b|>20^{o}$. At $|b|<20^{o}$ we can use the QMW IRAS Galaxy Catalogue (QIGC:
Rowan-Robinson et al 1991) to extend the search to $\sim$0.5 Jy. The
completeness of QIGC is given as 80$\%$ in the flux-range 0.5-0.6Jy (Rowan-
Robinson et al 1991). I therefore searched for unidentified IRAS PSC sources
with 60 $\mu$m fluxes in the range 0.5-0.6 Jy (from the QMW IRAS Galaxy
Catalogue, see Fig 5), with single HCON associations at separations in the
range of interest for Planet 9. 90 sources have 1HCON associations, 14 are
unidentified: of these, 6 are spurious (Scanpi), 2 are extended at 60 $\mu$m,
7 were candidates for having missing HCONs, and 2 of these are of interest
(Table 1). For 114.505836-23.892778 the QIGC source and the single HCON have
the appropriate pattern of detected and non-detected HCONs for a mover, in
terms of 3-$\sigma$ detections in Scanpi. However the non-detected HCONs are
not very convincing because each has several 2-$\sigma$ detections. For
276.900848+22.634167 the single HCON is not unidentified.

Figure 6:  Distribution of 60 $\mu$m single HCON sources close to ecliptic
plane (shown as a red locus). At least 12 main-belt asteroid tracks can be
seen. The triplet of points labelled U are due to Uranus.

###  6.4 Cross associations of single HCONs

If Planet 9 lay at a distance 200 - 400 AU, it could be present in the IRAS
Reject File of single HCONs as a triple of single HCONs with two separated by
a few arc minutes and a third at 10-35 arc mins. The signature would be that
the source would only be detected on one HCON pass each time, i.e. there must
be non-detections as well as detections. In the areas which received only 2
HCONs there could be two single HCONs separated by a few arc minutes (see
below). Finally in the case of a faint planet 9 candidate at 280-1000 AU in
the 3 HCON sky there could be two single HCONs separated by 6-25 arc minutes
if the source failed one of its first two HCONs. For completeness I have
explored all of these possibilities.

There are 15162 warm 60 $\mu$m single HCONs (including those undetected at 100
$\mu$m). Many are spurious detections (Scanpi shows no pass with detection
$>3\sigma$), or are clearly extended, due to cirrus. To cut down on the large
number of uninteresting matches with cirrus, extended or spurious sources, I
require the 100 $\mu$m flux or upper limit to be $<$ 3.5Jy. This reduces the
probability of a chance association with a single HCON within 25 arcmin at any
given position on the sky to 7$\%$.

Some single HCONs are main-belt asteroids. From Fig 3 these would be fainter
(and smaller) asteroids, with S60 $<$ 3 Jy, otherwise they would be detected
at 100 $\mu$m and excluded as ‘hot’ movers. Fig 6 shows the distribution of
single HCONs in a 40 sq deg area close to the ecliptic plane in which the
tracks of at least 12 main-belt asteroids can be seen. The triplet of sources
labelled U are due to Uranus. The central source is Uranus itself. The two
much fainter flanking sources appear to be glints from Uranus (they are too
far from Uranus to be the latter‘s brighter moons). In fact of the 88 single
HCONs in this area of the sky as many as 74 can be attributed to asteroids.
These detections of Uranus and asteroids demonstrate the IRAS capability to
detect faint moving objects.

There are 532 pairs of single HCONs at separations 2-25 arcmin. I examined all
of these with Scanpi and found one possible case of interest,
275.30018+7.33028 (Table 1), seen on its first HCON pass but not on the second
six months later. However the Scanpi scans on the paired single HCON show it
to have been detected on two out of four scans, so not a mover.

My triples search, requiring one association at 1-6 arcmin, and a second at
10-35 arcmin, yields 75 triplets, all of which I examined with Scanpi and in
NED. One of these, 314.83499+64.21527, is definitely of interest as a
candidate for a mover and is discussed in the next section.

Finally a search for close pairs of single HCONs, with separations 2-6 arcmin,
yielded 76 pairs (after excluding Saturn glints). These were examined in
Scanpi without yielding any further cases of interest.

Table 1: Unidentified IRAS sources with a missing HCON and with a single HCON partner at separation 1-30 arc min name | RA (1950) | dec | b | S25 | S60 | S100 |  | comment
---|---|---|---|---|---|---|---|---
| (1950) |  |  | (Jy) |  |  | dist(‘) |
PSCz |  |  |  |  |  |  |  |
Q05414+2128 | 85.358749 | 21.475834 |  | 0.27 | 1.04 | 2.79 |  | undetected on 3rd HCON, close to predicted Planet 9 orbit
R05419+2156 | 85.48292 | 21.93889 |  | 0.326 | 0.631 | 2.490 | 28.6 | none of 7 scans show a convincing detections
RIIFSCz |  |  |  |  |  |  |  |
F20189-0408 | 304.745880 | -4.145520 |  |  $<$0.310 | 0.321 |  $<$1.736 |  |
R20195-0430 | 304.88586 | -4.50111 |  |  $<$0.250 | 0.419 | 1.304 | 22.9 | 2M gal at 9 arcsec, zsp=0.1106, not unidentified
F19499-2111 | 297.479980 | -21.197281 |  |  $<$0.195 | 0.340 |  $<$0.894 |  | faint sources, br star at 6 arcsec
R19515-2105 | 297.87750 | -21.09750 |  |  $<$0.458 | 0.565 |  $<$1.000 | 23.0 | br 2M gal at 36 arcsec, zsp=0.03360, not unidentified
F18264-6330 | 276.61043 | -63.50469 |  |  $<$0.113 | 0.497 |  $<$1.666 |  | not really undetected on 3rd HCON
R18242-6346 | 276.06082 | -63.78194 |  |  $<$0.250 | 0.925 |  $<$1.437 | 22.4 | undetected on first two HCONs but poor flux match
QIGC |  |  |  |  |  |  |  |
738014-235334 | 114.50584 | -23.89278 |  |  $<$0.25 | 0.55 |  $<$2.17 |  |
R07378-2349 | 114.46250 | -23.82944 |  |  $<$0.604 | 0.603 |  $<$2.641 | 4.5 | non-detection on first two HCONs unconvincing
1827362+223803 | 276.90085 | 22.63417 |  |  $<$0.25 | 0.56 |  $<$1.43 |  |
R18272+2246 | 276.81000 | 22.77472 |  |  $<$0.250 | 0.448 |  $<$1.274 | 9.8 | radio, br 2M gal at 9 arcsec, not undetected
1HCONs |  |  |  |  |  |  |  |
R18212+0719 | 275.30018 | 7.33028 |  |  $<$0.275 | 0.548 |  $<$1.671 |  | non-detection on 2nd HCON
R18216+0724 | 275.42291 | 7.41306 |  |  $<$0.250 | 0.513 |  $<$2.480 | 9.0 | source seen on 2 out of 4 scans on 1st HCON
R20593+6413 | 314.83499 | 64.21527 |  |  $<$0.250 | 0.558 | (2.509) |  | clear non-detection on 3rd and 4th HCON
R20592+6411 | 314.80624 | 64.18917 |  |  $<$0.250 | 0.627 |  $<$3.251 | 1.7 | clear non-detection on 3rd and 4th HCON
R20562+6408 | 314.07373 | 64.14889 |  |  $<$0.250 | 0.538 |  $<$2.680 | 20.3 | clear non-detection on first 3 HCONs, Planet 9 candidate

Figure 7:  L: Possible orbit for planet 9 candidate for the year 1983
(ecliptic coordinates 2000). The footprint of an IRAS 60 $\mu$m detector is
indicated at the lower right. R: Schlegel, Finkbeiner and Davis (1998)
extinction map centred on location of Planet 9 candidate. The estimated
$A_{V}$ at this location is $>$2 mag.  Figure 8:  Possible orbit of candidate
planet 9 (equatorial coordinates 2000) from 1983 to 2100. Filled circles show
IRAS detections.

###  6.5 A planet 9 candidate

The triplet of single HCONs R20593+6413, R20592+6411, R20562+6408, is
definitely of interest as a candidate for Planet 9. There were in fact four
hours-confirming passes over this area of sky: (1) June 27-28, 1983, (2) July
13, (3) July 23-24 (4) Sept 19-20. The Scanpi detection profile of the 3
single HCONs during these passes is shown in Table 2, which gives the ratio of
the number of detections $>2.5\sigma/$number of possible detections.

HCONs 1 and 2, separated by 1.7 arcmin, appear to be cross-scan split
detections of the same source, detected most strongly on pass 1. Neither were
detected on passes 3 or 4. HCON 3 was detected on pass 4 but not on passes
1-3. So the pattern of detections and non-detections matches that expected for
a slow-moving object, which has moved 20.3 arcmin in 11.9 weeks, or 1.7 arcmin
per week.

Before we consider what the orbit of such an object might be, it should be
noted that the source(s) lie at low Galactic latitude (b = 9 deg), in a region
strongly affected by cirrus (Fig 7R). The detections are not of high quality,
do not show a strong correlation with a point-source profile, and therefore
allow a suspicion of being extended and thus cirrus.

Table 2: Detection profile of the 3 single HCONs during the 4 HCON passes pass | (1) | (2) | (3) | (4)
---|---|---|---|---
R20593+6413 | 4$/$5 | 2$/4$ | 0$/$2 | 0$/$7
R20592+6411 | 6$/$6 | 3$/$5 | 0$/$4 | 0$/$5
R20562+6408 | 0$/$4 | 0$/$6 | 0$/$2 | 4$/$7

The expected orbit of Planet 9 is a combination of a parallactic ellipse with
major axis parallel to the ecliptic plane, semi-major axis
$\alpha(arcmin)=3438/d(AU)$ and semi-minor axis $\alpha cos(\beta)$, and the
proper motion, which we may approximate as a straight line of length
$d^{-1.5}$ per year, with position angle (in ecliptic coordinates) $\theta$.

This very approximate orbit fit gives d = 225 $\pm$ 15AU, $\theta=205\pm
80^{o}$, and centre of the parallactic ellipse at
($\lambda_{0},\beta_{0}$)(2000.0) = (13.62, 71.62) $\pm 1^{o}$, and is
illustrated in Figure 7L. Scanpi signal-to-noise values of all the possible
detections at the locations on the preferred orbit corresponding to the times
of the four HCON passes are shown in Table 3. For a slow-moving source on this
orbit we should ideally see good detections on the diagonal of this array and
none off the diagonal. While the outcome is not ideal, it is quite suggestive.
This is even more suggestive in the plots of individual scans (available
online), where 7 of the 8 off-diagonal scans do not show any evidence of a
source at the central location on the scan, while all of the on-diagonal ones
do. Despite the poorer quality of the IRAS data at this low Galactic latitude,
both the HCON data and the individual averaged detector scans are consistent
with the idea of the detection of a moving object.

Figure 8 shows a possible orbit from 1983 to 2100, calculated using The
Project Pluto package Find Orb. However the IRAS data are not accurate enough
to allow an orbit to be determined and we can only say that at 2100.0 the
candidate object should be within an annulus of radius 2.5-4o, centred on
($\lambda_{0},\beta_{0}$)(2000.0) = (13.62, 71.62).

Table 3: Scanpi signal-to-noise at the locations on the preferred orbit corresponding to the four HCON passes time |  $\lambda$(2000) | $\beta$ | scan (1) | scan (2) | scan (3) | scan (4)
---|---|---|---|---|---|---
1983 June 28 6.7hrs | 14.4214 | 71.5869 | 2.9, 3.1, 2.47, 0, 2.7, 2.8 | 2.7, 0, 2.7, 2.2, 2.47 | 0, 2.1, 0 | 0, 0, 0, 0
1983 July 13 17.6hrs | 14.4063 | 71.6551 | 2.9 2.1 2.4 2.6 2.46 2.8 | 0, 3.1, 0, 2.49 |  3.0, 0 |  3.6, 0, 0, 0, 0, 3.8, 0
1983 July 23 23.4hrs | 14.3722 | 71.6913 | 0, 0, 0, 0, 0, 2.49 | 0, 0, 0, 3.1, 0 | 2.9, 0 | 0, 0, 0, 0, 0
1983 Sept 20 5.7hrs | 13.7554 | 71.8560 | 0,0, 2.2, 0 | 0, 0, 0, 0, 0, 0 | 0, 0, 0, 0 |  3.1, 2.4, 2.3, 2.7, 3.4, 2.6

Figure 9 shows planetary temperature versus radius, with the expected locus of
a 60$\mu$m source of 0.57 Jy (red loci, corresponding to distances of 210, 225
and 240 AU) together with locations of the Fortney et al (2019) Planet 9
models. The strong cirrus in the vicinity of the source make a significant
100$\mu$m detection or upper limit impossible.

The candidate discussed here would be likely to have mass 3-5 $M_{E}$, with
the ephemerides limits (Batygin et al 2019) suggesting a mass at the lower end
of the range. However this candidate is rather different from the Batygin and
Brown (2019) proposal on which the ephemerides limit of Fienga et al (2016),
from which the Batygin et al (2019) constraint is extrapolated, is based. This
candidate is of significantly lower mass and at much higher ecliptic latitude.
It would clearly be desirable to reconsider the ephemerides limits for an
object with this orbit.

Figure 9:  Planetary temperature versus radius ($r_{E}$) for Fortney et al
(2019) planetary models (filled hexagrams). Red curves: loci on which S60 =
0.57 Jy blackbody source would lie, for distance 210 (lower locus), 225, 240
AU.

##  7 Recovery of Planet 9 candidate 1983-present

A planet of mass 3-5 $M_{E}$, radius 1.5-5 $R_{E}$, at a distance of 210-240
AU, should in principle be relatively easy to recover from survey data in the
intervening period. The major difficulty is the closeness of the candidate
planet to the Galactic Plane, making the detection of even a relatively bright
moving object difficult against the very high surface density of stars.

The Palomar Digital Sky Survey (DPOSS) surveyed the entire northern sky in the
late 1980s and early 1990s and released a catalogue but only for $|b|>10^{0}$
(Reid et al 1991, Djorgovski 1999).

Meissner et al (2018) carried out a search for moving objects in the WISE
survey data, but only for 70$\%$ of sky, and not including the area of
interest.

The best dataset for this search is the Panstarrs ‘3 $\pi$’ survey in bands
g,r,i,z,y of the entire sky north of dec = -30o, carried out in 2009-14
(Chambers et al 2019). Approximately half of the gri observations were carried
out in a series of 4 exposures, quads, all separated by about 15 minutes, so
completed within 1 hour. By analogy with IRAS I will refer to these sets of
observations taken within about an hour as HCONs. These observations were then
repeated several times each year over the 5 years of the survey. In one hour
our Planet 9 candidate would have moved about 0.6 arcsec, so would be
registered as a point-source, ie a single HCON, albeit with only 4 detections
out of the target 60 for the survey. Wainscott et al (2015) reported a search
for Near Earth Objects (NEOs) with Panstarrs, but not yet at high ecliptic
latitude or low Galactic latitude.

Mallama et al (2017) give predicted colours and magnitudes for solar system
planets, and we can use these to estimate colours and magnitudes for Neptune,
Jupiter or Pluto -like objects, scaled for r=3.66 $R_{E}$ at a distance 225 AU
, with an uncertainty of $\pm$ 0.3 dex in distance and radius. For Jupiter-
like colours, (g,r,i,z = 16.84, 16.26, 16.41, 16.73), for Neptune-like
colours, (g, r, i, z = 16.79, 16.95, 18.04, 18.90), and for Pluto-like
colours, (g, r, i, z = 18.02. 17.22, 16.99, 16.99), respectively. These
suggest the search can be restricted to (r-i) $<$ 1.0 and (g-r) $<$ 1.5.

I used VizieR to extract Panstarrs single HCONs within 2.5-4.0 deg of the 1983
position. Many are too bright to be of interest, so I restricted the search to
$i>14$. The majority are part of point sources seen on all 4 or 5 years of
observation, so I restricted the search to sources with less than 5
detections. I examined detections for each candidate using MAST. There were a
small number of genuine single HCONs but none form the expected radial line of
HCONs corresponding to the 5 years of Panstarrs observation. Moreover none of
the ‘single HCONs’ in this area of the sky are in fact true quads. Typically
one triplet of gri observations was carried out on a single night, within one
hour, while other repeat observations are carried out 4-10 nights later. Since
in 4-10 days our candidate object would have moved $\sim$ 1-3 arcmin,
different bands would be displaced by this amount and would not be combined
into a single source. However Planet 9 could still have been seen as sets of
triplet detections in a roughly radial line, a year apart in time. A search
for HCON triplets in the expected annulus did not result in any candidates of
interest.

The failure to recover the candidate in the Panstarrs data suggests that this
candidate object is not in fact real, though the fact that the quad strategy
was not implemented in the region of sky of interest makes this conclusion
uncertain. Given the poor quality of the IRAS detections, at the very limit of
the survey, and in a very difficult part of the sky for far infrared
detections, the probability of the candidate being real is not overwhelming.
However given the great interest of the Planet 9 hypothesis, it would be
worthwhile to check whether an object with the proposed parameters and in the
region of sky proposed, is inconsistent with the planetary ephemerides. Fienga
et al (2020) state that a 5 $M_{E}$ planet would have to be at a distance $>$
500 AU to be consistent with the planetary ephemerides, but their simulations
do not appear to include cases at such high ecliptic latitudes as my
candidate. If simulations showed that the candidate here is not inconsistent
with the planetary ephemerides, a more sophisticated search of the Panstarrs
database would be worthwhile.

##  8 Discussion

The analysis here takes the discussion of identifications of the PSCz galaxy
catalogue a little further than Saunders et al (2000), and the identification
of RIIFSCz sources significantly further than Wang et al (2014). 0.3$\%$ of
the PSCz galaxies at $|b|>5^{o}$ (52 sources) remain unidentified, and 3$\%$
of RIIFSCz sources remain unidentified.

I have given a fairly full discussion of the nature of the IRAS 60 $\mu$m
single HCON sources in the IRAS PSC Reject Catalogue. Most are main belt
asteroids, galaxies or cirrus. I do not find a convincing example of an
unidentified PSCz or RIIFSCz source associated with an unidentified single
HCON at a separation of 1-30 arc min, which would be a candidate for a ninth
planet. Iorio (2014, 2016) and Fienga et al (2016, 2020) have already given
dynamical arguments which severely constrain the type of object postulated by
Batygin and Brown (2016).

Searching for close pairs or triplets of single HCONs, I have found a
candidate for a $\sim 3-5M_{E}$ planet at $\sim$ 210-240 AU in the 1983 IRAS
data. It is at very high ecliptic latitude and at low Galactic latitude, which
may explain why it has not been found in previous searches for a 9th planet.
Dynamical studies are needed to check whether such an object is consistent
with the ephemerides of other solar system objects and whether this object can
account for the clustering of the orbits of Kuiper belt dwarf planets. The
IRAS detections are not of the highest quality but it would be worth searching
at optical and near infrared wavelengths in an annulus of radius 2.5-4 deg
centred on the 1983 position. This candidate could be ruled out if radio or
other observations confirmed the reality (and stationarity) of the IRAS
sources at the 1983 1HCON positions.

IRAS had a significant capability to detect a planet in the outer solar system
of mass 10 $M_{E}$ or greater, especially if it had an extended gaseous
envelope. For the 70$\%$ of the sky which had a third IRAS HCON coverage, the
region of Fig 2 to the left of the 60 $\mu$m limit is excluded. However even
within the areas that did not receive a third HCON, Planet 9 would have been
detectable as an unidentified PSCz source (for d $>$ 400 AU) or as a close
pair of single HCONs (for d =200-400 AU). IRAS would have been able to detect
a 10 $M_{E}$ planet to 380-600 AU, and could have detected a 20 $M_{E}$ planet
to 690-1000 AU (with the higher limit corresponding to an extended planetary
atmosphere). The 90$\%$ completeness limit for these searches at 60 $\mu$m is
0.45 Jy for $|b|>20^{0}$. Between $|b|=20^{0}$ and the IRAS mask (see Fig 5),
the completeness limit is 90$\%$ down to 0.6 Jy, and 80$\%$ between 0.5 and
0.6 Jy. These are worthwhile constraints for the Planet 9 search.

##  9 Acknowledgements

This work would not have been possible without the data tools and date
archives provided by IRSA, NED, HEASARC, VizieR and MAST.

##  10 Data availability

All data used in this paper have been taken from public archives.

##  11 Note added in proof

In a paper published after this paper was submitted, Brown and Batygin (2021,
AJ in press) have carried out new simulations of Planet 9 candidates capable
of explaining the clustering of orbits of Kuiper belt dwarf planets. Their
preferred planetary mass, 6.2 +2.2 -1.3 $M_{E}$, and perihelion, 300 +85 -60
AU, are now quite close to those of my Planet 9 candidate. However the
preferred direction on the sky for their Planet 9 object is very far from my
candidate, so my object, if real, may not be the cause of the orbit
clustering.

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