Australian Sky & Telescope — November-December 2017

22 AUSTRALIAN SKY & TELESCOPE November | December 2017

LEAH TISCIONE /

S&T

, SOURCE: E. BAILEY ET AL. /

ASTRONOMICAL JOURNAL

2016 (152:126)

planetbutremaininginsideitsorbitatalltimes.Bothof

thesearrangementswouldbestable.

Theplanet,ifmassiveenough,couldalsoexplainanother

SolarSystemquandary.Fordecadesplanetaryscientistshave

wonderedwhytheaverageplaneoftheplanets’orbits(known

as theinvariable plane)isinclinedtotheSun’sequatorbyabout

6°.Adistantplanetofroughly10Earthmasses,withanorbit

highly inclined to the ecliptic, could produce most of the tilt.

Perhapswe’renowinasituationsimilartowhenLeVerrier

predictedNeptune’slocationbasedonUranus’speculiar

motion. But astronomers are still very cautious about the

planethypothesis,andwhilemostbelieveithasmerit,there

couldalwaysbeabetterexplanationwehavenotyetthought

of.Somestillsuspectthatpoorlyunderstoodobservational

biasesarethecause,andwe’llneedtofindadditionalextreme

objectstoknowforsure.Thisisscienceatitsfinest.

Theraceison

Thebestwaytoshowtheplanetexistsistofindit.Successwill

dependonthisobjectappearingbrightenoughforourcurrent

telescopes to pick it up, which relies on three big unknowns.

Firstisitssize.Inordertoproducetheeffectsweseein

thedata,theplanetmustbemoremassivethanEarthand

perhapsevenmorethanNeptune.Manyrecentanalyses

suggest the planet is on the more massive end.

Second is itsalbedo,orreflectivity.Theobject’ssurfacecould

be as dark as coal or as bright as fresh snow. This factor is a

complete unknown, and it depends highly on the composition

andhowextensiveanatmospheretheplanetmighthave.

Finally,theplanet’sdistanceiscrucial.Anobjectthat

shines by reflected sunlight appears only^1 / 16 as bright at any

particular distance as it would if it were only half as far from

the Sun.

Iftheplanetisonthefaintendofallthreeofthese

unknowns—smaller,darkerandfarther—itmight not even

be easily detectable by our largest optical telescopes. If it were

onthebig,brightandcloseend,weprobablywould have

founditbynow.Mostlikelytheplanetissomewhere in the

middle,makingitbetween23rdand25thmagnitude, faint

butobservablewithpossiblyamedium-classprofessional

telescope(4minaperture)andlikelywithalarge-class

telescope(6.5to10m).Ourcurrentall-skysurveys have only

used 2-m or smaller telescopes, which can efficiently cover

the sky to only 22nd magnitude.

The planet could be anywhere in the basic orbit proposed

in2016,asearchareathatencompassessomethousands of

squaredegrees—some20,000fullMoonsofsky. Yet time on

largetelescopesislimited,andanall-skysurveyis impossible.

So we need to limit the playing field. Simple physics and

basic reasoning can help. The eccentric orbit predicted for the

planet means that it will spend most of its time near aphelion,

its orbit’s farthest point. Also, Brown and Batygin have shown

that many of the past large, shallow surveys should have

foundtheworldunlessitisnearaphelionandthus too faint.

Thisconclusionhassupportfromasurprising source. If

theplanetwerelargerthan10Earthmassesandsituated

withinafewhundreda.u.oftheSun,NASA’scareful

tracking of the Cassini spacecraft would have likely revealed

disturbances in Saturn’s orbital motion — disturbances that

have not been seen during Cassini’s 13-year exploration of

Saturn. Thus either the planet is on the less massive end or it

isnearaphelion,overseveralhundreda.u.away.

Ifitsperihelionisindeedanti-alignedwiththose of the

main extreme objects, then the planet’s aphelion would be

intheSeptembertoJanuarysky,inorneartheconstellations

Orion,Taurus,EridanusorCetus.Mostsimulations of

potential orbital resonances with the extreme objects also put

theplanetnearitsaphelionintheOctobertoDecember sky.

◗^ If the hypothesised planet is on the very massive end

of the scale, it likely has a significant atmosphere. Such

an atmosphere might still be radiating heat left over

from the world’s formation billions of years ago. This

thermal glow would make the planet bright at infrared

wavelengths from about 5 to 20 microns. NASA’s

WISE spacecraft recently completed an all-sky survey

at around these wavelengths, and although Kevin

Luhman (Penn State) and others have searched these

observations for giant planets in the outer Solar System

and found none, the planet could still be hiding in WISE’s

archives. You can become a citizen scientist and search

the spacecraft’s data even further using the tools at

zooniverse.org, in a project called Backyard Worlds.

Plane of Sun’s equator

Lx

Lp

LSun

ΔΩ

6°

Planet

s’o

rb

its

PlanetX

STILTED SUN?The plane of the known planets’ orbits is askew relative

to the Sun’s equator by 6°. A massive planet on a large, elongated, highly

inclined orbit could cause most of this tilt, pulling on the planets and

making everything wobble like a big top over billions of years. Here,L

indicates orbital angular momentum (essentially, an axis perpendicular to

each orbital or rotation plane), and the blue arrows show the change in

the orbits’ inclinations with respect to the plane of the Sun’s equator. The

red arrows indicate the change in the longitude of ascending node for

both Planet X and the giant planets, and the green angle is the difference

between those two, which changes with time. Dotted lines denote where

an orbital plane passes beneath the reference plane.

Join the hunt for Planet X

PLANET QUEST