All_About_Space_-_Issue_94_2020

expandedpopulationatjusttherightdistancefrom

thestar,whileatthesametime– bycapturing

a significantfractionofthestar’semittedlight–

yieldinga virtuallyunlimitedsupplyofenergy.

Soit’snotbeyondtheboundsofpossibility

thata sufficientlyadvancedcivilisationmight

createa megastructure.Thenextquestionis

whetherwecoulddetectsucha thingwithour

currentastronomicalinstruments.Thisties

intotherelativelynewastronomicalconcept

of‘technosignatures’– observableindications

ofadvancedextraterrestrialcivilisations.It

encompassesactiveemissionslikeradiowaves

andlasercommunications,aswellasmoreindirect

evidencethatmightindicatethepresenceofthings

likeDysonspheres.Dysonhimself,picturinga

more-or-lesscompletesphereenvelopinga star,

realisedthatitwouldabsorbmostofthelightfrom

thestarandreradiatethewasteenergyasheat–

asinfraredradiation.Sohesuggestedsearching

forstarswhichproduceanunnaturalamountof

infraredcomparedtovisiblelight.

Ontheotherhand,whatifthemegastructureis

anincompletesphere,orsomeothershapesuch

asa ring?Thisquestionwasthoughtthroughby

FrenchastronomerLucArnoldin2005,andthe

resultswrittenupina paperhecalledTransit

Light-CurveSignaturesofArtificialObjects. Arnold

drewananalogywithoneofthetechniques

astronomersusetofindplanetsaroundotherstars,

knownasthetransitmethod.It’sa methodthat

was Tabetha Boyajian – a fact

that resulted in KIC 8462852

acquiring the much nicer

nickname of Tabby’s Star.

There were two very odd

things about Tabby’s Star’s light

curve.The dips in it – presumably caused

by something transiting between the star and

us – occurred at random times, not at the regular

intervals that would be expected in the case of

an ordinary planet. And those dips were huge,

representing a dimming of the star’s light by as

much as 22 per cent. For comparison, a large,

Jupiter-sized planet would only cause a dimming

of 1 per cent.

That first paper put forward a number of possible

explanations for the large, irregular dimming,

ranging from a massive cloud of debris from a

planetary collision to a huge swarm of comets.

It was left for a follow-up paper the next month

by a research group from Penn State University

led by Jason Wright, to broach the topic of alien

megastructures. Once made, however, the speed

with which that suggestion was snapped up by the

mainstream media was eye-watering. Throughout

October 2015 the press was abuzz with attention-

grabbing headlines about the newly discovered

‘alien megastructure star’ in every imaginable news

outlet, from The Daily Express to The Guardian.

Wright was quoted in the latter as saying: “Aliens

should always be the very last hypothesis you

Which star is

the weirdest?

Bycomparison,HD 139139 may

justpipTabby’sStartothetitle

Whyisitweird?

Tabby’sStar’slightcurveshowsverydeep,

irregulardips,whileHD139139’sarelessdeep,

moreplanet-likeandjustasrandom.Both

observationswereunexpected.

Weightofevidence

Tabby’sStarhasbeenthesubjectofnumerous

independentstudies,buttodateonlyonestudy

hasfocusedonHD139139.

Coulditbeanalienmegastructure?

WhateverisorbitingTabby’sStar,itprobablyisn’t

a largesolidobject.ButHD139139’srandomdips

dolookliketheycomefromsomethingsolid.

Whatotherexplanationsarethere?

Severaldifferenttheorieshavebeenput

forwardthatmightexplainTabby’sStar,butthe

behaviourofHD 139139 remainsa mystery.

Scientificconsensus

Astronomersseemtobeconvergingontheidea

ofa dustringaroundTabby’sStar– butthere’sno

consensusyetonHD139139.

receiveda boosta fewyears

afterArnold’spaper,when

NASAlauncheditsplanet-

huntingKeplerspacecraft,which

appliedthetransitmethodonan

industrialscaleforalmosta decade

between 2009 and2018.Thebasicidea

issimpleenough:whena planettransits in front of

itsparentstar,weseea slightdimmingof the star’s

lightcurve.

Thebeautyofthistechniqueliesinthe fact that

thedimmingeffectproducedbya planet is well

understoodscientifically.Planetsmovein clearly

definedorbits,whichmeansthedimming occurs

atregularlyspacedintervals.Thephysics of orbits

iswellknown,soit’seasytoworkoutthe planet’s

distancefromthestarbasedonthetiming of its

orbit.Asfortheshapeanddepthofthedips, which

shouldbeessentiallythesameoneachorbit – these

cantellusthesizeoftheplanet.But,asArnold

pointedoutin2005,noneofthiswillbe true for

a complex,artificialmegastructure.Ifwe observe

a starwithtransit-likedipsthatcan’teasily be

explainedintermsofa planet– that’swhen an alien

engineeringprojectbecomesa distinctpossibility.

That’sexactlywhathappenedinthecase of the star

labelledKIC 8462852 intheKeplercatalogue.

TheresultswerefirstreportedinSeptember

2015 ina paperthatcameoutofa citizen science

initiativecalledPlanetHunters.Thecoordinator

ofthatinitiative,andtheleadauthoronthe paper,

Above:

Freeman

Dyson was the

first person

to speculate

seriously

on alien

megastructures

in 1960

Right

(clockw ise):

Artist’s

impression of

a hypothetical

semi-complete

‘Dyson sphere’

around

Tabby’s Star

Tabby’s

Star shows

a complex

pattern of dips,

which are given

whimsical

names

The most-

favoured

explanation is

that an uneven

ring of dust

orbits around

Tabby’s Star

© Science Photo Library

© ioerror

The weirdest star