http://www.skyandtelescope.com.au 25MINI-NEPTUNE: DAVID A. AGUILAR / CFA; FLARE: SOLAR DYNAMICS OBSERVATORY / NASA; DETAIL WORK: ORBITAL ATK
habitable zone, one whose atmosphere
follow-up observations reveal has water
vapour and oxygen. But there are several
other questions TESS data could help us
answer.
One of the most perplexing questions
concerning exoplanets today, for example,
is the composition of the so-called mini-
Neptunes,exoplanetstwotothreetimes
thesizeofEarth.Whencombinedwiththeslightlysmaller
super-Earths,mini-Neptunesappeartobethemostcommon
planetinourgalaxy,yetwehavenocounterpartintheSolar
System.Wethinktheseplanetsaretoolargeandtoofluffy
tobepredominantlymadeofrock,buttheycan’tbeJupiter-
likegiants,either.Sincewedon’tknowwhatthey’remadeof,
thereisnoconsensusonhowtheymightform.
ThehopeisthatTESSwilluncoverdozensofmini-
Neptunes around a variety of nearby host stars. We will then
be able to combine the exoplanet radii measured from transit
observations with masses from follow-up radial-velocity
studies to discover these worlds’ average densities. Together
with atmospheric measurements, the densities might reveal
patterns that help us understand mini-Neptunes as an
ensemble. This data-fusion approach may yield satisfying
resultsonavarietyofothertopics,too.
Flaresareanotherforefronttopicinexoplanets,because
manyMdwarfstarsareactiveandflarefrequently.Trappist-1,
forexample,flaredsome40timesinan80-dayobservational
period; Proxima Centauri has even more frequent
outbursts.We’llbeabletosearchtheentireTESSdataset
for stars that show evidence of both planets and flares at
opticalwavelengths.This,togetherwiththeatmospheric
compositionsoftheseplanetsandotherstellar-activity
indicators, may help shed light on whether active stars
completely erode rocky exoplanets’ atmospheres or not.Tr a n s i t s a r e t h e f u t u r e
TESSisbynaturea‘cataloguemachine’:Itsmaingoalisto
createabiglistoftransitingexoplanetsorbitingbright,nearby
stars. That might sound boring, but it’s not. By creating this
catalogue,TESSwillhelpopenthedoortoabreathtakingnew
era in exoplanet science. Exoplanet science’s first era, that
of 1995–2009, saw the discovery and initial characterisation
ofindividualobjects;inthenextdecade(2010–18),Kepler
usheredintheeraofbigdataandexoplanetstatistics.
TESSwillenableustomergethetwo,fosteringthestudyof
individual exoplanets’ characteristics on a big-data scale.
Space-basedexoplanettransitsurveysaresopopularan
endeavour,infact,thatastronomershaveseveralmorein
thepipelineonarangeofscales.Onthesmallendthere’s
theMITandJetPropulsionLaboratory’sCubeSatASTERIA,
whichisaprototypeforafleetofCubeSatsthatwouldsearch
forexoplanettransitsaroundtheverynearest,verybrightest
Sun-like stars. It launched in August 2017 and began working
inNovember2017.Onthelargeend,there’sthejointSwiss
andEuropeanSpaceAgencyCHEOPSmission(launch2018),
whichwillfocusononestarwithatransitingexoplanetata
time,andESA’sPLATO(launch2026),acompoundtelescope
with26camerasaffixedtooneplatformthataimstofind
longer-periodplanetstransitingnearby,Sun-likestars.
Despitethehugeamountofactivity,transitsarethefirst
partofalongerjourney.Onlyplanetsfortuitouslyaligned
will show up as transits, but this alignment not only limits us
tosystemstiltedjustrightwithrespecttoourperspective,it
alsofavoursplanetsclosetotheirstars(becausegeometrically
they’remorelikelytocoverthestellardisk).Thusthe
technique leaves out worlds in larger or non-transiting orbits.
Thesesystemswillhopefullybecaughtupbyprojectssuchas
NASA’s WFIRST microlensing survey (launch mid-2020s) and
bothground-andspace-baseddirectimagingcampaigns.But
transitsurveyslikeTESSareasupremesteppingstoneonour
waytounderstandingourgalaxy’splanets.SARA SEAGERis the TESS mission’s deputy science director
and a professor at MIT, which is one of eight partners on the
project. Learn more about the mission attess.mit.edu.SDATA-DRIVEN ANSWERS TESS’s observations may help
astronomers determine the compositions of mini-Neptunes (artist’s
concept of Kepler-19b at left) and how planets fare around stars that
release frequent, powerful flares.SDETAIL WORK Visible in this lab shot are TESS’s four cameras (top),
which appear to sit like eggs inside the nest of the sunshield.