PHYSICS & ASTRONOMY_Protoplanetary Disks

The Turbulent Birth of Stars and Planets

Exoplanets – planets that orbit stars other than the Sun – used to be a matter of science fiction. Some 15 years ago, with the first detection of an exoplanet, they became a matter of observational astronomy. Since then, exoplanet observations have provided astronomers with intriguing clues as to the formation of stars and planets. This is invaluable information for researchers interested in planetary and star formation, such as the team led by Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg. Photo: NASA/JPL-Caltech is why,oncecontractionstarts,there ly torotate,ifonlyeversoslightly.That perfectly motionless;instead,itislike- ity. Typically,sucharegionwillnotbe it beginstocontractunderitsowngrav- such acloudexceedscertaindensity, cades: whenasuitablylargepartof happens nexthasbeenknownforde- nurseries.Inbroadterms,what cosmic billions ofyears,theyhaveactedas lactic home,theMilkyWay,andfor T TEXT beginning toend.Intheir search for that canexplainplanetformation from and thereisasyetnophysical model of years.Butthedevilisin details, mation hasbeenaroundfor anumber come intobeing. how planets,suchasourownEarth, attract eachotherandcoalesce.Thisis ensures everfurthergrowthasobjects meters insize,gravitytakesoverand these objectshavereachedafewkilo- that growcontinuallyinsize.When ing objectsofrock-likeconsistency colliding andstickingtogether, form- swirl, withdustparticlesrepeatedly regions, gasanddustcontinuesto commence: astarisborn.Intheouter and temperaturefornuclearfusionto disk, thegasreachessufficientdensity In thecenterofthisprotoplanetary PREDEFINED CHOREOGRAPHY THE BIRTH FOLLOWS A cloud intotheshapeofadisk. centrifugal forcepullsthecollapsing the interplaybetweengravityand ette. Asthespeedofrotationincreases, her bodyinordertoexecuteapirou- ure skaterwhopullsherarmscloseto of angularmomentum–thinkafig- to whatphysicistscalltheconservation rotation becomesfasterandfaster,due overall size.Butatthesametime, contraction willreducethecloud’s are twosignificantphysicaleffects: This generalpictureofplanet for- THOMAS BÜHRKE be foundthroughoutourga- years insize.Suchcloudscan dust measuringmanylight- begins withcloudsofgasand he birthofplanetsandstars 4 | 11 MaxPlanckResearch MaxPlanckResearch 47 PHYSICS & ASTRONOMY 48 tion department. of theinstitute’s PlanetandStarForma- for AstronomyinHeidelbergandhead a DirectorattheMaxPlanckInstitute what happens,”saysThomasHenning, should haveamuchclearerpictureof –butwhenwearedone, served evaluate andanalyzewhatwehaveob- cellent newdata.Itwilltakeyearsto have supplieduswithawealthofex- Spitzer “The ever morerefinedobservations. answers, astronomershaveturnedto MaxPlanckResearch MaxPlanckResearch and Herschel spacetelescopes 4 | 11

_Protoplanetary Disks a mirrormeasuring3.5metersindiam- successor operation, inMay2009,itsEuropean out ofthecoolantitneedsforproper with warmobjects.Justas for radiationthatistypicallyassociated Spitzer its orbitaroundtheSuneversince. 2003 andhasbeentrailingtheEarthin waslaunchedin Great Observatories, The False-color imagebasedoninfrared datatakenwiththe 8,000 light-yearsfromEarth,andyoungerstars(white-edgedboxnear thetop). at adistanceof A portraitoftwogenerations:Older starsinthecentralregionofobjectRCW which alsofeatureskeycontributions ofexpertiseininfraredastronomy,tory camera Herschel the constructionofonethree such asplanetsandnebulae. infrared radiationemittedbyobjects izes incollectingandanalyzingthe scope yet,andjustlike eter, tronomy playedasignificantrolein The MaxPlanckInstituteforAs- Spitzer SpaceTelescope Herschel isaninfraredtelescope,sensitive PACS instruments,thefar-infrared Herschel –buildingonalonghis- isthelargestspacetele- waslaunched.With Spitzer , oneofNASA’s Spitzer , itspecial- ran and dustwiththenameG011.11-0.12, omers settheirsightsonacloud ofgas phases ofstarformation.”Theastron- work ofthe lier ideal opportunitytobuildontheirear- Henning andhiscolleaguesfoundan When INTO THE HEART OFDARKNESS WITH INFRAREDRADIATION spectroscopic data,”saysHenning. out practicallytheentireanalysisof disks,forwhichwecarried planetary ofproto- program fortheobservation project. “We wereinvolvedinakey research groupandastrictlyAmerican which isquite access to gained theHeidelberggroupgenerous lite. Incidentally, thisexpertisealso to Herschel Spitzer Spitzer Herschel

Space Telescope Spitzer observations intheframe- observations ’s predecessor, the Herschel unusual foraEuropean beganitsoperations, observational data, observational program“Early . 34 , ISO satel-

Photos: NASA/ESA (top); NASA / JPL-Caltech (bottom) Photo: Hardy Müller (right) made useofthisprinciplewhen observ- lengths. Henningandhis colleagues radiation isemittedatinfrared wave- about roomtemperature,most ofthis to minus273degreesCelsius) and lute zero(0Kelvin,whichcorresponds tion, andattemperaturesbetweenabso- physics, allbodiesemitthermalradia- pends heavilyonthewavelength. dust. Whatyouseeofsuchacloudde- the otherhand,canpenetrategasand background.Infraredradiation,on ry showing upassolidblackagainstastar- clouds likethisarecompletelyopaque, tions intheinfrared:Invisiblelight, phases ofstarformation. an idealtargetforstudyingtheearly number ofnewlybornstars,makingit tions hadshownthatG011containsa or G011forshort.Previousobserva- uses tosharethefascinationofastronomywithpublicand,inparticular,students. Max PlanckInstituteforAstronomyinHeidelberg,isinvestigatingtheearlyphasesofstarbirth–althoughnotwiththistele 100 timesasmassivetheSun.ThomasHenning 50 to the MilkyWay.Itcontainsatleastadozenyoungstars,eachofwhichis Heavenly glory:TheCarinaNebula,atadistanceofsome7,500light-yearsfromEarth,isonethemostbeautifulstarformat Following a fundamental law of Following afundamentallawof An idealtarget,thatis,forobserva- tronomers couldpeerevermoredeep- successively longerwavelengths,theas- at still appearsdark,butinobserving lengths: at8micrometers,thecloud some imagesatrelativelyshortwave- ing G011. ious masses.“We canstudyingreatde- star cluster, teemingwithstars ofvar- years, thecloudwilltransform intoa idently, withinthe nextmillionorso ranged from1to240solarmasses. Ev- 12 Kelvin.Themassesofthese cores ly warmerthantheirenvironment,at between 16and26Kelvin,onlyslight- derived fromtheinfrareddata,varied pre- orprotostellarcores. regions ofincreaseddensity, so-called vealed, hiddendeepintheinterior, 24 between 70and350micrometersre- obtained with ly intothecloud.Theuniqueimages The temperaturesofthesecores,as Spitzer Herschel had alreadyprovided atwavelengths external trigger, considerthe starforma- quires some lapse willhappenspontaneously orre- question remainswhethersuch acol- collapsing undertheirowngravity, the already takenthefirststepstoward tions ofstars. the formationofsubsequentgenera- stars, leavingplentyofrawmaterialfor the availablegasmassisturnedinto cal manner. Onlyaround10percent of economi- formation proceedsinavery Another interestingfindingisthatstar IN AHOT GAS BUBBLE 20 says ThomasHenning. beginning ofgravitationalcollapse,” tail thephysicalconditionsatvery YOUNG STARS YOUNG As tothepossiblenatureofsuch an While, inG011,afewregionshave external trigger. 4 | 11 MaxPlanckResearch MaxPlanckResearch scope, whichhe ion regionsin , Directoratthe 49 PHYSICS & ASTRONOMY_Protoplanetary Disks

» Since interstellar gas clouds can span up to several light-years, different regions within those clouds will move at different speeds, leading to turbulent flows of gas and dust.

tion region RCW 34, at a distance of Using spectroscopy, the astronomers particles. Since the disks are heated by some 8,000 light-years from Earth: a were able to determine the age of the the central object, the temperature of large, hot gas bubble containing some stars and stars-to-be in RCW 34. Sur- the disk’s inner regions is significantly 20 young stars. In fact, the stars are like- prisingly, there is a systematic trend, higher than at the outer rim. By the ly responsible for the existence of the with objects inside the bubble being a fundamental laws of thermal radiation, bubble, blowing away the surrounding few million years older than those at this means that infrared radiation from gas with their intense radiation and a the edge. The researchers hypothesize the interior region will be emitted steady stream of particles known as that stars of the first generation in this mostly at shorter wavelengths than is “stellar wind”. Near the top of the bub- nebula pushed matter outward, trigger- the case for the outer zones. In cases ble is a cloud of gas and dust in which ing the formation of a second genera- where emission at shorter infrared additional stars are being born. tion of stars near the edge of the cloud. wavelengths was missing, astronomers concluded that the disks in question PROTOSTELLAR DONUTS must feature a large central hole. Direct evidence for this model was, Are such external influences – candi- however, lacking – at least until last dates include not only young stars, but year, when an international research also expanding clouds of matter result- group headed by Christian Thalmann ing from so-called supernova explo- and Johan Olofsson from the Max sions, in which more massive stars end Planck Institute for Astronomy finally their lives – necessary in order for new succeeded in producing a direct image stars to form? Thomas Henning is con- of such a disk. Thalmann and his col- vinced that external triggers play a sup- leagues utilized a special camera on the porting role at best: “Turbulence in the Japanese Subaru telescope in Hawaii to clouds is perfectly sufficient to cause observe the young star LkCa 15. The 10,000 years affected regions to contract and achieve image showed a disk with a large cen- critical density.” But how does this tur- tral gap. In fact, the gap is so large that bulence come about? it could quite comfortably accommo- Stars and nebula in our cosmic date the orbits of all the planets in our neighborhood orbit the center of our own solar system. home galaxy, the Milky Way, with ob- A second set of observations with jects closer to the center moving faster Subaru, again involving the Heidelberg than their more distant cousins. Since astronomers, led to the discovery of a interstellar gas clouds can span up to smaller type of disk. The disk around several light-years, different regions the star AB Aurigae, at a distance of 470 within those clouds will move at differ- light-years from Earth, is about the size ent speeds, leading to turbulent flows of our solar system. It consists of nest- of gas and dust. ed rings that are arranged somewhat 100,000 years In fewer than one million years, the asymmetrically around the star. protostellar cores of G011 will have con- Another discovery, again by Max densed sufficiently for protostellar disks Planck researcher Olofsson and col- to form around nascent stars. These disks leagues, involves the young star T are the birthplaces of planets. Many oth- Chamaeleontis (T Cha), at a distance er young stars in the galaxy have already of 330 light-years from Earth. Using 1 million years reached this stage, and have been re- the Very Large Telescope of the Europe- warding targets for observation. an Southern Observatory (ESO) in In many cases, the existence of Chile, the astronomers found that a these disks can be deduced only indi- portion of the disk material has formed rectly from characteristic infrared radi- a thin dust ring at a distance of just 20 10 million years ation emitted by their constituent dust million kilometers from the star –

Stages of stellar evolution: First, a nearly spherical cloud collapses. Since it rotates, centrifugal forces will pull it apart, resulting in a disk. In the center, matter collapses further to form a star, while the disk becomes the birthplace of planets. 100 million years Graphic: Subaru

50 MaxPlanckResearch 4 | 11 Insights: Using the Japanese Subaru telescope in Hawaii, Heidelberg-based Max Planck astronomers and their colleagues have managed to capture astounding images of protoplanetary disks, such as the ring-shaped disk around LkCa 15 (above) and the smaller disk around the star AB Aurigae with its much more complicated structure.

about one-third the orbital radius of “That, of course, is the most interest- carbon dioxide in their atmospheres. Mercury, the planet closest to our Sun. ing possibility,” says Thomas Henning. But they have not yet succeeded in pin- Behind this ring extends a wide, dust- Early this year, an international ning down the evolutionary link – a free region, up to about 1.1 billion ki- team actually found evidence for this young planet inside the disk that lometers from the star. That is where latter scenario. Near the outer edge of served as its birthplace. the outer part of the disk begins. the large gap in the disk around T Until recently, the astronomers had Chamaeleontis, they found what could observed only the dust component of CLOSE TO THE be an orbiting body. It is located about protoplanetary disks. But this accounts LIMITS OF OBSERVATION a billion kilometers away from its cen- for only about a hundredth of the to- tral star, a distance comparable to that tal mass. Most of the disk mass is in Even before these direct sightings of between Jupiter and the Sun. But these the form of gas, which is very difficult donut-shaped disks, astronomers had observations are at the limits of what is to observe, since the radiation emitted discussed the possible causes for the technically feasible for the astronomers. by the atoms and molecules is so ex- formation of such gaps. One possibili- They were thus unable to determine be- traordinarily weak. ty involves the central star’s radiation yond all doubt exactly what this object Yet the gas plays a major role in the causing the dust particles to evaporate: is. It may be a newly formed planet. development of the overall disk and in Alternatively, a nearby (and as yet un- That planets grow in protoplane- the formation of planets. For instance, it discovered) star could have swept up tary disks is, by now, an undisputed exerts friction on the tiny dust particles, the dust. The most likely option, how- fact. In the case of some older stars, sci- slowing them down at different rates de- ever, is that a planet has formed entists have managed to observe a few pending on particle size. The resulting around the star, sweeping up gas and exoplanets directly. In a few cases, it different speeds of the particles increase dust and, in the end, incorporating was even possible to detect substances the likelihood of collisions, thus accel-

Photos: Subaru (3) whatever material it has attracted. such as water, sodium, methane and erating the growth of the particles. >

4 | 11 MaxPlanckResearch 51 PHYSICS & ASTRONOMY 52 disk matterlargelydisappeared with- in parallel,andratherquickly atthat: gas anddustdecreasedalmost exactly found wassurprising:theamounts of quantities inthedisks. years, theydeterminedthedust andgas tween about1millionand20 al starclusterswithmembersagedbe- tricks toaddressthisquestion.Insever- deep intotheirbagofobservational planet formation. strong constraintonthetimescaleof eous disks.Theresultwouldimposea ing thetypicallifetimesofsuchgas- interestedinlearn- tronomers arevery atmospheres. Forthesereasons,as- of gaswhenformingtheirenormous urn, whichgathersignificantamounts of giantplanets,suchasJupiterandSat- Furthermore, gasisthekeyingredient planets anddisks. plays akeyroleinthesearchforextrasolar of unparalleledsharpness.Adaptiveoptics turbulence, resultinginground-basedimages that canmitigatetheeffectsofatmospheric ments, includingadaptiveopticssystems in developingandconstructingnewinstru- astronomers andengineersarealsoinvolved At theMaxPlanckInstituteinHeidelberg, MaxPlanckResearch MaxPlanckResearch The astronomersinHeidelbergdug 4 | 11

_Protoplanetary Disks What they the youngplanetsunharmed? they possiblymakeittothesurfacesof ent inthecircumstellar disks,andcan es ofplanetformation?Aretheypres- theearlyphas- molecules oflifesurvive answered: Canthecomplexprecursor leaves highlyinterestingquestionsun- This compounds havebeenobserved. disks, where,sofar, nosuchchemical stances. Notsointheprotoplanetary cules, includingcomplexorganicsub- far foundaround150typesofmole- stellar clouds,astronomershavethus on Earthandelsewhere.Inlargeinter- on questionsabouttheoriginsoflife nents ofdisksthathasadirectbearing the analysisofgaseouscompo- Last butnotleast,thereisanaspectof INTERSTELLAR CLOUDS 150 igins, aswell. or- the timescaleofourownplanetary solar system,itimposesstrictlimitson ply equallytotheformationofourown And, asallthesestatementsshouldap- planet formation,”explainsHenning. strictlimitsonthetimescaleof very in, at by theEuropeanSouthernObservatory (ESO). ranal intheChileanAndes,andisoperated situated atopthe2,635-meter-high CerroPa- scope Interferometer ultra-high resolution–the effect creatingasinglevirtualtelescope with ed bythesetelescopescanbecombined,in ble lightuptothemid-infrared.Lightcollect- The VLTisdesignedforobservationsinvisi- plemented byseveralauxiliarytelescopes. mirror measuring8metersindiameter,com- An arrayoffourtelescopes,eachwithamain Very LargeTelescope(VLT) globular clusters. gion (theso-calledhalo)thatcontains150 central bulgeissurroundedbyasphericalre- the MilkyWaysitsagiganticblackhole.The about 1,000light-yearsthick.Attheheartof years. Viewedfromtheside,diskisonly with adiameterofabout100,000light- viewed fromthetop,wouldlooklikeadisk clouds. Itfeaturesseveralspiralarmsand,if suns, aswellinterstellargasanddust gala The MilkyWayGalaxy(fromtheGreekword to whichourownplanetarysystembelongs. The collectionofbillionsandstars Galaxy GLOSSARY TYPES OFMOLECULESIN most, 10millionyears.“Thatputs formilk)consistsof150to200billion (VLTI).Theobservatoryis Very LargeTele- Space Telescope in thecomingyears. and willgraduallybeputintooperation currently underconstructioninChile moved intodifferentpositions.ALMAis separate radiotelescopesthatcanbe pound telescopeconsistingofnearly70 limeter Array scopes, notablythe their hopeinthenextgenerationoftele- disks.Nowtheyareputting toplanetary as carbonmonoxideandwaterinpro- simplemoleculessuch aged toobserve baby stepstowardthatgoal:theyman- my haveatleastbeenabletotakesome at theMaxPlanckInstituteforAstrono- as some sensitiveradiotelescopes,aswell of today’s telescopes.However, using blocks oflifeliesbeyondthecapabilities Detecting themorecomplexbuilding observational surprise. observational pected tobegoodformorethanone medium infraredrange–andisex- instrument, whichwillworkinthe veloping keycomponentsoftheMIRI are involvedinthisproject,too,de- delberg-based MaxPlanckresearchers shift incosmicperspective.TheHei- the intendedsuccessorto The James WebbSpaceTelescope birthplace offutureplanets. known asaprotoplanetarydisk:the will typicallycontracttoformwhatis tion. Aroundsuchaprotostar,matter is aprotostarthatemitsinfraredradia- gravitational energyintoheat.Theresult and contractseverfurther,transforming velops, whichattractsadditionalmatter of massknownasaprotostellarcorede- own gravity.Ultimately,aconcentration gas anddustthatcollapseundertheir Stars forminsideinterstellarcloudsof Protostellar cores JWST canstartasplannedin2018. NASA. Itisquestionablewhetherthe hold bytheAmericanspaceagency, sons, theprojectwasrecentlyput on galaxies. However,duetofinancialrea- the-making, aswellthemostdistant frared lightfromplanetarysystems-in- space thanitspredecessor,receivingin- ror, itissettopeermuchdeeperinto Space Telescope Spitzer Hubble James WebbSpaceTelescope and ’s successor, the . Withits6.5-meter-mir- (ALMA). ALMAisacom- Herschel , promisesanother Atacama LargeMil- , theresearchers

(JWST)is Hubble James Webb

Photo: Hardy Müller