SPAT IUMPublished by the Association Pro ISSI

No 6, October 2000

INTERNATIONAL SPACESCIENCEINSTITUTE

From Dust to PlanetsFrom Dust to Planets

The study of earth-like planetsoutside our solarsystem is one of thegreat scientific,technological andphilosophicalundertakings of our time.

The study of earth-like planetsoutside our solarsystem is one of thegreat scientific,technological andphilosophicalundertakings of our time.

Chance or necessity? Careful ob-servation of our world provides usa fascinating picture where chanceand necessity play the key role.Weather evolution may be one ofthe most prominent examples inour daily life: While short-termforecasts have become fairly pre-cise, thanks to a wealth of datagathered by spacecraft andground-based stations and power-ful computers processing thesedata, long-term forecasts remain adream, because non-linear, chaot-ic processes become dominant.Interestingly, teachers at everylevel tend to carefully avoid thetopic of chaos, perhaps becausethey prefer the rigour of order tothe freedom of chaos . . . But orderis only half of the truth, as weshould have learnt in the last 150or so years since the publication ofthe “On the Origin of Species" byCharles Darwin.

The orbits of planets around acentral star are another exampleof a short term deterministic andlong term chaotic process. Whileit is possible to predict for exam-ple the orbit of Saturn and itsmoon Titan over a decade –which is of crucial importance forthe common E SA / NASA mis-sion Huygens /Cassini, which iscurrently on its seven years jour-ney to Saturn and Titan – it is notpossible to investigate the forma-tion process of our solar systemsimply by extrapolating back overbillions of years to the era whenour sun began to shine.

In recent years, it has become pos-sible to observe – at least indirect-ly – planets circling around other

stars. M. Mayor and D. Queloz ofthe Observatory of Geneva werethe first to announce such a spec-tacular finding. Observing andcomparing solar systems yields anew picture of the star and planetformation process, which itself isthe product of interaction be-tween chaos and necessity. It isthis topic to which the presentissue of S PATI UM is devoted.Prof. Willy Benz of the Physi-kalisches Institut of the Universityof Bern gave the members of ourassociation a fascinating lecture onthe subject of planet formation.We are pleased to submit here-with the revised version of his lec-ture to our readers.

Bern, October 2000Hansjörg Schlaepfer

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Impressum

S PATI UMPublished by theAssociation Pro ISSItwice a year

Association Pro ISSIHallerstrasse 6, CH-3012 BernPhone ++41 31 631 48 96Fax ++41 31 631 48 97

President Prof. Hermann Debrunner, University of BernPublisherDr. Hansjörg Schlaepfer, Contraves Space AG, ZurichLayoutMarcel Künzi, marketing · kom-munikation, CH-8483 KollbrunnPrinting Druckerei Peter + Co dpcCH-8037 Zurich

FrontcoverBirth and death of stars in theNGC 3603 nebula (W. Brander/JPL/IPAC, E.K.Grebel / U.Wash.,You-Hua Chu / UIUC, HST/NASA)

INTERNATIONAL SPACESCIENCEINSTITUTE

Editorial

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Introduction

While the search for planets andlife outside the solar system hasbeen going on for decades, the dis-covery in 1995 of the first extra-solar giant planet by GenevaObservatory astronomers MichelMayor and Didier Queloz fol-lowed within months by the dis-covery of 2 new giant planets byGeoffrey Marcy and Paul Butlerof Lick Observatory (USA) hassparked a real revolution. Fiveyears later, over 50 such giantplanets have been found (Figure 1).implying that at least 3–5% of allsun-like stars have giant planets.

Since this represents only the frac-tion of stars having planets thatcould be detected with current in-struments, we must conclude thatplanet formation is not an extraor-dinary event but rather quitecommon occurrence.

Our solar system forms the basisfor most of our information abouthow planetary systems must de-velop. However, the degree towhich it is actually representativeof all planetary systems is unclear.It now appears to be very differentfrom all those discovered thus far.Indeed, contrary to the giant plan-ets in our own system (Jupiter,Saturn, Uranus, Neptune), thenewly discovered planets havemuch smaller orbits many withsizeable eccentricities. Althoughthere is clearly a strong observa-tional bias against detecting dis-tant and/or small planets, it is sig-

nificant that none of these newlydiscovered objects should haveexisted according to conventionalformation theory!

Does this mean that our solar sys-tem is unusual or maybe evenunique or have we simply not yetbeen able to detect the right kindof systems elsewhere? To answerthis central question requires get-ting a full measure of the possiblediversity between existing systemsas well as a much better under-

standing of the physical processesunderlying planet formation andevolution.

From Dust to Planets *)

Willy Benz, Institute for Physics, University of Bern

Figure 1Mass of the extra-solar giant planets discovered thus far. Since more massiveplanets are easier to detect, the paucity of massive objects indicates that jupiter-sized(or smaller) objects are by far the most abundant ones.

*) Pro I S S I lecture, Bern, 3rd November 1999

Planetformation: Theconventionalpicture

Planets are likely nothing elsethan a by-product of star forma-tion stemming from the necessityof conserving angular momen-tum. Indeed, stars form throughthe gravitational collapse of inter-stellar matter over more than 8orders of magnitude in size. In thepresence of rotation and /or mag-netic field, the collapse must resultsin a star /disk structure with thestar having most of the mass andthe disk most of the angular mo-mentum. Thanks to the incredibleresolution of the Hubble SpaceTelescope (H ST) , a few of thesedisks orbiting nearby young starscould even be imaged (Figure 2).

In the case of the solar system, thedisk is generally taken to have amass of a few percent of a solarmass and to be less than 100 AU(1 AU is the distance between theEarth and the sun) in size.

Planets subsequently form in thisdisk probably mostly through col-lisions at first between dust grainsand as time goes by between larg-er and larger bodies. Earlier theo-ries in which planets formthrough the gravitational collapseof patches in the disk have grownout of favor. The flow diagram inFigure 3 illustrates these concepts.

This picture provides a simple ex-planation why all planets in oursolar system not only orbit the sunnearly in the same plane but alsoin the same direction. The nearlycoeval formation of the planetsand other small bodies and the sunis actually supported by compar-ing the ages of the oldest Moonrocks and the sun.

The basic challenge of planet for-mation consists therefore of assem-bling in a disk orbiting a centralstar micron-sized or smaller dust

grains in bodies with over 104 kmin diameter (Figure 4), a growth bynearly a factor 1013 in size or 1040

in mass! Since giant planets aremainly gaseous planets, their for-mation must take place while gassupply lasts. From studies of disksaround other young stars, it is be-lieved that typical lifetime of disksare of order a few million years.Hence, as paradoxical as it sounds,giant planets must be formed inless than ten million years whileforming terrestrial planets maytake much longer.

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Figure 2Images of circumstellar disks around a few nearby young stars imaged by HST.Note the the bipolar jets emerging on either side of the disk.

HH 30

200 AU

DG Tau B

Haro 6-5B HK Tau

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Figure 3Planetary formation as a by-product ofstar formation. In the standard model,the planets form entirely through colli-sions (solid line) while in other modelsgravitational collapse is invoked at vari-ous stages of the formation process. Themain physical processes at play areshown in red.

Figure 4The challenge of planetary forma-tion: Assembling micro-meter dustgrains in planets through collisions in anamazingly short timescale.

On their path to becoming a plan-et, dust grains reach the size ofcomets and asteroids which, ifthey can avoid being incorporatedin a larger object, are left behindlike crumbs on a table after a goodmeal.

The early phases: The first million year

The growth of planet-sized bodiesin this disk is thought to occur es-sentially through collisions. Earli-er models relied on gravitationalinstabilities in the dust layer torapidly grow objects several kilo-meters in size (dotted line in Figure3). However, it has been pointedout that the velocity shear be-tween the dust and the gas will stirup the dust sufficiently to makeinstabilities impossible. While theextend of this turbulence of thedust layer is still debated, colli-sional growth from the smallestsizes on has become the favoritescenario.

At first, dust grains collide at rela-tively gentle velocities which aredetermined by size and shape de-pendent gas drag. As bodies growlarger, the importance of gas dragdiminishes to vanish completelyby the time bodies reach severaltens of meters in size. With subse-quent increase in mass, the colli-sional cross section of these plan-etesimals increases due togravitational focusing yielding tothe so-called runaway growthphase during which the largerbodies sweep-up all the smallerones within their gravitationalreach.

This phase is not without prob-lems. Laboratory experimentshave shown that at the very smallscale dust aggregates readily (Fig-ure 5).

On the very large scales, variousimpact simulations have shownthat self-gravity will ensuregrowth. However, the situation ismuch less clear for objects rangingin size from a centimeters to kilo-meters since in this size range noreal “sticking" mechanism has yetbeen found. Indeed, at this size,the forces operating at the micronsize level are no longer effectiveand gravity is still much too weak.

The escape velocity of a 1 meter-sized rock is of order 1 mm /swhile the typical collisional velcitybetween these objects is of order100 m /s. Hence, for sticking thebodies involved have to be able todissipate all but 10–10 of the in-coming kinetic energy. Whetherthis can be achieved by purelymechanical structures or requiresthe presence of a “glue" whithspecial visco-elastic properties re-mains to be seen.

Figure 6 summarizes the main threestages of growth, the relevant phys-ical mechanism operating and themain study tools.

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Figure 5Aggregate obtained in laboratory dust coagulation experiments by J. Blumand collaborators. The scale is given by the 10 µm black line.

The late phases: 100 million years

This early planetary accretionphase was, over a few millionyears, replaced by an even moreviolent period when growingbodies encountered one anotherat increasingly high velocitiesboosted by mutual gravitationalinteractions. This phase last foranother 100 to 200 million yearsuntil all remaining bodies havebeen swept up by the planets.Collisions occur in a random fash-ion involving objects of differentmasses, structures, compositionand moving at different speeds.Thus, this phase of planet forma-tion must not be viewed as a mo-notonic process by which materialis incrementally added to a grow-ing planet. Instead, accretion mustbe viewed as a long chain of sto-chastic events in which non-dis-ruptive infall exceeds, over time,violent dispersal.

The so-called giant impacts inwhich proto-planets of compara-ble size collide represent the ulti-mate in violence during planetaryaccretion. While they can lead tothe total destruction of the planetsinvolved they can also leave scaresarguably the best evidences re-maining today of such a violentpast. The Earth's Moon, for exam-ple, is believed to originate fromthe debris ejected after such agiant impact and subsequently re-accreted in Earth's orbit (Figure 7).

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Figure 6Planetary growth stages. In green the main physical mechanism ensuring growthand in blue the main tool used to study these phases. Note that the growth mecha-nism in the meter size range is still unknown.

Figure 7The Moon may have originated from the debris ejected in orbit following a giant im-pact of the sort depicted in this painting by W. Hartmann.

Simulations of both impact andre-accumulation have not onlyshown that such a scenario is pos-sible but have made it today's fa-vorite theory of lunar origin.Studies of lead and tungsten iso-topic composition of the silicateEarth have even allowed to datethe giant impact to about 50 mil-lion years after the start of thesolar system!

Mercury's anomalous composi-tion can also be explained in termsof a giant impact which ejectedmost of the mantle of the planetleaving behind essentially the ironcore. A similar event could havecaused the large obliquity ofUranus. Giant impacts, by ex-plaining many individual plane-tary characteristics as outcome ofa general process rather than theresult of unique and ad hoc localconditions, have undoubtedly be-come a central characteristic ofthe modern paradigm of planetaryformation.

Giant planets

If a body grows beyond a criticalmass of about 10 times the Earth’smass while still embedded in agaseous disk, it will be able to ac-crete dynamically a considerableamount of surrounding gas even-tually becoming a giant gaseousplanet such as Jupiter or Saturn(Figure 8).

In comparison to terrestrial plan-ets, giant planets formation mustproceed very rapidly since obser-vations of many young stars aswell as some theoretical consider-

ations imply that circumstellardisks have lifetimes ranging fromone to ten million years. The timeavailable is therefore relativelyshort especially since the envelopeaccretion begins rather slowly atfirst (Figure 8). It is therefore im-portant that the seed body reachescritical mass rapidly hence rela-tively high surface densities ofsolids are required. This explainswhy giant planets were believedto form only sufficiently far awayfrom the star where ices and notjust silicates are present.

Planet formation: The problem

With the discovery of the firstextra-solar giant planet, we havelearned that some stars have giantplanets orbiting at distances up to10 times closer to their star thanMercury to the sun. While not allare that close, a significant num-ber of them orbit within 0.1 AUof their star! In addition, exceptfor these very close planets forwhich tides circularize the orbit,the eccentricity of all extra-solar

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Figure 8Accretion history of Jupiter. Note the rapid accretion of the gaseous envelope(green) onto the core (red) once the system has reached the critical state. (adaptedfrom Pollack et. al 1996). With the surface density adopted here (10 g/cm2), Jupiterreaches its final mass in about 8 million years.

planets is rather large. To illustrateto what extend these systems dif-fer for our own solar system, wehave plotted in Figure 9 the eccen-tricity of all extra-solar giant plan-ets as well as solar system planetsas a function of their semi-majoraxis.

The presence of these giant plan-ets at close orbital distances re-quires significant modificationsand /or extensions to the standardformation model outlined abovefor two major reasons. First, themass of typical proto-planetarydisk within the orbit of the closestobjects observed would notamount to a jupiter mass by alarge factor even assuming 100%efficiency in collecting the matter.Second, even if there was suffi-cient mass available, the young 51Peg B for example would be tornapart by the star's gravitationalforces at its current location.

To reconcile theory and observa-tions different mechanisms havebeen considered which essentiallyallow planets to migrate fromtheir birth place to where they areobserved today. This planetarymigration is not a new idea, but itwas never considered before as anessential ingredient in planet for-mation.

Most migration scenarii considerthe gravitational interactions be-tween the growing planet and thegaseous disk. When a massive ob-jects orbits inside a gaseous disk,gravitational interactions betweenthe two give rise to significantperturbations in the disk. In par-ticular, if the planet is massive

enough a gap in the disk openswhile density perturbations ex-tend further inwards and out-wards (Figure10).

The tides raised in the disk by theplanet result in a non-axisymmet-ric density distribution which inturn exerts a torque on the planet.The magnitude as well as the signof the net torque is determined ina relatively complicated mannerby the overall structure of the diskitself. Sophisticated multi-dimen-sional numerical simulations are

required to actually compute thistorque. Figure 11 displays an exam-ple the torque exerted on a planetfrom the different regions of thedisk.

The result is a transfer of angularmomentum from the disk insidethe planet's orbit to the planet aswell as a transfer from the planetto the disk outside its orbit. As aresult of this transfer, the planetopens a gap in the disk and spiralsslowly inwards.

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Figure 9Eccentricity as a function of semi-major axis for giant extra-solar planets (bluedots), giant (red dots) and terrestrial planets (red crosses) of our own solar system.Note the difference in orbital parameters of giant planets in and outside our ownplanetary system. The absence of extra-solar giant planets beyond 3 AU is due to ob-servational biases.

While migration appears to solvesome of the problems raised bythe new systems, other issues re-main puzzling and may hint tomore fundamental problems inour understanding. For example,the migration timescale appears tobe quite short (a few 100’000years) therefore, why didn't theplanets “fall" into their star butstop after having traveled 99% ofthe distance? A hint for the exis-tence of a “stopping" mechanismis given by the apparent pile-up ofplanets visible in Figure 9 at theshortest radii. Even more puzzlingmaybe is the fact that there are nosigns of extensive inward migra-tion in our own solar system. Inparticular, Jupiter does not appearto have migrated significantly.

In summary, while a few years agoit was believed that a relativelyconsistent working paradigm forthe formation of planetary sys-tems existed, today we are leftwith pieces of theories that do nolonger provide a physically coher-ent picture!

For our understanding to makeprogress, further detections ofextra-solar planets are required inorder to have a statistically mean-ingful sample of objects to ana-lyze. Ideally, this sample mustinclude planets of all sizes not just giant planets so that the fullextend of the existing diversityamong planetary systems can begrasped.

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Figure 10Simulation of a giant planet embedded in a gaseous disk. The local surface densityis represented as the third dimension with red implying a high density. Notice thenon-axisymmetric density perturbations in the disk induced by the presence of theplanet (from G. Bryden).

Figure 11Torques exerted on a planet from various regions of the disk at different timesduring the simulation during which the planet was not allowed to move (units are ar-bitary). The net torque determining the radial migration of the planet is the sum ofthe local torques (from W. Kley).

Finding andstudying earth-like planets

Indirect detections

So far the discovery of extra-solargiant planets has been made onlythrough indirect methods inwhich the perturbations in themotion of the star induced by thepresence of a planet are detected.The fact that these perturbationscorrespond indeed to orbitingplanets has been confirmed re-cently by the detection of a transit,that is the decrease in stellar lightas the planet passes between thestar and us. These observationsalso yielded the radius and meandensity of the object confirmingits giant gaseous planet nature.

Today, the most successful planetdetection method remains the de-tection of radial velocity varia-tions. The best measurements todate have an accuracy of approx 4 m /s (the speed of a casualbiker). HARP S, a new instrumentdeveloped for E SO by a consor-tium including the Physics Insti-tute of the University of Bern andled by Geneva Observatory, hasbeen designed to reach an accura-cy of 1 m /s (the speed of a pedes-trian). Improving accuracy furtheris thought to be pointless because

stellar surface motions induce anintrinsic noise in velocity meas-urements of this order. For com-parison, the sun's reflex motiondue to the presence of the Earth isabout 8 cm /s, or more than anorder of magnitude smaller.

Radial velocity searches are mostsensitive to massive planets closeto their parent star. To detectmore distant and /or smaller plan-ets, other methods are required.One of the most promising con-sists at measuring not the radial

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Figure 13Detection limits of extra-solar planets for radial velocity searches (red line) and as-trometric searches (blue line). The limit has been computed for the instrumental pre-cision indicated. For reference, a few planets of the solar system are indicated. Thebrown box delineates the area of current discoveries.

Figure 12Light curve of HD 209458 duringtransmit as measured by HST (D. Char-bonneau).

wobble of the star but its periodicdisplacements in the plane of thesky. In other words, high precisionastrometry is used to detect againnot the planet itself but the mo-tion of the star. Since for a givenmeasurement precision, onemethod is most sensitive to ob-jects close to stars while the other to distant objects, both methodsare actually complementary (Fig-ure13).

To measure the difficulties in-volved in making these astro-metric measurements, one has to

realize that angles as small as 1micro-arsecond have to be meas-ured. This represents an anglesmaller than the one sustained bya hair more than 100 km away!While just a dream a few yearsago, such measurements will soonbe possible using long baselineoptical interferometers. Astrome-try with an interferometer con-sists at combining the light of twoor more separate telescopes andmeasuring angles (by adjustingoptical delay lines) between thetarget star and a nearby referencestar. To actually build such instru-

ments is a very challenging tech-nological task. For example, to getan efficient beam combination re-quires active distance control at afew tenths of nanometer precisionlevel over more than 100 m oflight path!

Despite these difficulties, longbaseline optical interferometers,are scheduled to become standardfacilities at the largest obser-vatories in Europe as well as in the US. The European SouthernObservatory (E SO) Very LargeTelescope Interferometer (VLTI)located in Chile will be within thenext three to five years the mostpowerful instrument of this sort(Figure 14).

Direct detection

While indirect methods will cer-tainly yield a wealth of data aboutextra-solar planetary systems, thedirect detection of photons origi-nating from the planet itselfwould enable much more detailedphysical studies. Examples includedetermining the chemical compo-sition and temperature of theplanet's atmosphere through spec-troscopy, and studying surfacestructure and rotation by analyz-ing the light-curve. Even the pres-ence of life could be tested bysearching, for example, for oxygenlines in the spectrum. Indeed, the ozone absorption feature near10 mm in the Earth's spectrum butabsent in Venus' or Mars' onlyexists because of the photo-syn-thetic activities taking place onEarth.

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Figure 14ESO’s Very Large Telescope (VLT) array on the Paranal mountain in Chile consistsof four 8.5 m diameter mirror telescopes. These telescopes can be coherently com-bined together with auxiliary telescopes (not on the picture but the rails are visible)to provide an interferometer with a baseline of up to 200 m).

The major observational chal-lenge in direct detections residesin observing a very faint object ex-tremely close to a very bright one.Indeed, the most favorable bright-ness ratio (in the infra-red) be-tween a planet and a sun is oforder 107 and both objects are typ-ically separated by 0.01 to 0.1 arc-sec. While there are no direct de-tection of planets as of today,Figure 16 illustrates the importance

of resolution when it comes to de-tect faint objects close to brightones. Such direct imaging of afaint object close to a bright one isagain possible using optical inter-ferometry. By combining the lightof two or more arms of the inter-ferometer in such a way that de-structive interferences occur on-axis, the star is literally eclipsedthus revealing the fainter objectsnearby.

To combine the light in such away to “null" the star's light re-quires the exact knowledge of thephase of each light beam. Unfor-tunately, atmospheric turbulencerenders this task extremely diffi-cult and thus these so-called“nulling interferometer" will haveto be flown in space. Such space-based instruments are on E SA's(Figure 18) as well as NASA's list ofpossible missions of the nextdecade albeit none has been defin-itively selected yet.

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Figure 16The direct detection of brown dwarf Gliese 229 from the ground (left) and fromspace using HST (right). This example illustrates how important high resolution iswhen it comes to detect faint objects close to bright ones. Detecting planets is ordersof magnitudes more difficult and will be only possible from space.

Figure 15In this colourful galaxy NGC 4214star formation has taken place since bil-lions of years ( J. MacKenty/ STScI et al.& the Hubble Heritage Team /AURA,STScI, NASA).

Conclusions

The detection and study of earth-like planets outside our solar sys-tem will be one of the great scien-tific, technological, andphilo-sophical undertakings ofour time. Considered yesterdayby most as a wild dream, thesearch for and studies of planetsoutside the solar system will be-come reality within the nextdecade or two. From the groundand later from space incrediblypowerful instruments will searchfor and analyze the light of distantplanets in an unprecedentedworld-wide effort to understandthe origin and evolution of planetsand maybe most importantly tocheck for the presence of evenprimitive life elsewhere in theuniverse.

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Figure 18DARWIN: The InfraRed Space Interfer-ometer. Six free-flying telescopes areconnected interferometrically to detectEarth-like planets orbiting nearby solar-type stars.

Figure 17Star formation in the Omega Nebula(European Southern Observatory).

SPAT IUM

The author

research focused on giant impactsand stellar explosions. He ex-plored the possibility of formingthe earth moon from the ejectathrown into orbit following theimpact on the young earth of amars-sized body.

In 1991, he joined the rank of thefaculty of the University of Ari-zona in Tucson were he became aprofessor of astronomy and plan-etary sciences. It was the timewhen comet Shoemaker-Levy 9fell into Jupiter and thus a greattime for someone interested inthe physics of collisions!

After 13 years in the USA, he re-ceived a call from the Universityof Bern in 1997, where he wasoffered the position of physicsprofessor in the Group of SpaceResearch and Planetology at thePhysikalisches Institut.

Willy Benz is currently living inNeuchâtel on the foothills of theJura mountain range. He is mar-ried and has three daughters.Mountain biking in the Jura andlong walks together with his dogare among his favourite activitiesallowing him to dive deeply intothe loneliness of a wonderfulnature.

A recognized teacher, Willy Benzaims at conveying his fascinationof nature and the intense pleasure

provided by the understanding ofsome aspects of it to his studentsand to the general public as well.Space research has always benefit-ed from the interest of the laterand conveying to him the impor-tance of the latest discoveries isseen by Willy Benz as an impor-tant task of a scientist. Hissparkling lectures never fail theaudience.

For him, despite its small sizeSwitzerland plays an importantrole in the global scientific com-munity. In many areas of space re-search Swiss scientists are at theforefront, as for instance in thediscovery of planets outside thesolar system. However, there is al-ways the danger that Switzerlandmay loose this excellent position ifthe political will to support ourstars dwindles, if we somehowfear to be ahead.

Willy Benz was born in Neuchâ-tel, Switzerland in 1955. Fascinat-ed by natural sciences, he went to the University of Neuchâtel,where he acquired the diploma inphysics. He carried his studiesfurther as a research assistant atGeneva Observatory working inthe group of Prof. Mayor (thewell- known Swiss scientist hav-ing discovered the first planet or-biting a star other than the sun). Itis during this period that his inter-ests lead him from observations(he has spent many nights observ-ing from the South of France andfrom the European Southern Ob-servatory at La Silla, Chile) to amore theoretically oriented re-search. A doctoral thesis on thesubject of star formation and rota-tion concluded his Geneva years.

After his doctorate, he went fortwo years to the Los Alamos,National Laboratory, USA, to holda postdoctoral position in the The-oretical Division working with S. Colgate with whom he is stillcollaborating. There, he becameinterested in collision theorieswhich are at the heart of our un-derstanding of planet formation.A visit by A. Cameron triggeredanother collaboration that lastedmany years and that led to hismove to Harvard Universitywhere he was an assistant andlater an associate professor for 5 years. During these years, his