INTERNATIONAL SPACE SCIENCE INSTITUTE No 6, October 2000 SPATIUM Published by the Association Pro ISSI

FromFrom DustDust toto PlanetsPlanets

The study of earth-like planets outside our solar system is one of the great scientific, technological and philosophical undertakings of our time. Editorial

Chance or necessity? Careful ob- stars. M. Mayor and D. Queloz of Impressum servation of our world provides us the Observatory of Geneva were a fascinating picture where chance the first to announce such a spec- and necessity play the key role. tacular finding. Observing and Weather evolution may be one of comparing solar systems yields a SPATIUM the most prominent examples in new picture of the star and planet Published by the our daily life: While short-term formation process, which itself is Association Pro ISSI forecasts have become fairly pre- the product of interaction be- twice a year cise, thanks to a wealth of data tween chaos and necessity. It is gathered by spacecraft and this topic to which the present ground-based stations and power- issue of SPATIUM is devoted. INTERNATIONAL SPACE ful computers processing these Prof. Willy Benz of the Physi- SCIENCE data, long-term forecasts remain a kalisches Institut of the University INSTITUTE dream, because non-linear, chaot- of Bern gave the members of our Association Pro ISSI ic processes become dominant. association a fascinating lecture on Hallerstrasse 6, CH-3012 Bern Interestingly, teachers at every the subject of planet formation. Phone ++41 31 631 48 96 level tend to carefully avoid the We are pleased to submit here- Fax ++41 31 631 48 97 topic of chaos, perhaps because with the revised version of his lec- they prefer the rigour of order to ture to our readers. President the freedom of chaos... But order Prof. Hermann Debrunner, is only half of the truth, as we University of Bern should have learnt in the last 150 Bern, October 2000 Publisher or so years since the publication of Hansjörg Schlaepfer Dr. Hansjörg Schlaepfer, the “On the Origin of Species" by Contraves Space AG, Zurich Charles Darwin. Layout Marcel Künzi, marketing · kom- The orbits of planets around a munikation, CH-8483 Kollbrunn central star are another example Printing of a short term deterministic and Druckerei Peter + Co dpc long term chaotic process. While CH-8037 Zurich it is possible to predict for exam- ple the orbit of Saturn and its moon Titan over a decade – which is of crucial importance for the common ESA/ NASA mis- sion Huygens /Cassini, which is currently on its seven years jour- ney to Saturn and Titan – it is not possible to investigate the forma- tion process of our solar system simply by extrapolating back over billions of years to the era when Frontcover our sun began to shine. Birth and death of stars in the NGC 3603 nebula (W. Brander/ In recent years, it has become pos- JPL/IPAC, E.K.Grebel /U.Wash., sible to observe – at least indirect- You-Hua Chu / UIUC, HST/ ly – planets circling around other NASA)

SPATIUM 6 2 From Dust to Planets *)

Willy Benz,Institute for Physics, University of Bern

Introduction

While the search for planets and life outside the solar system has been going on for decades, the dis- covery in 1995 of the first extra- solar giant planet by Geneva Observatory astronomers Michel Mayor and Didier Queloz fol- lowed within months by the dis- covery of 2 new giant planets by Geoffrey Marcy and Paul Butler of Lick Observatory (USA) has sparked a real revolution. Five years later, over 50 such giant planets have been found (Figure 1). implying that at least 3–5% of all sun-like stars have giant planets.

Since this represents only the frac- tion of stars having planets that could be detected with current in- struments, we must conclude that planet formation is not an extraor- dinary event but rather quite Figure 1 common occurrence. Mass of the extra-solar giant planets discovered thus far. Since more massive planets are easier to detect, the paucity of massive objects indicates that jupiter-sized (or smaller) objects are by far the most abundant ones. Our solar system forms the basis for most of our information about how planetary systems must de- velop. However, the degree to nificant that none of these newly standing of the physical processes which it is actually representative discovered objects should have underlying planet formation and of all planetary systems is unclear. existed according to conventional evolution. It now appears to be very different formation theory! from all those discovered thus far. Indeed, contrary to the giant plan- Does this mean that our solar sys- ets in our own system (Jupiter, tem is unusual or maybe even Saturn, Uranus, Neptune), the unique or have we simply not yet newly discovered planets have been able to detect the right kind much smaller orbits many with of systems elsewhere? To answer sizeable eccentricities. Although this central question requires get- there is clearly a strong observa- ting a full measure of the possible tional bias against detecting dis- diversity between existing systems tant and/or small planets, it is sig- as well as a much better under-

*) Pro ISSI lecture, Bern, 3rd November 1999

SPATIUM 6 3 Planet HH 30 DG Tau B formation: The conventional picture

Planets are likely nothing else 200 AU than a by-product of star forma- tion stemming from the necessity Haro 6-5B HK Tau of conserving angular momen- tum. Indeed, stars form through the gravitational collapse of inter- stellar matter over more than 8 orders of magnitude in size. In the presence of rotation and /or mag- netic field,the collapse must results in a star /disk structure with the star having most of the mass and the disk most of the angular mo- mentum. Thanks to the incredible resolution of the Hubble Space Telescope (HST), a few of these Figure 2 disks orbiting nearby young stars Images of circumstellar disks around a few nearby young stars imaged by HST. Note the the bipolar jets emerging on either side of the disk. could even be imaged (Figure 2).

In the case of the solar system, the disk is generally taken to have a mass of a few percent of a solar This picture provides a simple ex- grains in bodies with over 104 km mass and to be less than 100 AU planation why all planets in our in diameter (Figure 4), a growth by (1 AU is the distance between the solar system not only orbit the sun nearly a factor 1013 in size or 1040 Earth and the sun) in size. nearly in the same plane but also in mass! Since giant planets are in the same direction. The nearly mainly gaseous planets, their for- Planets subsequently form in this coeval formation of the planets mation must take place while gas disk probably mostly through col- and other small bodies and the sun supply lasts. From studies of disks lisions at first between dust grains is actually supported by compar- around other young stars, it is be- and as time goes by between larg- ing the ages of the oldest Moon lieved that typical lifetime of disks er and larger bodies. Earlier theo- rocks and the sun. are of order a few million years. ries in which planets form Hence, as paradoxical as it sounds, through the gravitational collapse The basic challenge of planet for- giant planets must be formed in of patches in the disk have grown mation consists therefore of assem- less than ten million years while out of favor. The flow diagram in bling in a disk orbiting a central forming terrestrial planets may Figure 3 illustrates these concepts. star micron-sized or smaller dust take much longer.

SPATIUM 6 4 Figure 3 Planetary formation as a by-product of star formation. In the standard model, the planets form entirely through colli- sions (solid line) while in other models gravitational collapse is invoked at vari- ous stages of the formation process. The main physical processes at play are shown in red.

Figure 4 The challenge of planetary forma- tion: Assembling micro-meter dust grains in planets through collisions in an amazingly short timescale.

SPATIUM 6 5 On their path to becoming a plan- This phase is not without prob- The escape velocity of a 1 meter- et, dust grains reach the size of lems. Laboratory experiments sized rock is of order 1 mm /s comets and asteroids which, if have shown that at the very small while the typical collisional velcity they can avoid being incorporated scale dust aggregates readily (Fig- between these objects is of order in a larger object, are left behind ure 5). 100 m /s. Hence, for sticking the like crumbs on a table after a good bodies involved have to be able to meal. On the very large scales, various dissipate all but 10–10 of the in- impact simulations have shown coming kinetic energy. Whether that self-gravity will ensure this can be achieved by purely The early phases: growth. However, the situation is mechanical structures or requires The first million year much less clear for objects ranging the presence of a “glue" whith in size from a centimeters to kilo- special visco-elastic properties re- The growth of planet-sized bodies meters since in this size range no mains to be seen. in this disk is thought to occur es- real “sticking" mechanism has yet sentially through collisions. Earli- been found. Indeed, at this size, Figure 6 summarizes the main three er models relied on gravitational the forces operating at the micron stages of growth,the relevant phys- instabilities in the dust layer to size level are no longer effective ical mechanism operating and the rapidly grow objects several kilo- and gravity is still much too weak. main study tools. meters in size (dotted line in Figure 3). However, it has been pointed out that the velocity shear be- tween the dust and the gas will stir up the dust sufficiently to make instabilities impossible. While the extend of this turbulence of the dust layer is still debated, colli- sional growth from the smallest sizes on has become the favorite scenario.

At first, dust grains collide at rela- tively gentle velocities which are determined by size and shape de- pendent gas drag. As bodies grow larger, the importance of gas drag diminishes to vanish completely by the time bodies reach several tens of meters in size. With subse- quent increase in mass, the colli- sional cross section of these plan- etesimals increases due to gravitational focusing yielding to the so-called runaway growth phase during which the larger bodies sweep-up all the smaller Figure 5 ones within their gravitational Aggregate obtained in laboratory dust coagulation experiments by J. Blum reach. and collaborators. The scale is given by the 10 µm black line.

SPATIUM 6 6 The late phases: 100 million years

This early planetary accretion phase was, over a few million years, replaced by an even more violent period when growing bodies encountered one another at increasingly high velocities boosted by mutual gravitational interactions. This phase last for another 100 to 200 million years until all remaining bodies have been swept up by the planets. Collisions occur in a random fash- Figure 6 ion involving objects of different Planetary growth stages. In green the main physical mechanism ensuring growth and in blue the main tool used to study these phases. Note that the growth mecha- masses, structures, composition nism in the meter size range is still unknown. and moving at different speeds. Thus, this phase of planet forma- tion must not be viewed as a mo- notonic process by which material is incrementally added to a grow- ing planet. Instead, accretion must be 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 in which proto-planets of compara- ble size collide represent the ulti- mate in violence during planetary accretion. While they can lead to the total destruction of the planets involved they can also leave scares arguably the best evidences re- maining today of such a violent past. The Earth's Moon, for exam- ple, is believed to originate from the debris ejected after such a giant impact and subsequently re- accreted in Earth's orbit (Figure 7).

Figure 7 The 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.

SPATIUM 6 7 Simulations of both impact and re-accumulation have not only shown 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 silicate Earth have even allowed to date the giant impact to about 50 mil- lion years after the start of the solar system!

Mercury's anomalous composi- tion can also be explained in terms of a giant impact which ejected most of the mantle of the planet leaving behind essentially the iron core. A similar event could have caused the large obliquity of Uranus. Giant impacts, by ex- plaining many individual plane- tary characteristics as outcome of a general process rather than the result of unique and ad hoc local conditions, have undoubtedly be- come a central characteristic of Figure 8 the modern paradigm of planetary Accretion history of Jupiter. Note the rapid accretion of the gaseous envelope (green) onto the core (red) once the system has reached the critical state. (adapted formation. from Pollack et. al 1996). With the surface density adopted here (10 g/cm2), Jupiter reaches its final mass in about 8 million years.

Giant planets

If a body grows beyond a critical ations imply that circumstellar Planet formation: mass of about 10 times the Earth’s disks have lifetimes ranging from The problem mass while still embedded in a one to ten million years. The time gaseous disk, it will be able to ac- available is therefore relatively With the discovery of the first crete dynamically a considerable short especially since the envelope extra-solar giant planet, we have amount of surrounding gas even- accretion begins rather slowly at learned that some stars have giant tually becoming a giant gaseous first (Figure 8). It is therefore im- planets orbiting at distances up to planet such as Jupiter or Saturn portant that the seed body reaches 10 times closer to their star than (Figure 8). critical mass rapidly hence rela- Mercury to the sun. While not all tively high surface densities of are that close, a significant num- In comparison to terrestrial plan- solids are required. This explains ber of them orbit within 0.1 AU ets, giant planets formation must why giant planets were believed of their star! In addition, except proceed very rapidly since obser- to form only sufficiently far away for these very close planets for vations of many young stars as from the star where ices and not which tides circularize the orbit, well as some theoretical consider- just silicates are present. the eccentricity of all extra-solar

SPATIUM 6 8 planets is rather large. To illustrate to what extend these systems dif- fer for our own solar system, we have plotted in Figure 9 the eccen- tricity of all extra-solar giant plan- ets as well as solar system planets as a function of their semi-major axis.

The presence of these giant plan- ets at close orbital distances re- quires significant modifications and /or extensions to the standard formation model outlined above for two major reasons. First, the mass of typical proto-planetary disk within the orbit of the closest objects observed would not amount to a jupiter mass by a large factor even assuming 100% efficiency in collecting the matter. Second, even if there was suffi- cient mass available, the young 51 Peg B for example would be torn apart by the star's gravitational forces at its current location. Figure 9 Eccentricity as a function of semi-major axis for giant extra-solar planets (blue dots), giant (red dots) and terrestrial planets (red crosses) of our own solar system. To reconcile theory and observa- Note the difference in orbital parameters of giant planets in and outside our own tions different mechanisms have planetary system. The absence of extra-solar giant planets beyond 3 AU is due to ob- been considered which essentially servational biases. allow planets to migrate from their birth place to where they are observed today. This planetary enough a gap in the disk opens required to actually compute this migration is not a new idea, but it while density perturbations ex- torque. Figure 11 displays an exam- was never considered before as an tend further inwards and out- ple the torque exerted on a planet essential ingredient in planet for- wards (Figure10). from the different regions of the mation. disk. The tides raised in the disk by the Most migration scenarii consider planet result in a non-axisymmet- The result is a transfer of angular the gravitational interactions be- ric density distribution which in momentum from the disk inside tween the growing planet and the turn exerts a torque on the planet. the planet's orbit to the planet as gaseous disk. When a massive ob- The magnitude as well as the sign well as a transfer from the planet jects orbits inside a gaseous disk, of the net torque is determined in to the disk outside its orbit. As a gravitational interactions between a relatively complicated manner result of this transfer, the planet the two give rise to significant by the overall structure of the disk opens a gap in the disk and spirals perturbations in the disk. In par- itself. Sophisticated multi-dimen- slowly inwards. ticular, if the planet is massive sional numerical simulations are

SPATIUM 6 9 While migration appears to solve some of the problems raised by the new systems, other issues re- main puzzling and may hint to more fundamental problems in our understanding. For example, the migration timescale appears to be quite short (a few 100’000 years) therefore, why didn't the planets “fall" into their star but stop after having traveled 99% of the distance? A hint for the exis- tence of a “stopping" mechanism is given by the apparent pile-up of planets visible in Figure 9 at the shortest radii. Even more puzzling Figure 10 maybe is the fact that there are no Simulation of a giant planet embedded in a gaseous disk. The local surface density is represented as the third dimension with red implying a high density. Notice the signs of extensive inward migra- non-axisymmetric density perturbations in the disk induced by the presence of the tion in our own solar system. In planet (from G. Bryden). particular, Jupiter does not appear to have migrated significantly.

In summary, while a few years ago it was believed that a relatively consistent working paradigm for the formation of planetary sys- tems existed, today we are left with pieces of theories that do no longer provide a physically coher- ent picture!

For our understanding to make progress, further detections of extra-solar planets are required in order to have a statistically mean- ingful sample of objects to ana- lyze. Ideally, this sample must include planets of all sizes not just giant planets so that the full extend of the existing diversity among planetary systems can be grasped.

Figure 11 Torques exerted on a planet from various regions of the disk at different times during 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 of the local torques (from W. Kley).

SPATIUM 6 10 Finding and studying earth- like planets

Indirect detections

So far the discovery of extra-solar giant planets has been made only through indirect methods in which the perturbations in the motion of the star induced by the presence of a planet are detected. The fact that these perturbations correspond indeed to orbiting planets has been confirmed re- cently by the detection of a transit, that is the decrease in stellar light as the planet passes between the star and us. These observations also yielded the radius and mean density of the object confirming its giant gaseous planet nature. Figure 13 Detection 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. The brown box delineates the area of current discoveries.

Today, the most successful planet stellar surface motions induce an detection method remains the de- intrinsic noise in velocity meas- tection of radial velocity varia- urements of this order. For com- tions. The best measurements to parison, the sun's reflex motion date have an accuracy of approx due to the presence of the Earth is 4 m /s (the speed of a casual about 8 cm /s, or more than an biker). HARPS, a new instrument order of magnitude smaller. developed for ESO by a consor- tium including the Physics Insti- Radial velocity searches are most tute of the University of Bern and sensitive to massive planets close led by Geneva Observatory, has to their parent star. To detect been designed to reach an accura- more distant and /or smaller plan- Figure 12 cy of 1 m /s (the speed of a pedes- ets, other methods are required. Light curve of HD 209458 during transmit as measured by HST (D. Char- trian). Improving accuracy further One of the most promising con- bonneau). is thought to be pointless because sists at measuring not the radial

SPATIUM 6 11 wobble of the star but its periodic realize that angles as small as 1 ments is a very challenging tech- displacements in the plane of the micro-arsecond have to be meas- nological task. For example, to get sky. In other words, high precision ured. This represents an angle an efficient beam combination re- astrometry is used to detect again smaller than the one sustained by quires active distance control at a not the planet itself but the mo- a hair more than 100 km away! few tenths of nanometer precision tion of the star. Since for a given While just a dream a few years level over more than 100 m of measurement precision, one ago, such measurements will soon light path! method is most sensitive to ob- be possible using long baseline jects close to stars while the other optical interferometers. Astrome- Despite these difficulties, long to distant objects, both methods try with an interferometer con- baseline optical interferometers, are actually complementary (Fig- sists at combining the light of two are scheduled to become standard ure13). or more separate telescopes and facilities at the largest obser- measuring angles (by adjusting vatories in Europe as well as in To measure the difficulties in- optical delay lines) between the the US. The European Southern volved in making these astro- target star and a nearby reference Observatory (ESO) Very Large metric measurements, one has to star. To actually build such instru- Telescope Interferometer (VLTI) located in Chile will be within the next three to five years the most powerful instrument of this sort (Figure 14).

Direct detection

While indirect methods will cer- tainly yield a wealth of data about extra-solar planetary systems, the direct detection of photons origi- nating from the planet itself would enable much more detailed physical studies. Examples include determining the chemical compo- sition and temperature of the planet's atmosphere through spec- troscopy, and studying surface structure and rotation by analyz- ing the light-curve. Even the pres- ence of life could be tested by searching, for example, for oxygen lines in the spectrum. Indeed, the ozone absorption feature near 10 mm in the Earth's spectrum but Figure 14 absent in Venus' or Mars' only ESO’s Very Large Telescope (VLT) array on the Paranal mountain in Chile consists exists because of the photo-syn- of 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) thetic activities taking place on to provide an interferometer with a baseline of up to 200 m). Earth.

SPATIUM 6 12 Figure 15 In this colourful galaxy NGC 4214 star formation has taken place since bil- lions of years (J. MacKenty/ STScI et al. & the Hubble Heritage Team/AURA, STScI, NASA).

The major observational chal- of resolution when it comes to de- To combine the light in such a lenge in direct detections resides tect faint objects close to bright way to “null" the star's light re- in observing a very faint object ex- ones. Such direct imaging of a quires the exact knowledge of the tremely close to a very bright one. faint object close to a bright one is phase of each light beam. Unfor- Indeed, the most favorable bright- again possible using optical inter- tunately, atmospheric turbulence ness ratio (in the infra-red) be- ferometry. By combining the light renders this task extremely diffi- tween a planet and a sun is of of two or more arms of the inter- cult and thus these so-called order 107 and both objects are typ- ferometer in such a way that de- “nulling interferometer" will have ically separated by 0.01 to 0.1 arc- structive interferences occur on- to be flown in space. Such space- sec. While there are no direct de- axis, the star is literally eclipsed based instruments are on ESA's tection of planets as of today, thus revealing the fainter objects (Figure 18) as well as NASA's list of Figure 16 illustrates the importance nearby. possible missions of the next decade albeit none has been defin- itively selected yet.

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

SPATIUM 6 13 Conclusions

The detection and study of earth- like planets outside our solar sys- tem will be one of the great scien- tific, technological, and philo-sophical undertakings of our time. Considered yesterday by most as a wild dream, the search for and studies of planets outside the solar system will be- come reality within the next decade or two. From the ground and later from space incredibly powerful instruments will search for and analyze the light of distant planets in an unprecedented world-wide effort to understand the origin and evolution of planets and maybe most importantly to check for the presence of even primitive life elsewhere in the universe.

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

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

SPATIUM 6 14

SPATIUM

The author

Willy Benz was born in Neuchâ- research focused on giant impacts tel, Switzerland in 1955. Fascinat- and stellar explosions. He ex- ed by natural sciences, he went plored the possibility of forming to the University of Neuchâtel, the earth moon from the ejecta where he acquired the diploma in thrown into orbit following the physics. He carried his studies impact on the young earth of a further as a research assistant at mars-sized body. Geneva Observatory working in the group of Prof. Mayor (the In 1991, he joined the rank of the well- known Swiss scientist hav- faculty of the University of Ari- ing discovered the first planet or- zona in Tucson were he became a biting a star other than the sun). It professor of astronomy and plan- is during this period that his inter- etary sciences. It was the time ests lead him from observations when comet Shoemaker-Levy 9 (he has spent many nights observ- fell into Jupiter and thus a great ing from the South of France and time for someone interested in provided by the understanding of from the European Southern Ob- the physics of collisions! some aspects of it to his students servatory at La Silla, Chile) to a and to the general public as well. more theoretically oriented re- After 13 years in the USA, he re- Space research has always benefit- search. A doctoral thesis on the ceived a call from the University ed from the interest of the later subject of star formation and rota- of Bern in 1997, where he was and conveying to him the impor- tion concluded his Geneva years. offered the position of physics tance of the latest discoveries is professor in the Group of Space seen by Willy Benz as an impor- After his doctorate, he went for Research and Planetology at the tant task of a scientist. His two years to the Los Alamos, Physikalisches Institut. sparkling lectures never fail the National Laboratory, USA, to hold audience. a postdoctoral position in the The- Willy Benz is currently living in oretical Division working with Neuchâtel on the foothills of the For him, despite its small size S. Colgate with whom he is still Jura mountain range. He is mar- Switzerland plays an important collaborating. There, he became ried and has three daughters. role in the global scientific com- interested in collision theories Mountain biking in the Jura and munity. In many areas of space re- which are at the heart of our un- long walks together with his dog search Swiss scientists are at the derstanding of planet formation. are among his favourite activities forefront, as for instance in the A visit by A. Cameron triggered allowing him to dive deeply into discovery of planets outside the another collaboration that lasted the loneliness of a wonderful solar system. However, there is al- many years and that led to his nature. ways the danger that Switzerland move to Harvard University may loose this excellent position if where he was an assistant and A recognized teacher, Willy Benz the political will to support our later an associate professor for aims at conveying his fascination stars dwindles, if we somehow 5 years. During these years, his of nature and the intense pleasure fear to be ahead.