Introduction

The widely accepted picture of stellarformation tells us that a planetary sys-tem is a simple by-product of the stellarformation process. When a cloud of gasand dust contracts to form a star, con-servation of angular momentum in-duces the formation of a flat diskaround the central newborn “sun”. By aprocess still not fully understood, thisdisk is believed to be the stage for theplanetary formation. According to thetraditional paradigm, dust particles andice grains in the disk are gathered toform the first planetary seeds (e.g.Pollack et al. 1996). In the “outer” re-gions of the disk, where ices can con-

densate, these “planetesimals” arethought to grow in a few million years.When such a “planetesimal” achievesenough mass (about 10 times the massof the Earth), its gravitational pull is suf-ficiently strong for it to start accretinggas in a runaway process that givesorigin to a giant gaseous planet similarto the outer planets in our own SolarSystem. Later on, in the inner part ofthe disk, where temperatures are toohigh and volatiles cannot condensate,silicate particles are gathered to formthe telluric planets like our Earth.

In the past decade, images taken bythe NASA/ESA Hubble Space Tele-scope (HST) have revealed a multitudeof such proto-planetary disks in the

Orion stellar nursery, showing thatdisks are indeed very common aroundyoung solar-type stars. This supportsthe idea that extra-solar planets shouldbe common. However, such systemshave escaped detection until very re-cently.

In fact, it was not until 1995, followingthe discovery of the planet orbiting thesolar-type star 51Peg May95, that thesearch for extra-solar planets had itsfirst success1. This long wait was main-ly due to the difficulty of detecting suchbodies. Planets are cold objects, andtheir visible spectrum results basicallyfrom reflected light of the parent star. Asa result, the planet/stellar luminosity ra-tio is of the order of 10–9. Seen from adistance of a few parsec, a planet is nomore than a small “undetectable”speckle embedded in the diffractionand/or aberration of the stellar image.

The detection of exoplanets has thusbeen based, up to now, upon “indirect”methods. In particular, all the planetarydiscoveries were only possible due tothe development of high-precision radi-al-velocity techniques. These methods,that measure the motion of a star alongthe line of sight (by measuring theDoppler shift of spectral lines), havenow achieved precisions of the order ofa few m s–1 (∆λ/λ ∼ 10–8). Such a highprecision is indeed needed to find aplanet: for example, Jupiter induces aperiodic perturbation with an amplitudeof only 13 m s–1 on the Sun!

In this article we will review the cur-rent status of planetary searches, pre-senting the major challenges that weare facing at this moment. We will thendiscuss how new and future generationinstruments and missions will help toanswer the most important questions.We will concentrate mostly on the re-sults we can expect from future radial-velocity campaigns with state-of-the-artinstruments like HARPS on the ESO3.6-m telescope (see article by Pepe etal. in this issue).

A Diversity of Planets

Today, about 100 extra-solar plane-tary systems have been unveiled

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REPORTS FROM OBSERVERS

Extra-Solar Planets N.C. SANTOS, M. MAYOR, D. QUELOZ, S. UDRY

Observatoire de Genève, Sauverny, Switzerland

Figure 1: Schematic orbital configurations for some of the newly found extra-solar planets inthree different scales. In the upper-left panel we represent the orbits of the shorter-periodcompanions. The Sun (yellow circle) is drawn to scale. This plot illustrates well both the prox-imity of these planets to their host stars, and the complete lack of planetary companions or-biting closer than a certain distance (see text for more details). The upper-right and lower pan-els illustrate the situation concerning longer orbital period planets. In these two plots, the or-bits of the Earth and Jupiter are also drawn for comparison (with the usual symbols). Thesethree panels clearly illustrate the huge variety of orbital parameters presented by the extra-solar planets.

1Before this discovery, only planets around apulsar had been detected (Wolszczan & Frail 1992);however, these are probably second-generationplanets. In this article we will concentrate on plan-ets around solar-type stars.

Figure 2: Distribu-tion of minimummasses for the cur-rently discoveredlow-mass compan-ions to solar-typestars. Although theradial-velocitymethod has a higher sensitivity tohigher-mass com-panions, theobserved distribu-tion rises verysteeply towards thelow-mass domain.From this mass upto the stellarregime, only a fewobjects weredetected; thisregion is usuallydenominated the“Brown-Dwarfdesert”. This gap inthe mass distribu-tion of low-masscompanions to so-lar-type stars supports the view that the physical mechanisms involved in the formation ofthese two populations (planets vs. stars) are very different.

around stars other than our Sun2.These discoveries, which include ∼ 10multi-planetary systems, have broughtto light the existence of planets with ahuge variety of characteristics, openingunexpected questions about theprocesses of giant planetary formation.

The diversity of the discovered extra-solar planets is well illustrated inFigure 1. Unexpectedly, they don’thave much in common with the giantplanets in our own Solar System.Contrarily to these latter, the “new”worlds present an enormous and unex-pected variety of masses and orbitalparameters (astronomers were basical-ly expecting to find “Jupiters” orbiting at∼5 A.U. or more from their host stars inquasi-circular trajectories). The majori-ty of the discovered planets were noteven supposed to exist according to thetraditional paradigm of giant planetaryformation (e.g. Pollack et al. 1996).Their masses vary from sub-Saturn toseveral times the mass of Jupiter.Some have orbits with semi-major axessmaller than the distance from Mercuryto the Sun, and except for the closestcompanions, they generally follow ec-centric trajectories, contrarily to thecase of the giant-planets in the SolarSystem.

These findings have put into questionthe former planetary formation para-digm. However, the relatively large num-ber of discovered planets is alreadypermitting us to undertake the first sta-tistical studies of the properties of theexoplanets, as well as of their host stars.This is bringing new constraints to themodels of planet formation and evolu-tion. Let us then see in more detail whatkind of problems and information thesenew discoveries have brought.

The Period Distribution

One of the most interesting problemsthat appeared after the first planetswere discovered has to do with theproximity to their host stars. The firstplanet discovered (around 51Peg –Mayor & Queloz 1995) is exactly thefirst such example: it orbits its star onceevery 4.2 days, corresponding to an or-bital radius of only 0.05 A.U. This ismuch less than the distance fromMercury to the Sun.

The problem that was raised with thefinding of these 51Peg-like planets(usually called “hot-Jupiters”) resides inthe fact that at such close distances thetemperatures are too high for ices tocondensate, and there does not seemto exist enough available material toform a Jupiter-mass planet. It is thusvery difficult to imagine that suchworlds could be formed so close to thecentral stellar furnace.

In order to explain the newly foundsystems, several mechanisms havethus been proposed. Current resultsshow that in situ formation is very un-likely, and we need to invoke inward mi-gration, either due to gravitational inter-action with the disk (Goldreich &Tremaine 1980) or with other compan-ions to explain the observed orbital pe-riods. In other words, the observedclose-in planets could simply havebeen formed far from their host star,and then migrated inwards.

But the migration mechanisms, thathave broken long-lasting ideas of “sta-bility” of the planetary systems, havesome problems to solve. According tothe models, the timescales of planetarymigration in a disk are particularlyshort. This means that more than wor-rying about how planets migrate after orduring their formation, we need to un-derstand how migration can be stopped(and/or slowed down)!

One particularly interesting cluecomes from the observation that thereis a clear pile-up of planetary compan-ions with periods around 3 days, ac-companied by a complete absence ofany system with a period shorter thanthis value. This result, which is in com-plete contrast with the period distribu-tion for stellar companions (we can finddouble stars with periods much shorterthan 3 days), means that somehow theprocess involved in the planetary mi-gration makes the planet “stop” at a dis-tance corresponding to this orbital peri-od. To explain this fact, several ideashave been presented. These invoke dif-ferent mechanisms like e.g. a magne-tospheric central cavity of the accretiondisk (once the planet gets into this cav-ity it will no longer strongly interact with

the disk and consequently stops the mi-gration), photo-evaporation, tidal inter-action with the host star, or Roche-lobeoverflow of the young inflated giantplanet (processes resulting in an in-crease of the orbital radius of theplanet, thus opposing the migrationtendency)3 .

In any case, even if these mecha-nisms are able to explain how the short-er period planets have stopped migra-tion, they do not explain how the longerperiod ones (like Jupiter itself) did notmigrate to distances closer to their hoststars. The key to this might have to dowith a combination of parameters, likethe disk masses, lifetimes, viscosities,and initial planetary masses and/ornumber of bodies formed, that will af-fect the final orbital configuration of aplanet. Some of these parameters are notwell known (maybe the planetary for-mation itself is controlling them), some-thing that complicates the discussion.

The Mass Distribution

Very important information is broughtto us by the analysis of the mass spec-trum of the planetary companions. Inparticular, a plot showing the mass dis-tribution of companions to solar-typestars (as shown in Figure 2), shows aclear discontinuity for the mass regimebetween about 30 and 50 times themass of Jupiter: there are basically nocompanions found to date having thosemasses. This result is even more strik-ing if we note that the radial-velocity

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2For a complete and updated list of the knownexoplanets, see e.g. table at http://obswww.unige.ch/exoplanets.

3Planets migrating more than this approximatelimit might “simply” also evaporate/transfer materi-al to the host star, and thus disappear or becometoo low-mass to be detected.

Figure 4: Eccentricityvs. orbital period (inlogarithmic scale) forthe discovered extra-solar planets (redpentagons), stellarbinaries (filled dots),and for the giantplanets in our SolarSystem (green stars).The earth is repre-sented by the usualsymbol. Althoughsome long-period ex-oplanets exist havinglow values for theeccentricity, most ofthe systems presentmuch higher eccen-tricities than thoseobserved for theSolar System giantplanets. Possible ex-planations invokemechanisms capableof pumping theeccentricity, likegravitational interactions between planets in a multi-planetary system, or with a distant stellar companion.

Figure 3: Distributionof exoplanet masses.The histogram repre-sents the observedminimum mass distri-bution, while the redline represents thestatistical true plane-tary mass distributionresulting from a de-convolution of the un-known orbital inclina-tions. The distributionreaches “zero” at amass of about 10times the mass ofJupiter, which probablycorresponds to theupper limit for the massof a giant planet. Thenature of the objectswith masses between10 and ∼17 times themass of Jupiter is stillan open question. Asin Jorissen et al.(2001).

technique is more sensitive to massivecompanions than to their lower masscounterparts.

This gap, usually called the “browndwarf desert”, separates the low mass“planetary” companions from their highmass “stellar” counterparts, and isprobably telling us something very im-portant about the physical processesinvolved in the formation of these twopopulations: stars, even the low massones, are thought to be formed as theresult of the gravitational collapse andfragmentation of a cloud of gas anddust. On the other hand, a planet formsin a circumstellar accretion disk.

More information is provided if weanalyse the shape of the distribution forthe planetary mass regime (Fig. 3).This distribution is observed to de-crease smoothly with increasing mass,reaching “zero” at about 10 Jupitermasses (Jorissen et al. 2001). This lim-it is clearly not related to the Deuterium-burning mass limit of ∼ 13 MJup, some-times considered as the limiting massfor a planet (this latter value is in fact anarbitrary limit used as a possible “defi-nition”, but it is not related to the plane-tary formation physics). As it was re-cently shown by several authors (e.g.Jorissen et al. 2001), this result is notan artefact of projection effects (the un-known orbital inclination implies that wecan only derive a minimum mass for thecompanion from the radial-velocitymeasurements), but a real upper limitfor the mass of the planetary compan-ions discovered so far.

The Period-Mass Relation

Recent results have also unveiledsome interesting correlations betweenthe planetary mass and its orbitalperiod. In fact, there seems to exist apaucity of high-mass planetary com-

panions (M > 2 MJup) orbiting in shortperiod (lower than ∼ 40-days) trajecto-ries. Similarly, there seems to be a lackof long-period and very low-massplanets.

These results are further helping usto understand the mechanisms of plan-et migration, since they are compatiblewith the current ideas about planetaryorbital evolution (either due to an inter-action with the disk or with other com-panions), that suggest that the higherthe mass of a planet, the more slowly(and less) it will migrate.

The Eccentricity

One of the most enigmatic resultsfound to date has to do with the analy-

sis of the orbital eccentricities of theplanetary-mass companions. Accord-ing to the traditional paradigm of plane-tary formation, a planet (formed in adisk) should keep a relatively circular(low eccentricity) trajectory. Currentmodels shown indeed that the interac-tion (and migration) of a low mass com-panion within a gas disk has the effectof damping its eccentricity (Goldreich &Tremaine 1980). However, opposite toexpectations, if we look at the eccen-tricities of the planetary companions wecan see that they are spread throughvalues that go from nearly zero to morethan 0.9 for the planet orbiting the starHD80606!

In Figure 4 we plot the eccentricity asa function of orbital period for the plan-etary companions to solar-type stars,as well as for the stellar companions.First of all, it is important to describe thegeneral tendencies observed in theplot. The low eccentricity observed forshort-period binaries is the result of awell known effect: the proximity to theirprimary stars induces tidal interactionsthat have the effect of damping the ec-centricity. Since the tidal effect de-creases very fast with distance, abovea given orbital period (about 10 days fordwarf star binaries), tidal circularizationis no longer effective, and all compan-ions having periods longer than a givenvalue simply keep their “initial” orbitaleccentricity.

While both distributions show the sig-nature of tidal effects on the eccentrici-ty, a first glimpse also tells us that thereis no clear difference between the twogroups of points: stars and planetshave a similar distribution in this dia-gram. This poses the problem of un-derstanding how planetary companionsformed in a disk can have the same ec-

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centricity distribution as their stellarmass counterparts. And how can thisbe fit into the picture of a planet forming(and migrating) in a disk?

The explanation for these facts maybe other processes capable of excitingthe eccentricity of the planetary orbits.These include the interaction betweenplanets in a multiple system or betweenthe planet and a disk of planetesimals,the simultaneous migration of variousplanets in a disk, or the influence of adistant stellar companion. All or at leastsome of these physical processesmight play an important role in definingthe “final” orbital configuration.

Although still not clear, a close inspec-tion of Figure 4 permits us to find a fewdifferences between the eccentricitiesof the stellar and planetary compan-ions. In particular, for periods in therange of 10 to 30 days (clearly outsidethe circularization period by tidal inter-action with the star), there are a fewplanet hosts having very low eccentric-ity, while no stellar binaries are presentin this region. The same and evenstronger trend is seen for longer peri-ods, suggesting the presence of agroup of planetary companions with or-bital characteristics more similar tothose of the planets in the Solar Sys-tem (with low eccentricity and longperiod). On the other hand, for thevery short period systems, we can seesome planetary companions with ec-centricities higher than those found for“stars”. This features may be telling usthat different formation and evolutionprocesses took place: for example, theformer group may be seen as a sign forformation in a disk, and the latter oneas an evidence of the influence of alonger period companion on the eccen-tricity.

Clues from the Planet Hosts:the Stellar Metallicity

Up to now we have been reviewingthe results and conclusions obtained di-rectly from the study of the orbital prop-erties and masses of the discoveredplanets. But another fact that is helpingastronomers understand the mecha-nisms of planetary formation has to dowith the planet host stars themselves.Indeed, they were found to be particu-larly metal-rich, i.e. they have, on aver-age, a higher metal content than thestars without detected planetary com-panions (see Santos et al. 2001 for themost recent results) – see Figure 5.

A possible and likely interpretation ofthis may be that the higher the metallic-ity of the cloud that gives origin to thestar/planetary system (and thus thedust content of the disk), the faster aplanetesimal can grow, and the higherthe probability that a giant planet isformed before the proto-planetary diskdissipates. In other words, the metallic-ity seems to be playing a key role in the

formation of the currently discoveredextra-solar planetary systems.

Boss (1997) has suggested that be-sides the core accretion scenario (seeintroduction), giant planets might alsobe formed as a result of disk instabilityprocesses: by the formation and con-densation of clumps of gas and dust inthe protoplanetary disk. This processis, however, not very dependent on themetallicity. In other words, if the disk-in-stability models were the most impor-tant mechanism involved in the forma-tion of giant planets, we should not ex-pect to see a strong dependence on therate of planet detection as a function ofthe metallicity. The huge dependenceobserved is thus probably a sign thatthe core accretion scenario is the im-portant mechanism involved in the for-mation of giant planets.

It is, however, important to stress thatit is not precisely known how the metal-licity is influencing the planetary forma-tion and/or evolution; for example, themasses of the disks themselves, whichcan be crucial to determine the efficien-cy of planetary formation, are notknown observationally with enoughprecision.

A Case of Stellar “Cannibalism”

Recent observations also suggestthat planets might be engulfed by theirparent stars, whether as the result oforbital migration, or e.g. of gravitationalinteractions with other planets or stellarcompanions. Probably the clearest evi-dence of such an event comes from thedetection of the lithium isotope 6Li in theatmosphere of the planet-host star HD82943 (Israelian et al. 2001). This frag-ile isotope is easily destroyed (at only1.6 million degrees, through (p,α) reac-tions) during the early evolutionarystages of star formation. At this stage,the proto-star is completely convective,and the relatively cool material at the

surface is still deeply mixed with the hotstellar interior (this is not the case whenthe star reaches its “adulthood”). 6Li isthus not supposed to exist in stars likeHD 82943, and the simplest and mostconvincing way to explain its presenceis to consider that planet(s), or at leastplanetary material, have fallen into HD82943 sometime during its lifetime.

The most recent and detailed analy-sis seems to clearly confirm the pres-ence of this isotope. The question is thenturned to know whether this case is iso-lated or if it represents a frequent out-come of the planetary formation process.How much can this process increasethe observed metallicity of the planethosts? Current results suggest that atleast the degree of stellar “pollution” isnot incredibly high (Santos et al. 2001).

Black Sheep

When measuring the spectrum of astar we are obtaining the integratedlight of the whole stellar disk, and gath-ering photons coming from differentpoints in the stellar surface. Each indi-vidual point has its own spectrum, witha different Doppler shift that is a func-tion of the velocity field in that specificregion of the stellar photosphere. As aconsequence, any phenomenon capa-ble of changing the velocity field of agiven region in the stellar surface willchange the global spectrum Dopplershift, and consequently the measuredradial-velocity.

This result has an important impactwhen dealing with radial-velocity meas-urements: the radial-velocity techniqueis not sensitive only to the motion of astar around the centre of mass of astar/planet system, but also to eventualvariations in the structure of the stellarsurface.

In fact, phenomena such as stellarpulsation, inhomogeneous convectionor the presence of dark spots (e.g. Saar

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Figure 5: Left: Metallicity ([Fe/H]) distributions for stars with planets (yellow histogram) com-pared with the same distribution for field dwarfs in the solar neighbourhood (open red his-togram). In this panel, both distributions are normalized by the total number of points. The[Fe/H] scale is logarithmic, and the Sun has by definition [Fe/H] = 0. Most planet hosts aremore metal-rich than our Sun; Right: the percentage of stars that have been found to harboura planet for each metallicity bin, plotted as a function of the metallicity. This plot clearly showsthat the probability of finding a giant planet increases with the metallicity of the star. As inSantos et al. (2001).

& Donahue 1997) are expected to in-duce radial velocity variations. Further-more, contamination from the spectrumof other stellar companions can also in-duce spurious radial-velocity signals.These cases can prevent us from find-ing planets (if the perturbation is largerthan the orbital radial-velocity varia-tion), but perhaps more importantly,they might give us false candidates ifthey produce a periodic signal (e.g. arotating stellar spot). The radial-veloci-ty technique is thus most efficient forlow chromospheric activity single “old”dwarfs.

A few good examples of spurious pe-riodic radial-velocity variations can befound in the literature. The first to bedescribed was the periodic signal ob-served for the dwarf HD 166435, thatwas shown to be due to the presence ofa dark spot rather than to the gravita-tional influence of a planetary compan-ion.

At the current (and increasing) levelof precision obtained using the radial-velocity techniques, such kinds of ex-amples might become more and morecommon. It is thus very important to de-velop ways to disentangle e.g. activity-related phenomena from real planetarycandidates. Such methods might bebased on the study of the shape of thespectral lines (usually called the “bisec-tors”), the photometric variability of thestar, and/or of chromospheric activityindexes. This is very important for proj-ects like HARPS, for which the barrierof the 1 ms–1 will be achieved.

Guidelines for the Future

The study of extra-solar planetarysystems is just beginning. After only 7years, we can say that at least 5% ofsolar-type dwarfs have giant planetarycompanions in relatively short periodorbits (≤ 3 years). However, and as wehave seen in the previous sections,many interesting but troubling problemsstill await a solution. In fact, the newly-found planets have clearly disturbedthe long-standing theories of giant plan-etary formation and evolution. The def-

inition of a planet itself is currently un-der debate.

To solve this problem, there are highexpectations from new or soon-to-beavailable instruments (see the paper onthe HARPS spectrometer in this issue).The incredible precision gain achievedby these radial-velocity “machines” willbe crucial in this aspect. But what areexactly the important lines of study tofollow? And what kind of answers canwe expect in the next few years?

Increasing the Statistics

As we have seen above, the ob-served correlations between the orbitalparameters of the newly found planetsare giving astronomers a completelydifferent view on the formation and evo-lution of planetary systems. We nolonger have the Sun as the only exam-ple, and today we have to deal with thepeculiar characteristics of the “new” ex-tra-solar planets: a huge variety of peri-ods, eccentricities, masses.

To clear up current uncertainties weneed more data. Only a large and sta-tistically uniform set of data may enableus to clarify the current situation. In thissense, the hundreds of new planets ex-pected to be discovered in the next fewyears will have a very important out-come.

Lower and Lower Masses

One clear result of the increase inprecision of the current and future radi-al-velocity surveys is the ability to findlower-mass planets.

As we have seen above, the massfunction for planetary companionsaround solar-type stars is rising towardthe low-mass regime. But given that theradial-velocity technique is more sensi-tive to the more massive companions,the lower mass bins in the plot ofFigure 2 are definitely not well repre-sented. How then does this function be-have for the lower-mass regime? Whatis the minimum mass of a giant planet?How does this depend on the orbital pe-riod?

The increase in long-term precision,and the continuation of the currenthigh-precision radial-velocity pro-grammes, will also give us the opportu-nity of finding more and more long-pe-riod planets. Current models predictthat the planetary formation and evolu-tion processes should produce morelong-period planets than their short pe-riod counterparts. In fact, the currentsurveys are just starting to discoverexo-planets with periods comparable tothe ones found for the Solar System gi-ants. This is an essential goal in orderto improve the statistics of these ob-jects, and to check if the Solar Systemis anomalous or common.

Of course, other problems will ariseand become more important as a con-sequence of the dramatic increase inprecision. In particular, the intrinsic stel-lar radial-velocity “jitter” produced bye.g. chromospheric activity related phe-nomena (e.g. Saar & Donahue 1997)might impose serious (and still un-known) limits on the final precision, andon the consequent ability to find verylow-mass (and long-period) systems.This is particularly true for the young-est stars. Some effort should thusbe put into the development of diag-nostics capable of confirming the plan-etary nature of the radial-velocity sig-nal. Furthermore, we might even imag-ine that the spurious radial-velocityvariations caused by activity might bemodelled and corrected, leaving onlythe real gravitational effects on thesignal.

Planets Around M Dwarfs

Although more than 100 exoplanetcandidates are now in the lists, only twoplanetary companions around M dwarfshave been detected (both in the solelysystem Gl 876). The very low numberof M-dwarf planetary companions canin fact be largely explained by observa-tional biases: the very low mass dwarfsare faint and it is difficult to obtain ac-curate radial velocities for them.

However, to constrain the variousscenarios of planetary-system forma-tion and evolution, it is now crucial toobtain better statistics for planetsaround the most numerous stars in ourgalaxy. M dwarfs compose 80% of themain sequence stars. How many ofthem have planets? How these planetsdiffer from those orbiting the moremassive G dwarfs is totally unknown.These questions await the future capa-bilities of instruments like HARPS.

The Chemical Link

Planet host stars seem to be, on av-erage, particularly metal-rich. This in-teresting result probably reflects the im-portance the quantity of available rockymaterial in the disk has on the forma-tion of giant planets.

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Figure 6: Radial-ve-locity measurementsand best 2-planetKeplerian solution forHD 82943. The radi-al-velocity measure-ments show that thisstar has a system oftwo resonant plan-ets, orbiting it withperiods of ∼ 220 and440 days, respec-tively. This same starwas found to have6Li in its atmosphere,a signature of possi-ble planet engulf-ment (Israelian et al.2001).

As discussed above, this link mighthold the key to understanding how gi-ant planets are formed. The two com-peting theories (core accretion and diskinstability) should have different sensi-tivities to the metallicity. If most planetswere formed by the latter of these twoprocesses we should not expect anyspecial metallicity sensitivity.

Although the metallicity trend isclearly seen (Santos et al. 2001), thereare nevertheless a few planet hosts thatare particularly metal poor (see e.g. caseof HD 6434). How can this be explained?One elegant way of solving this puzzleis to look for the frequency of planetsaround metal-poor halo dwarfs. If, asexpected, no planetary mass objectsare found around such a sample ofstars, then the disk instability model isclearly put into question. However, ifsome are found, giant planets might beformed by different processes.

There are also some traces of stellar“pollution” among the planet host stars(Israelian et al. 2001). This opens thequestion of understanding how muchplanetary material might fall into theconvective envelope as a consequenceof the planetary formation process it-self. How much can this change themetallicity of the star? If important, thiscould have consequences even forstudies of the chemical evolution of thegalaxy.

Although current results seem to re-fute any strong generalized stellar pol-lution among planet hosts, it is impor-tant to cross-check. One interestingway of addressing this problem mightthen be the study and comparison ofthe chemical contents of stars in visualbinary systems composed of similarspectral type solar-type stars, one (orboth) of which having planetary-masscompanions. Strong differences foundwould be interpreted as a sign that stel-lar pollution is quite common. However,only one clear case has been studied todate: the double visual system 16 CygA and B, where the B component isknown to harbour a planet. This casedoes not show any clear difference inthe iron abundance, while a curiouslithium abundance difference is found.

Transit Candidates

Another important result of instru-ments like HARPS will be their ability tofollow up planetary-like transit signa-tures detected by photometry.

There are currently more than 20groups around the world trying to lookfor signatures of the presence of plan-ets around field dwarfs by looking forthe brightness dimming as a putativeplanet crosses its disk. In spite of theefforts, only very few results have beenannounced, and none of these wasconfirmed to have a planetary origin.The only clear case of a real planetarytransit detected so far was found for the

star HD 209458, a dwarf that was pre-viously discovered to host a very-shortperiod planetary companion (Charbon-neau et al. 2000).

This detection, and the subsequentrelated studies, have had an enormousimpact for the understanding of thesesystems. For example, it was possibleto estimate the mean density of theplanet, and to prove that it is orbiting inthe same direction and plane as thestar’s equator.

In the near future we can expect thatother such events might be brought tolight. In particular, many hundreds ofphotometric transit candidates are ex-pected from space missions likeCOROT, Kepler or Eddington. How-ever, based only on photometry wecannot determine whether the ob-served transiting body is a planet orsimply a low-mass star (since the effectis of similar magnitude because of thelarge degeneracy of the radius of theseobjects). The follow-up of the photo-metric observations by radial-velocitysurveys is thus essential and will permitus to obtain the real mass of the planet.

With such data we can hope to deriveempirical relations between variablessuch as the planet’s mean density andits distance from the star, its mass, andthe stellar metal abundance. In thissense it is important to say that the veryhigh precision of HARPS, together withthe relatively large aperture of the ESO3.6-m telescope, will play an importantrole, since it will give the opportunity toobtain masses (or at least meaningfulupper limits) for the least massive planetsdetected by the photometric missions.

Multiple Systems: DynamicalInteractions

Among the many planets that are ex-pected to be found, some will surely be-long to multi-planetary systems. Today,only about 10 such cases are known,but many stars that are already knownto harbour a planet also show system-atic trends in radial velocity, indicatingthat at least a second companion ispresent in the system. While for the ma-jority of cases this tendency might besimply due to the presence of low-massstellar companions, in some othersthey might be the telltale signatures ofa multi-planetary system. The gain inprecision with instruments like HARPSwill definitely permit us to search the al-ready known planet hosts for otherplanet-mass companions, and to in-crease the number of known multiplesystems.

There is in fact much interesting in-formation that can come from thesecases. Current results have shown thatplanets in multiple systems come fre-quently in resonant orbits (see e.g.HD82943 – Figure 6). This is telling usa lot about the formation and migrationof the exoplanets.

On the other hand, the strong inter-action between planets in such sys-tems will be reflected as an observableevolution of their orbital parameters. Adynamical analysis of this will give usthe opportunity to obtain information onthe masses and relative orbital inclina-tions for the companions.

Planets in Binaries

To date, several planets have beendiscovered in known multiple stellarsystems. Moreover, a fraction of starsknown to host planets exhibit a drift inthe γ-velocity indicating the presence ofan additional distant companion.

These observations show that de-spite the gravitational perturbation ofthe stellar companion, planets may formand survive around stars in multiple sys-tems. The properties of such planetshold important clues on the mecha-nisms of planetary formation. For ex-ample, according to the standard coreaccretion model of planetary formation,a giant planet is formed by the accre-tion of gas around a ∼ 10 earth masscore of rocky material. This is supposedto take place at distances comparableto the Jupiter–Sun separation (∼5 A.U.).Opposing this model, Boss (1997) hasproposed that giant gaseous planetsmight also be formed from the conden-sation of clumps resulting from gravita-tional instabilities in the disk. How canwe distinguish these two scenarios?

One of the keys may come from thestudy of planets in binaries. The pres-ence of a stellar companion possiblyplays an important role in the formationof planets. It has been shown, for ex-ample, that a stellar companion cantruncate the proto-planetary disk at aradius that depends mainly on the dis-tance between the two stars. If so, andconsidering the core accretion sce-nario, we should not be able to seeplanets around stars members of bina-ry systems that are closer than a givenlimit. How close can a star have a com-panion and still have planets? The an-swer to this question is very importantto understand how giant planets areformed.

Concluding Remarks

As the planet search programmesare on their way, many more planetarycompanions are expected to be discov-ered in the next few years. Many hopesare now placed on instruments likeHARPS, that will provide radial veloci-ties of stars with a precision of 1 ms–1

or better. This will give us the opportu-nity to dramatically improve the sam-ples.

Other major contributions will comefrom future space missions likeCOROT, Eddington, or Kepler, whichwill unveil thousands of short-periodplanets around stars in the solar neigh-

37

bourhood. And, of course, the use ofhigh-precision astrometric measure-ments with instruments like the VLTI orthe Keck interferometer will survey“nearby” stars for long-period systems.Altogether, these coming observationalfacilities will definitely help us to con-struct a new and more complete view ofhow planetary systems are born andhow they evolve.

References

Boss A.P., 1997, Science 276, 1836. Charbonneau D., Brown T.M., Latham D.W.,

Mayor M., 2000, ApJ 529, L45. Goldreich P., Tremaine S., 1980, ApJ 241,

425. Israelian G., Santos N.C., Mayor M., Rebolo

R., 2001, Nature 411, 163. Jorissen A., Mayor M., Udry S., 2001, A&A

379, 992.

Mayor M., Queloz D., 1995, Nature 378,355.

Pollack J.B., Hubickyj O., Bodenheimer P.,Lissauer J.J., Podolak M., Greenzweig Y.,1996, Icarus 124, 62.

Saar S.H., Donahue R.A., 1997, ApJ 485,319.

Santos N.C., Israelian G., Mayor M., 2001,A&A 373, 1019.

Wolszczan A., Frail D. A., 1992, Nature 355,145.

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FIRES: Ultradeep Near-Infrared Imaging withISAAC of the Hubble Deep Field SouthI. LABBÉ1, M. FRANX1, E. DADDI3, G. RUDNICK2, P.G. VAN DOKKUM4, A. MOORWOOD3, N.M. FÖRSTER SCHREIBER1, H.-W. RIX5, P. VAN DER WERF1, H. RÖTTGERING1, L. VAN STARKENBURG1, A. VAN DE WEL1, I. TRUJILLO5, andK. KUIJKEN1

1Leiden Observatory, Leiden, The Netherlands; 2MPA, Garching, Germany; 3ESO, Garching, Germany; 4Caltech, Pasadena (CA), USA; 5MPIA, Heidelberg, Germany

1. Introduction

Between October 1999 and October2000 an undistinguished high-galacticlatitude patch of sky, the Hubble DeepField South (HDF-S), was observedwith the VLT for more than 100 hoursunder the best seeing conditions. Usingthe near-infrared (NIR) imaging mode

of the Infrared Spectrometer and ArrayCamera (ISAAC, Moorwood 1997), weobtained ultradeep images in the Js(1.24 µm), H (1.65 µm) and Ks (2.16µm) bands. The combined power of an8-metre-class telescope and the high-quality wide-field imaging capabilities ofISAAC resulted in the deepest ground-based NIR observations to date, and

the deepest Ks-band in any field. Thefirst results are spectacular, demon-strating the necessity of this deep NIRimaging, and having direct conse-quences for our understanding ofgalaxy formation.

The rest-frame optical light emittedby galaxies beyond z ∼1 shifts into thenear-infrared. Thus, if we want to com-pare 1 < z < 4 galaxies to their present-day counterparts at similar intrinsicwavelengths – in order to understandtheir ancestral relation – it is essentialto use NIR data to access the rest-frame optical. Here, long-lived starsmay dominate the total light of thegalaxy and the complicating effects ofactive star formation and dust obscura-tion are less important than in the rest-frame ultraviolet. This therefore pro-vides a better indicator of the amount ofstellar mass that has formed. Com-pared to the selection of high-redshiftgalaxies by their rest-frame UV light,such as in surveys of Lyman BreakGalaxies (LBGs, Steidel et al. 1996a,b),

Figure 1: Three-colour composite image ofthe ISAAC field on top of the WFPC2 main-field, outlined in white, and parts of threeWFPC2 flanking fields. The field of view isapproximately 2.5 × 2.5 arcminutes andNorth is up. The images are registered andsmoothed to a common seeing of FWHM ≈0.46″, coding WFPC2 I814 in blue, ISAAC Jsin green and ISAAC Ks in red. There is astriking variety in optical-to-infrared colours,especially for fainter objects. A number ofred sources have photometric redshifts z > 2and are candidates for relatively massive,evolved galaxies. These galaxies would notbe selected by the U-dropout technique be-cause they are too faint in the observer’s op-tical.