illion :l as- erial !r, in nical time final simi- fther and mto Probingthe GiantPlanets rtion f 106 have very eccentric orbits while our and own giant planets orbit the Sun in se- date circles. Could it be that "otdi- nary" circular orbits are in fact quite extraordinary by Galactic standards? Is our solar system an unlikely out- come of the general mechanism of TristanGuillot planet formation? Or perhaps, might extrasolar planets be formed by a dif- ferent mechanism? llefore 1995, most astronomers expected giant extra- These are but a few ofthe questions that confront us. lJsolar planets to orbit their stars in quasicircular or- There's no shortage of suggested answers, but none thus bits at a distance of more than a few astionomical units. far has given satisfaction. A large part ofthe problem is (1AU is the mean b distance between Earth and the Sun.) that the so-calledradiovelocimetry method used until now qr tt our Solar System, the orbits of the four giant planets- to discover extrasolar planets provides only a lower limit Jupiter, Saturn, IJranus, and Neptune-have semimajor on the planetary mass M. Relying on the tiny Doppler mod- axes ranging from 5.2 to 30 AU and eccentricities no ulation as the star is tugged to and fro by its orbiting la larger than 5.6Vo. planet, the method determines the orbit's diameter and ec- Since then, more than 100 extrasolar planets have centricity. But the inclination angle i between the normal been discovered, all ofthem giants with at least 107othe to the orbital plane and the line of sight is unknown, and mass of Jupiter (0.1 M,, about twice the mass of Uranus radiovelocimetry only determines the product M sini. r-a or Neptune). Much smaller Earthlike ("terrestrial") extra- Of course, we do have interesting additional meas- ral solar planets would not be massive enough to be detected urements. For example, it is becomingclear that h6 stars with by current methods. planets tend to be enriched in heavy elements-meaning, Ein The newly found planets are strange-not at all what loq in astronomers'jargon, anything other than hydrogen and we expected.A signifrcant fraction ofthe extrasolar giants rcm helium. That tells us something about planetary forma- tDr, detected thus far orbit extremely close to their star, that tion, namely that planets probibly grew more rapidly in is, at less than 0.1AU. Some semimajor axes are as small environments rich in dust. (Most heavy elements form n,K as 0.04AU, implying a period of revolution around the star chemical speciesthat condenseat low temperatures.) But ofonly about three Earth days. The archetype ofthese so- "hot we do not know two crucial properties ofthe extrasolar gi- called Pegasi planets (also called Jupiters") is the first ants: their exact masses and their sizes. Except for one M7 one to have been discovered: 51 Pegasi b, a roughly serendipitous casea-the planet HD209458b. flx, Jupiter-mass giant orbiting at only 0.05 AU from a sun- like star in the constellation Pegasus.l A planet in transit ,219 The other fraction of extrasolar planets discovered HD209458 is a faint star in Pegasus,barely visible to the since 1995 may be even stranger. As shown in frgure 1, naked eye. Like 51 Pegasi, its planet has a mass close to many of them exhibit very eccentric orbits,2 quite unlike that of Jupiter, and a tiny semimajor axis, a mere 0.045AU. 54if our own giant planets. It may be, however, that Solar Sys- The particularity of the HD209458 system is that, as seen tem analogs are common rigin but hard to discover.After all, from Earth, the planet passesin front ofthe star every 3.5 Jupiter and Saturn U. of are so far from the Sun that they take days. Therefore, we can measure the time it takes for the 12 and 29 years, respectively, to complete an orbit. A few planet to transit the stellar surface and the consequent :M. planets in frgure t have large orbits with low eccentricity. dimming of the star (about 2Vo).Thus we can also calcu- Discovering more objects like these will require patience late the planet's size, the inclination ofits orbit, and there- and improved observational techniques. fore its mass.5 Ithasbeen very dillicult to 'the). construct a coherent sce- The radius measured for the giant planet HD20g4b8b nario that would explain the formation of giant planets as is about SSVolargerthan that ofJupiter, although its mass \riz. we seethem both within and beyond our solar system. The is 30Voless than Jupiter's. This apparent contradiction is presenceofclose-in planets is generally thought to be due understood by realizing that gaseousgiant planets ;, P. tend to to early migration of planets forming in a disk of gas and cool slowly by infrared emission and thus contract. and dust surrounding the young central star.3 But that's not that planets 952 heavily irradiated by their stars contract the only possibility, and it doesn't explain why Jupiter and more slowly. Jupiter itself is estimated to be contracting its sisters didn't share such a fate. at a rate ofabout 3 mm per year. nof Similarly, it is not clear why extrasolar planets often The observations of HD209458b, a hundred times TX closerto its star than is Jupiter, confrrmed the general the- oretical picture and showed that the planet, discoveredin tuf,, 200O,is made mostly of hydrogen and helium.6 The study I of this transiting Jovian giant was the first confirmation

@ 2004 American Instituteof Physics,S-003i-9229-0404-040-2 April 2004 PhysicsToday 63 and coworkers,s in 200L, detected the presence of sodium in the atmosphere of HD209458b. The sodium, it turned out, was less abun- dant than expected. But be- cause we know so little about the atmospheres of these ex- otic planets, it's not yet clear what that means. The sodium shortfall could, for example, be due to colder atmgspheric temperatures than expected. Or it might result from at- mospheric dynamics or non- equilibrium effects. In any case, the observation was a milestone in the study of ex- trasolar planets, .because it showed that we can detect constituents in the atmos- pheres of planets millions of times further from us than Jupiter. Another crucial recent discovery is that HD209458b is slowly losing mass. Last year,Alfred Vidal-Madj ar and coworkers found that when one observes the transiting planet at the LIV wavelength of the Lyman-a absorption line of hydrogen, it appears three times larger than at other wavelengths.eThis suggests the presence of an ex- tended, tenuous, envelope ofhydrogen escaping from the planet as a result ofheating ofthe upper atmosphere and bombardment by ionized stellar wind. The magnitude of this mass loss appears to be consistent with estimates based on Jupiter, but scaled up by a factor 10 000 because the star is 100 times closer to the planet than the Sun is to Jupiter.GThe Lyman-a measurements give us the first possibility of quantiffing such processes more precisely planet that our models of formation were not completely and examining how gaseousplanets manage to suryive so off-track. Had HD209458b been found to have a smaller close to their stars. diameter than Jupiter, it would have meant that the new Impressive as they are, the HD209458b measure- planet consists mostly of heavy material, which would ments tell us only about the upper atmospheres of giant patently contradict our formation models. gas planets. How can we better constrain their interior However, the hope that we would be able to learn more compositions?To do so,we have to learn how surface meas- precisely the composition of HD209458b has dwindled rap- urements relate to interior compositions.And to that end, idly. The planet's radius turned out to be slightly larger we must look more carefully to the gas giants closestto us. than expected. So we're probably missing some enerry source that prevents the planet from cooling faster.TIt ap- Our own giant planets pears that tides may be dissipating heat into the planet's We can, of course, measure very accurately the masses, interior and thus slowing its contraction. The energ'y sizes, and hence the mean densities'of Jupiter, Saturn, source might be the orbital enerry of an unseen eccentric IJranus, and Neptune. The densities turn out to be quite giant companion planet, or it might be winds generated in low-ranging from 0.7 glcml for Saturn to 1.6 g/cm3 for the planet's atmosphere by the strong stellar irradiation. Neptune. For comparison, the four inner terrestrial plan- Alternatively, the atmosphere may be hotter than most ets have mean densities near 5 glcml. One might have at- models predict (seefigure 2). We know little about meteor- tributed the difference to the conjecture that the giant ology and tides on gaseousplanets. Tens ofEarth masses planets have the same composition as Earth but very much of heavy elqments could be mixed in with the hydrogen and hotter. Alternatively, one could supposethat they are very helium without our being able to tell. Better understand- much colder but made of light material. Of course, we've ing the giant planets will require more examples of tran- known for a hundred years that only the second explana- siting extrasolar planets, with different massesand orbital tion is valid. distances. That's an important goal of spacemissions such Indeed, a look at the atmospheres ofthe giant planets as COROT, Kepler and, we hope, Eddingtoa (seebox 1). shows that they are made mostly of hydrogen and helium, In the meantime, the power of transit observations with only traces of heavier elements. Exactly how much of has been demonstrated. Because we know exactly when these trace elements is present is a big question. Remote the planet is due to transit in front of its star, we can make sensing cannot probe very deep, becausethe atmospheres very accurate measurements and compare the on-transit become too opaque and because cloud formation sequesters to off-transit results. In just that way, David Charbonneau a number of important elements in the deep regions. The

64 April 2004 PhysicsToday http://www physicstoday.org t-, lo1, of eof t, it )tur- be- tout ex- lear ium Lple, eric ted. at- ton- any tsA i ex- eit rtect nos- their abundancesin the Sun. sof sequestered elements include water, the main carrier of measuring its abun- han oxygen. Being the third most abundant element in the uni- Ammonia was also detected, but verse, oxygen is a critical but hidden component of the danceposed problems becauseofits tendency to stick to the cent giant gas planets. walls of the probe's mass spectrometer. The most elusive 158b The Galil.eoprobe, which was sent down into the Jov- species,however, proved to be the one that was most sought Last ian atmosphere in September 1995, was desigrredto detect after-water. The probe did detect some water, but much and some of the hidden atmospheric ingredients. It success- less than expected,and its abundancewas still rising in the ting fully probed Jupiter's atmosphere down to a pressure of 22 last measurements before the probe frnally fell silent.10 tion bar. (1 bar = 105 pascals, roughly the mean sea-level at- What does that deficit mean? In the frigid outermost nat mospheric pressure on Earth.) Thus the probe was able to precincts ofthe Jovian atmosphere,the water is condensed. I ex- measure accurately the abundance ofconstituents such as To properly measure its bulk abundance, the probe had to the helium, methane, hydrogen sulfur, neon, argon, krypton, reach levels deep and hot enough for the water to be en- and and xenon. Except for helium and neon, all these species tirely vaporized. It had been thought that probing down to le of appear to be enriched by about a factor ofthree relative to 5 bar would suffice. Obviously, that was too optimistic. ates tuse mis first 1-he formidablepromise of the transit rsely planetshas re I methodof studyingextrasolar so led to the developmentof severalspace mis- sionsdedicated to the discovervand meas- ;ure- urementof moreextrasolar planets transiting iant in frontof their stars.Canada's 15-cm MOST )rior telescope,launched in August2003, can reas- follow giantextrasolar planets closely orbit- end, ing their stars,even if they don't transitthe DUS. stellardisk. Two other missions.the French SpaceAgency's COROT and NASA'sKepler, are due to be launchedin 2006-02.The fu- '{les, ture of the EuropeanSpace Agency's Ed- llrn, dington mission,originally scheduled for a prte 2008 launch,is unclear. I for These missions are designed to search rlan- hundreds of thousandsof stars for planets e at- ranging from Pegasigiants down to Earth- !ant sized planets.To do all that, the instruments ruch will haveto achievevery stableand accu- very rate photometric measurements.The dim- rete ming of a sunlikestar by a transitinggiant ana- planetis only of order 1%; for a transiting Earth-sizedplanet, the dimming is 100times weakerstill. nets - ium, By comornrngtne space-Daseoooserya- rs'with"grouni-b*"b radiovelocimetry,we will be able to ch of te radii and massesof gasgiants and ice giants.For note t$tramlarplanets thatlrrsr areors alsodrJv veryvcl closeLlvJs to rv theirfllsll reres Prdlq) )t be ableto measurethe modulationof the storls vters as its phase changesfrom our vantage aboundin: The y.org srrir?gg+ planet After the fact, the consensus of the community is that the gravitational potential V just'outside the is probe fell into a rare dry hot spot. If,s as ifan alien race de- given by iigned a probe for Earth's atmosphere and it just happened to fall onto the Sahara desert. Very likely, there is more v(r, cos o)= - "r,,e,,(.o,e)], water elsewhere on Jupiter, or at greater depths. rylt 2(h)'' The only secure conclusion is that we don't under- polar andM stand the meteorology ofgiant planets. That's becausethe where r and 0 are the radial and coordinates radius. The meteorology is inherently complex. We don't know how it's and Ro are the planet's mass and equatorial J*the tied to the planet's internal structure, and data are functions PrdcoJ0) are Legendre polynomials and parameterize painfully scarce. Visible and IR observations, and even corresponding gravitational moments that the deepestprobe we could design, only provide skin-deep the cylindrically symmetric mass distribution. by incursions into the interior. To really understand the The -ass and gravitational moments determined potential giant gas planets, we have to avail ourselves of another satellite measurements of the gravitational density profrle technique. translate into a constraint on the interior p(r,0). Theseconstraints can be written Gravimetry = One possible method is seismology. Solar astronomers M ! o(r,e)d, have learned much from observing helioseismic modes' Unfortunately, it is not clear that the giant planets can os- t = - p(r,o)r2iPr,(cos g)dr, cillate like the Sun. Attempts to measure seismic oscilla- r, h I tions on the giant planets ofthe Solar System have thus far been inconclusive.l: where dr is a volume element and the integrations are per- Our last resort is gravimetry. The giant planets rotate formed over the entire volume of the planet. quite rapidly. The fastest is Jupiter, with a diurnal period Because of the form of the Legendre polSmomials, of t hours, 55 minutes; the slowest is lJranus, with a pe- gravitational moments of progressively higher order con- riod of 17 hours, 14 minutes. itrain the density profrle of regions closer to the planet's Therefore, all the Solar System giants are signifi- atmosphere. Unfortunately, only J" and Jn are presently cantly flattened by centrifugal acceleration. The conse- known well enough to provide useful constraints on the in- quent departure of the gravitational field from spherical teriors ofJupiter, Saturn, IJranus, and Neptune. symmetry can be measured by careful monitoring of the The density p is itself a function of several thermody- trajectory ofa spacecraftcoming closeto the planet, prefer- namic variables: pressure, temperature, and composition. ably on a polar orbit. Roughly speaking, when the space- The pressure can be calculated quite accurately becagse craft is near the equator, its trajectory is less influenced gianf planets are always very close to hydrostatic equilib- by the planet's core, and more by the outer regions, than iium; internal pressure equilibrates gxavity and inertial when it flies close to either pole. forces at all points. Determination of the internal temper- For a fluid planet in the absence of tidal forces, the ature is more problematic. Fortunately, the giant planets radiate more energ-Jr'than they receive from the Sun. We can measure the rate of energy loss and estimate how they cool. Jupiter, for example, cools bY about 1 K per million years. We can then infer how heat is transported within the planetary interiors. Convection appears to be the dom- inant form oftransport in Jupiter, Sat- urn, and Neptune, and probably also in IJranus. Becauseconvection is very ef- ficient in these fluid planets, tempera- ture structure should be close to an adiabat-that is, a system that ex- changes no heat with its environment. The temperature distribution can then be calculated as a function of pressure, composition and the known atmos- pheric boundary conditions. Composition is the knotty Prob- lem. It affects the temperature struc- ture and can also affect convection. That leaves an infinite number of

66 April 2004 PhysicsToday http://www.physicstoday. org 165-170K 1 bar 135-145K 6300-6800 2 Mbar u be hr 4 100K !v nl ile gen in which helium-rich droplets form and fall to deeper regions. Such 15 000-21000 10000 K phase separation should occur at high 40 Mbar Mbar pressures and low temperatures. But neither laboratory experiments nor calculations have yet confirmed that it really does happen under conditions :,, relevant to Jupiter and Saturn. Inter- :r- ,' estingly, the Galileo probe found a ,, lower concentration of neon in the ls, outer reaches ofJupiter than the Sun n- has. That's consistent with the predic- t's tion that neon should efficiently dis- llv solve into the fallinghelium droplets.l2 E. Other constituents, such as ices and silicates, may have been delivered ly- to the forming planet very early on, as )n. IJRANUS N.EPTUNENEPftJ ingredients of a central solid core. EE They may have arrived in small pieces b and mixed with the mostly hydro- ial solutions describing how pressure, temperature, density, gen-helium envelope, or they may have come as large- lr- and composition might depend on r. The true solution may say, Earth-sized-protoplanets. In the latter case,the ma- )ts depend on the initial conditions. terials would have penetrated deep into Jupiter. YE Tb make matters even more complicated, any primor- rte The GedankenCaf6 dial central core might either have remained largely intact ey To illustrate the conundrum, let's assume you're sitting until now or, like the sugar cube in our espresso cup, it by comfortably in the sun at a caf6 and you're served an might have been eroded by convection. The spoon in the an espresso.I like it sweet and, in any case,our gedanken ex- cup plays the role ofconvection in the giant planets. De- ed periment requires sugar. Ifyou drop a sugar cube into the pending on the vigor and depth ofthe convection, it either precincts. cup and let it dissolve, most of the sugar will stay at the will or will not scoopcore material up into higher ln- you bottom. If you stir the liquid gently (let's say for fear of But independently ofhow stirred the espresso,so rt- a little bit, you expect it to be spilling it on your laptop), only part of the sugar will mix long as it's been stirred even in homogeneous. Only the last sip might be signifr- with the coffee.You get a homogeneoussystem only if you fairly ef- cantly sweeter, and there might be a residue of sugar left stir vigorously enough. !a- in the bottom of the cup. In the same way, we expect our An alternative method would be to drop finely granu- an giant planets, which are mostly convective, to be reason- lated sugar instead ofcubes into the coffee.Ifthe sugar is )x- ably well-mixed. That simplifies the problem enorrnously. frne enough, it will dissolve before reaching the bottom, ot. We have to consider only three principal layers. For is needed. But that's only if the en and very little stirring Jupiter and Saturn, they are a central core and an enve- rre, espressois hot enough. If it's only lukewarrn, the sugar has lope that is split into a helium-rich interior and a helium- )s- a hard time dissolving; it tends to sink to the bottom. Fi- poor outer shell (seefigures 3 and 4.) For Uranus and Nep- nally, after all this preparation, let a friend arrive at the tune, the two much smaller giants, it is less clear that this rb- table and try to guess, from his first sip, how much sugar approach is valid. Nonetheless, astronomers often'divide lc- there is in the cup. their interiors into a central rock core, a surrounding ice )n. For giant planets, the problem is similar, only more mantle, and an outermost hydrogen-helium envelope. of complex. Ifthe coffeeitselfis a good analog for the hydro- Models of the planetary interiors that match all the gen, the sugar is replaced by a mixture of helium, water, observational constraints conclude that Jupiter and Sat- methane, ammonia, neon, and many more chemical urn consist mostly of hydrogen and helium. Saturn ap- species,all ofwhich behave differently. They could, for ex- pears to require a dense central core of L0-25 Earth ample, either be mixed with the hydrogen or sequestered masses, while Jupiter's core has no more than 12 Earth in the deepest regions. masses.13lJranus and Neptune are quite different in struc- Part ofthe helium is believed to have fallen into the ture from their larger cousins. While Jupiter and Saturn interiors ofJupiter and Saturn. Less of it is observed in are called gas giants, the other two are really ice giants. their atmospheres than must have been present when the Indeed, for more l};ran 70Voof their interiors, the density Solar System was formed. This sequestration is thought to profile appears to be consistent with that of compressed be due to a phase separation between helium and hydro- water ice. Their outer precincts appear to be gas envelopes

http://www.physicstoday. org April 2004 PhysicsToday 67 n 1 July2004, Saturn will acquirea \lnew satellite.The Cassinispacecraft will fire its enginesfor orbit insertioninto biq the Saturniansvstem. For the next four years,it will studySaturn's moons, rings, ;fl and magnetosphere,and alsothe planet's ur{ intriguing meteorology. The spacecraft "4 will remotelymeasure the compositionof inl Saturn'satmosphere. or.{ Cassiniwill'also determinethe gravity field much more accuratelvthan has been done before. Cravitationalmo- { ments at least up to ,1,should be deter- minedwith highprecision, which would r4H allow a much betterdetermination of Sat- b4 urn's internalstructure. We should then begin to understandthe planet'sinterior rotation. At present, we don't know H whether Saturn'satmospheric winds are tr superficialor deep-rooted Early next yeat Cassini will drqp a probe, called Huygens,into the atnegi* phereof Titan,Saturn's largest moon. The Cassini-Huygensmission is an impres. f, siveexample of a successfulcooperation "c between NASA, the EuropeanSpace Agency,and the ltalianSpace Agency. H consisting mostly of a few Earth masses of hydrogen and of argon at three times the solar abundance inside H helium.la The few measurements we have don't allow a Jupiter's atmosphere.loArgon is a noble gas that is ex- ad more accurate estimate of the structures of l]ranus and pected to condensein the protosolar nebula only at very tH Neptune. low temperature-no higher than 30 K. One therefore ex- For Jupiter and Saturn, it is important also to con- pects that the ratio ofargon to hydrogen should be about sider their total content of heavy elements. Present mod- the same in Jupiter and in the Sun. The unexpectedly els estimate the heavy-element component at 20-30 Earth large Jovian argon abundance indicates that some physi- masses for Saturn and 10--40 Earth masses for Jupiter. cal process led to the separation of argon and hydrogen The uncertainty is signifrcant. Most of it is due to our lim- during the planet's formation. That points to relatively low ited understanding of the behavior of hydrogen at ultra- temperatures at the time of Jupiter's formation. high pressures on the order ofa megabar. One possibility is that small ice grains grabbed some Shock compression experiments can now reach these of the argon in the protosolar nebula (in a process called high pressures in the laboratory. However, two sets of such clathration) and that the grains were somehow efficiently experiments have yielded conflicting results.ls The 1998 brought into Jupiter.l6 This process would imply that laser-induced shock experiments at Lawrence Livermore water is abundant inside the planet. Unfortunately, pres- National Laboratory measured a maximum density com- ent models ofthe Jovian interior are too uncertain to tell I pression ofabout a factor of6 for hydrogen at 1 Mbar. The us whether clathration is the correct answer. more recent experiment by Marcus Future high-pressure laboratory d Knudson and cowork- experiments on deu- \ ers with magneticallj' accelerated plates at Sandia Na- terium compression and numerical calculations of the be- : tional Laboratories found a maximum compressionof only havior of the hydrogen-helium mixture at the relevant I about 4 at the same high pressure. pressures and temperatures will give us a clearer picture I For Jupiter, the greater hydrogen compressibility im- of the interiors of the giant planets. So will t}re Cassini 8j plies a significantly smaller component of heavy elements. satellite's precise determination of Saturn's gravitational E Hence the wide uncertainty range of 10-40 Earth masses. moments. But we'll still be missing cr-ucial pieces of 10d For Saturn, on the other hand, the total amounts ofheavy the puzzle: the abundance ofwater in Jupiter and Saturn, l elements appear to be more or less independent ofhydro- and a precise knowledge ofJupiter's gravitational and mag- lil gen's equation of state at ultrahigh pressure. That's netic fields. Uranus and Neptune will remain mysterious. mostly because the pressures in Saturn's interior are Jupiter is the next giant planet on the agenda. It is 12. smaller. Our hopes rest on the Cassini mission, whieh easier to reach than the other Solar System giants, and it's : should allow a much better determination of Saturn's the one from which we can learn the most. How, specifi- 13i gravity field (see box 2). cally, should we proceed?One possibility, inspired by the L4-t Galileo mission, is to send several more probes into the at- Lessonsand prospects mosphere. They should be able to penetrate deeper than 151 Our knowledge ofthe interior structure ofthe giant plan- the Galileo probe-down to at least 100 bar. The Galileo ets remains vag'ue, which severely limits our progress in probe taught us that Jupiter's atmosphere is complex.Al- 1E understanding how they formed. For example, one of the though the new probes would provide unique local data, I unexpected results from the Galileo probe is the presence the measurements could be diffrcult to interpret without L7:

68 April 2004 PhysicsToday http://www.physicstoday.org prior knowledge of deep winds, meteorological structures, and so forth. Therefore we must, first of all, learn more about the global structure of Jupiter's deep atmosphere. Using an or- biter that comes within 4000 km of the planet's cloud tops on a polar orbit, one can get very accurate measurements of the planet's gravitational and magnetic fields. The meas- urement of higher-order gravitational moments would re- veal whether or not the zonal winds are rooted deep in the interior.l? That issue is critical for understanding the mete- orology and internal structure of rapidly rotating gaseous planets. With a radiometer added to the spacecraft, one can measure variations in the deep atmosphere'stemperature and its ammonia and water abundances as a function of latitude and longitude, down to pressures ofa few hundred bar. Using meteorological and radiative-transfer models together with the most up-to-date laboratory measure- ments of molecular opacities at high temperature, one can then disentangle the different satellite data to determine constraints on the abundance ofwater. Thus equipped, the next generation ofmissions to the giant planets, probably including a few well-designed probes to be dropped into specifrclocations, should do much to unlock their secrets. Giant planets, extrasolar as well as our solar system neighbors, retain most ofthe keys to understanding how I planets form and evolve. Their exploration began with the Pioneer, Voyager, and Galileo missions. The scheduled Cassini-Huygens mission to Saturn promises great scien- tific returns. But the most signifrcant void in the inven- tory of Solar System data is probably Jupiter's unknown composition. To understand how the Solar System formed and to acquire fundamental data that will be crucial for generation astronomers, let the explo- the following of Circle number 26 on ReaderService Card ration continue! References M. Mayor, D. Queloz, Noture 378,355 (1996). G. W. Marcy, R. P. Butler, Annu. Reu.Astron. Astrophys.36, 57 (1998); D. A. Fischer et al.,Astrophys. J. 586, 1394 (2003); C. Perrier et al.,Astron.Astrophys.,4l0, 1039(2003). 3. D. N. C. Lin, P. Bodenheimer, D. C. Richardson,NatureSSO, 606 (1996). 4. There are other transiting candidates, but they have yet to be confrrmed. See M. Konacki et al., Nature 421,507 (2003). l 5. D. Charbonneau, T. M. Brown, D. W. Latham, M. Mayor, As- trophys. J. Lett.529,lA5 (2000); G. W. Henry G. W. Marcy, R. P. Butler, S. S. Vogt, Astrophys.J. Lett.529,IAl(2000); T. Brown et al., Astrophys. J. 552,699 (2001). 6. T. Guillot et al., Astrophys. J. Lett. 460, L993 (l-996). 7. P. Bodenheimer, D. N. C. Lin, R. Mardling,Astrophys. J.548, 466 (2001); T. Guillot, A. P. Showrnan, Astron. Astrophys. fi$taxriffi 385, 156 (2002); l. Baraffe et al., Astron. Astrophys. 4O2,7Ot (2003); A. Burrows, D. Sudarsky, W. B. Hubbard.,Astrophys. J.594.545 (2003). 8. D. Charbonneawet al., Astrophys. J. iffi,377 (2002). #bi,$ruMij$rtt":.1 9. A. Vidal-Madjar et al., Nature 422, L43 (2003). r,M 10. T. Owen et al, Nature 4O2,269 (1999); S. K. Atreya et al., rffi:;r Planet. SpaceSci. 51,105 (2003). *.rr 11. B. Mosser, J. P. Maillard, D. M6katnia, Icarus l/U, L04 LqJ+ (2000). 12. See abstract, M. S. Roustlon, D. J. Stevenson EOS Tlans. Arn. Geophys.Union 76,343 (1995). 13. T. Guillot, Science 2a6,72 (1999); D. Saumon, T. Guillot, Astrophys. J. (in press). 14. M. Podolak, J. I. Podolak, M. S. Marley, Planct. Space Sci.48, 143 (2000). 15. G. W. Collins et a1.,Science 281, 1178 (1998); M. D. Knudson et al.,Phys. Reu. Lett.90, 035505(2003). 16. D. Gautier, F. Hersant, O. Mousis, J. I. Lunine, Astrophys.'J. 55O,227(20Ot). 17. W.B. Hubbard,lcorus 137,196(1999). I

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