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NEWS AND VIEWS PLANETARySCIENCE------teresting possibility is that if it were not for How special is ? the , the would not be habitable, at least for advanced organ­ George W. Wetherill isms that think about questions of this kind. Maybe we must observe a Jupiter in BUILDING Jupiter has long been a prob­ young GM Aurigae2, which is roughly our because otherwise we lem to theorists. Observations presented 2 million years old. wouldn't be here. Instead of producing on page 494 of this issuel confirm that the This early loss of gas could make prob­ of roughly 300 Earth , problem is indeed a serious one. lems for the formation of Jupiter and perhaps lack of synchronization of the The idea that and planetary sys­ . Theoretical models for the forma­ independent processes of planetary tems including our Solar System form tion of Jupiter usually first require the growth and gas loss would usually lead to simultaneously from a disk-shaped nebula growth of a core roughly 10 times the -poor 'failed' Jupiters of about of gas and dust is an old one, dating back ofthe Earth, upon which the gas necessary the mass of and . to the days of Kant and Laplace. In recent for Jupiter to grow to its full 317 Earth In such systems the outer Solar System decades this hypothesis has finally re­ masses will accrete from the disk in a few would not be cleansed of residual ­ ceived considerable observational and million years. The requirements for the ary material, including bodies the size of theoretical support. , millimetre formation of Saturn are similar. Obvious­ or even larger. Instead, most of and optical wavelength observations of ly this entire process must take place these bodies could be stored in nearby very young stars demonstrate the pres­ before the gas has been lost from the disk. trans-Neptunian orbits, from whence they ence of infalling circumstellar disks of It has proved awkward to develop a could easily penetrate into the inner Solar dust and gas associated with these stars. quantitative model that permits a suffi­ System over the entire age of the Solar 7 Theoretical calculations have supported ciently large Jupiter core to grow during System • Catastrophic impact events of plausible mechanisms whereby, in a sym­ the short time that gas is present, and at the kind believed to have caused the mass biotic manner, the excess angular momen­ the same time is consistent with the abs­ extinctions at the CretaceouslTertiary tum associated with a 'core' of a parental ence of large planets in the belt, bounda~ might then have occurred about interstellar molecular may be parti­ although this may be possible for seeming­ every 10 years instead of about once in tioned between the newly formed central ly ad hoc distributions of mass of gas and 108 years. Stabilization of the Earth's star and a residual disk of gas and dust. dust in the disk. An 'easy' way out of this obliquity and therefore its climate and The star thus obtains the mass it needs to dilemma could be to hypothesize that the habitabili~ by the has been function as a hydrogen-burning thermo­ special conditions necessary to form Jupi­ suggested ; we may be more dependent nuclear furnace, while losing the excess ter and Saturn represented an unusual on other members of our Solar System angular momentum that would prevent it stochastic circumstance, and that ­ than previously thought. from collapsing into a compact body. ary systems rarely include gas giant It could justifiably be argued that the Much of this angular momentum, associ­ planets, even if their theoretically based suggestion that Jupi­ ated with only a small part of the mass, can regions are populated by bodies generally ters are rare is simply an excuse for our be transferred to a residual centrifugally similar to those of our Solar System. ineptitude in producing an adequate supported cirsumstellar disk from whi I:h There is some observational support for theory for the formation of the outer planets may form. this. Conventional models for the forma­ planets. The answer must come from At present, our con­ tion of the '' of , roughly observation. Theory can stitch together tains no more than a few per cent of the 104_105 astronomical units (AU) distant, threads of observations into a fabric of gas, primarily hydrogen, that should have predict that this process is inefficient and understanding. The observational data accompanied a preplanetary disk of cos­ that many more comets were ejected from are not yet definitive. Taken altogether, mic chemical composition. How and when the Solar System than were preserved in the evidence does raise the possibility that was this excess gas lost? Our well behaved the Oort cloud. Jupiter (and Saturn) are what was thought to be the sine qua non of middle-aged is not capable of 'blow­ primarily responsible for this ejection. If a 'real' planetary system, the presence of ing away' such large quantities of gas. It were always accompanied planets like Jupiter and Saturn, may seems most likely that the necessary loss by ejection of comets, interstellar space actually be unusual. In contrast, more of gas from the plane of the disk would would be well populated by these bodies, easily made terrestrial planet systems occur during the less stable first few which from time to time should penetrate physically similar to ours may be abundant million years of a star's life. the inner Solar System and be discovered but hazardous unless protected by gas Zuckerman et at. l report radiofre­ as comets with distinctive hyperbolic giant planets. 0 quency observational data supporting this orbits. No such comets have been picture for 20 nearby young stars, all observed, whereas it has been argued that George W. Wetherill is in the Department of 3 estimated to be between 1 and 10 million at least one should have been . Terrestrial Magnetism, Carnegie Institution years of age, using methods of varying More direct evidence for a paucity of of Washington, 5241 Broad Branch Road reliability. They determined the gas densi­ extrasolar Jupiter-like planets is suggested Northwest, Washington DC 20015, USA. ty from the intensity of monoxide by telescopic searches for perturbations of molecular transition lines, direct deter­ the motion of nearby stars by possible 1. Zuckerman, B., Forveille, T. & Kastner,J. H. 373, mination of Hz being impossible because Jupiter-mass companions4-6. Although 494-496 (1995). 2. Koerner, D. W., Sargent, A. I. & Beckwith, S. V. W.lcarus of the absence of suitable Hz radiofre­ more than 20 stars have been well studied, 106,2-10 (1993). quency transitions. Concentrations of H2 no evidence has been found for compan­ 3. Weissman, P. R. in Comets in the Post-Halley Era (eds Newburn, R. L. et al.) 463--498 (Kluwer, Dordrecht, were then obtained by assuming the stan­ planets down to a limit of 1 to 3 Jupiter 1994). 4 dard interstellar ratio (10 ) between the masses at a distance of 5 AU. 4. Walker, G. H. etal.lcarus (in the press). number of Hz and CO molecules. Subject If 'Jupiters' are rare, why should we 5. Cochran, W. D. & Hatzes, A. P. in Planets Around Pulsars (edsPhilips,J.A. etal.) 267-274 (Astr. Soc. Pac. Coni. to many caveats, the authors conclude happen to live in an unusual planetary Vol. 36, 1993). that within a few million years after the system that contains a Jupiter (and a 6. Marcy, G. W. & Butler, R. P. Publ. astr. Soc. Pacif. 104, 270--277 (1992). formation of a star, the mass of H2 gas Saturn as well)? One answer could be that 7. Wetherill, G. W. in Planetary Systems: Formation, remaining is typically very small, much our having a Jupiter is just a random Evolution and Detection (edsBurke, B., Rahe,J. H. & Roettger, E. E.) 23--32 (Kluwer, Dordrecht, 1994). less than the mass of Jupiter. An excep­ chance. If gas can occasionally form 8. Laskar,J. & Robutel, P. N. Nature 361, 608--615 tion to this has been found for the still somewhere, why not here? A more in- (1993). 470 NATURE· VOL 373 . 9 FEBRUARY 1995