Planetary Diversity

David J. Stevenson, Guest Editor

fince upon a time, on a planet far away, a guest editorV for the local equiualent of PHYSICS ToDAY was puzzlingouer the challenge ofan issue deuoted to planetary science.For it seeme!. to her that planets are so conl.rlon yet so di-uerse in character that, euen omitting planetary biology, onecould not hope to couer their richness and accompanyingscientifi.c challenges. Should one talk about the recentlydiscouered planets with neon oceans, or the purely ironplanets, stripped of their primordial rocky mantles? Or thesystems whose disks are continuously recycled so that theirplanets keep falling into the central stari only to be replacedby copies inthe outer suburbs? What of those systems wherebiology has consumed all the planetary m.atter becausethere are a uery large number of small planets? (Such sys-terns optimize entropy production because they maximizethe interception oflight from the central star.) Or the plan-ets composed of giant single quanturn states?

From her uantage point on an ice-couered world orbit'ing a giant planet that fills much of the spectacular nightshy, she thought back to the time when scientists had onlyto be concerned with their planetary neighbors and the in-direct euidence ofplanets around other stars. Science thenhad seemed simple4 a rlere generalization of the conceptsthat gouerned the homeland. The diuersity presented bystellar and galactic astronisrny had seemed so great becauseone had not yet fully recognized the diuersity of planets.[Fade to black. . . .]

Much of the universe is unknown still, at least to uson Earth, although most of the known baryonic mass is ev-idently in a simple form. Most of it is hydrogen-dominatedgas or plasma, simple in behavior though bearing the seedsof elemental diversity through thermonuclear reactions instars. The predominance of hydrogen and the simplicity ofits material properties allow astronomers to collapse mostof the behavior of stars into a two-dimensional plot, theHertzsprung-Russell (H-R) diagram of color versusbrightness. And most stars fall along the main sequence ofthat diagram. Even the degenerate stars-the whitedwarfs and neutron stars-are simple, though in a differ-ent way because so much of their matter exists in an idealextreme form (as a Fermi gas). Only neutron stars com-pete with planets for the phenomenological richness andcomplexity of their behaviors.

That richness arises from imperfection and fromchemistry, here broadly defined to embrace the range of

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behaviors that can occur when mate-rials are alloys or physical mixtures.Planets lack the equivalent of the low-dimensional H-R diagram because oftheir complexity. But the richness ofplanets also arises from their choreog-raphy-that is, the remarkable rangeofpatterns in which they form and in-

teract. And, ofcourse, life exists on planets, though we donot yet know at what frequency.

What is a planet?This question must be asked, even if only to explain whyit is unimportant. Planetary scientists study what is inorbit around stars but not doing what a star does (or usedto do in an earlier life), which is fusion. All scientific la-beling is secondary to the essence of science, and labelssuch as "planet" exist only to help us talk to each other.Dust, asteroid belts, and cometary belts or clouds are justas important in the grand scheme of things. Mike Brown'sarticle on page 49 concerns some of the new things we arelearning about such objects. But for purposes of this dis-cussion, a planet could be anything from the range ofaboutVro the mass of Earth's Moon to about 10 Jupiter masses.That is a range of almost six orders of magnitude iir mass.On the low mass end, the range extends roughly to a pointat which the effects of gravity and insulation allow the in-side ofa body to be different from the outside. (Even thatis a rather arbitrary distinction, because the inside of acomet or asteroid can differ from its outside). The highmass end extends roughly to a point at which the fusionprocess is possible from the deuterium in a hydrogen-dom-inated body.

Within that range, a break in the spectrum of massesoccurs among those objects found so far in orbit aroundmain sequence stars.l Scientists have discovered manymore planets with masses below 10 Jupiter masses thanthey have planets in the brown dwarf range-from 10 to70 or 80 Jupiter masses (see frgure 1). The lowest mass forsustained fusion of light hydrogen is roughly 80 Jupiter.masses.

Why do planets exist?On one level, this is an easy question to answer. Planetsexist because of angular momentum. When a star formsfrom the interstellar medium, the collapsing cloud of gasand dust has unavoidable vorticity, far too great to be ac-commodated in the hydrostatic star that gravity tries tomake from the available mass. The excess angular mo-mentum can be accommodated by fragmenting the cloud,a process that leads to a binary or multiple-star system.But, in general, even those fragments will have too muchangular momentum and must each form a disk and a cen-tral stellar concentration of mass. From those Kepleriandisks, planets form.

Angular momentum does not guarantee planets, though.Gas giants can form in a manner somewhat analogous to

April 2004 Physics Today 43

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stars by a gravitational instability ofthe gas disk, providedit is sufficiently cold and dense. No one knows whetherthat mechanism actually works and played a significantrole in our solar system or other planetary systems. Themore popular view continues to be that planets like Jupiterform through nucleation-the formation of a solid body fol-lowed by gas inflow onto that body.

Solid bodies undoubtedly formed, but there is no con-sensus on this process either. Late stages in the growth ofbig objects were surely dominated by gravity. However, therelative importance of sticking (that is, weak chemicalforces) and gravity is uncertain for bodies just a few kilo-meters in size. The lack of consensus comes not so muchfrom ignorance ofprocess as from the complex interplay oftime scales and rates. One suspects that different plane-tary systems will have different outcomes, probably in-cluding the case in which material fails to accumulate intobig bodies. In her article on page 56, Robin Canup dis-cusses our current understanding ofplanet formation.

What determines diversity?When we cpnsider what exists and what is possible, threeissues are paramount: stability, cosmochemistry, and ori-gin. Stability prompts the question, Is the body able to sur-vive a long time-say, billions of years? Cosmochemistryprompts the question, Is the body's composition plausible,given the starting materials that the universe provides?Origin prompts the question, Is there a dynamical path-way that can make such a body? These three factors arestrong, moderate, and wehk, respectively, in their power toconstrain the formation of possible planets.

Stability considerations exclude a wide range of ob-jects or characteristics that cannot persist. For example, aself-gravitating one-Earth-mass body ofhydrogen and he-lium with the entropy of Jupiter could not exist in Earth'sorbit. It would literally disassemble by tidal action and theaction of external IIV radiation. Somewhat less dramati-cally, stability considerations also preclude small bodiesclose to the parent star from retaining substantial atmos-pheres. Stability should be regarded as a strong constrainton hypothetical planets because the physical considera-tions are frrmly based and well understood. Of course, itis usually much easier to prove that something is unsta-ble than to prove it stable, so even this consideration hasits limitations.

Cosmochemistry is the legacy of thermonuclear synthe-sis. It varies with cosmological epoch and locality, but the

44 April 2004 Physics Today

broad consequences are invariant. The elements can bedivided into three classes on the basis of nuclear

r physics and physical chemistry: gases, ices, and rocks; (see figure 2). Gas refers primarily to hydrogen and he-

lium, the most abundant elements in the universe.These elements also happen to comprise molecules that donot condense as solids or liquids under conditions that areencountered during planet formation. The material deepwithin a planet such as Jupiter can be called a liquid metalbut it has a much higher entropy than the value at the con-ventional critical point of molecular hydrogen, where liquidand gas merge. Thistan Guillot discusses our current under-standing ofthe giant planets in his article on page 63.

Ice refers to the compounds that form from the nextmost abundant elements: oxygen, carbon, and nitrogen.These compounds are primarilyhydrides (water, methane,and ammonia) but also include carbon monoxide, carbondioxide, and molecular nitrogen, among others. The icelabel does not mean that the constituents are necessarilyfound as solid materials, although that is indeed the formin which they would provide the building blocks for plan-ets. However, condensation requires low temperatures,corresponding to the orbit of Jupiter and beyond in oursolar system. Ice is roughly two orders of magnitude lessabundant than gas, by mass.

Rock refers to everything else-primarily magrtesium,silicon, and iron, and the oxygen that would naturally com-bine with those elements. Rock is less abundant than iceeven though it includes most of the periodic table. Re-markably, nuclear physics orders the elements accordingto abundance in a way that also naturally groups the ele-ments into chemical classes.

Such elementary considerations enable us to under-stand why a Jupiter-mass planet made mostly of iron, say,would probably not exist. Tlrpically, insu{ficient iron wouldbe around during the formation ofa planetary system forscientists to imagine that happening. Our inclination to in-terpret Jupiter-mass extrasolar planets as gas balls (evenwithout information on the size or composition) is not mereSolar System prejudice, but is guided by cosmochemicalprinciples. Likewise, we could explain the known proper-ties ofEarth's core by an alloy ofniobium but choose notto do so because a much more abundant element-iron-can do the job.

On the other hand, we have learned to be suspiciousof arguments that relate expected planet composition to lo-cation. Although it is tempting to argue that planets form-ing close to stars might tend to have a composition domi-nated by materials that condense at high tempelature, weprobably cannot exclude formation of gas balls at smallradii. Orbital migration cannot be overlooked as an im-portant process in the structure of many planetary sys-

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tems, probably including most of those extrasolar systemsdiscovered so far. Orbital migration arises through an ex-change of angular momentum between a massive gaseousdisk and an embedded giant planet and can cause a largechange in the planet's orbit in only 100 000 years. Thepresence ofwater on Earth and heavy noble gas excessesin Jupiter also cause us to question whether all the mate-rial in planet$ condensed near their current orbit. Much ofEarth's water was probably delivered from the Jupiterzone or beyond. Thus cosmochemistry although helpful, isonly a moderate constraint on what exists, because mate-rials can end up in different places from where they firstaccumulated.

Origrn scenarios lead one to "explain" particular out-comes. The test of a good theorist is the ability to explainany outcome, even when the data are wrong. Ttris ironyhighlights a fundamental difficulty in explaining howthings came to be: The basic physics-dynamies of many-body systems interacting through gravity-is better under-stood than most things in planetary science. But that doesnot mean one can severely limit the possible outcomes of anevent! There are too many contingencies to account for, andthe outcome of an event may depend on a particular se-quence of processes with different (but variable) time scales.An example of a crucial time-scale issue is the timing re-quired for the loss of nebular gas; that is, the time at whichmost ofthe hydrogen and helium that is not grabbed by thegiant planets or the Sun is sent back into the interstellarmedium. The outcome for a particular planetary systemmight be wildly different if the nebular gas is expelledsooner or later than in our system. Even for a particular or-

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dering of major events, the outcome still depends on chance,in the sense of a deterministieally chaotic system.

The problem is reminiscent of a current debate in bi-ological evolution between those who argue that Darwin-ian natural selection on Earth can lead to an extraordi-nary range ofpossible outcomes sensitive to contingencies2and those who say that the range of outcomes is quite nar-rowly limited.s In the debate over biology, just as over plan-etary formation, scientists agree about underlyingprocess-most believe Darwin was essentially correct-yetmuch room exists for disagreement about consequence.

Irrespective of who is right about life on Earth and else-where, it seems likely that the universe of planetary systemswill, like Darwinian evolution, seek out all ecological nichesand exhibit a remarkably wide range of patterns, even whileconforming to the physics and chemistry exhibited in ourneighborhood. For that reason, origin scenarios can proba-bly only weakly constrain the evolution ofplanets.

How unique is our solar system?The current observational evidence of extrasolar planetscertainly contains many instances that are markedly dif-ferent from our solar system.l That finding is possibly anobservational biasl Systems with giant planets in orbitswith periods less than a year are much easier to discoverthan systems like our own, especially using the currentlydominant technique based on the Doppler shift of lightfrom the parent star. This unexpected outcome is appar-ently quite common, occurring for roughly LOVo of neatbySunlike stars.l

The most likely reason that giant planets exist in

April 2004 Physics Today 45

near-star orbits is that they migrated there from muchmore distant locations, where they frrst formed. The trans-fer of angular momentum between a planet and the moremassive gas disk in whieh it sits-and which accounts forthe migralion-is well understood in principle. Still, manyaspects of the phenomenon remain mysterious. In partic-ular, scientists don't know why or whether our solir sys-tem avoided this process. Agiant planet sweeping throughthe natural zone of terrestrial planet formation might ex-clude the formation of planets like Earth. Does thai meanEarth is uncommon, oimerely that we have not yet foundits analogs? We expect to answer that question in the nextdecade or two.

What has our solar system taught us?In the past 40 years, planetary science has evolved from aminor branch of astronomy to an activity that embracesall the physical sciences and part of biological science.Planetary science is not a scientific discipline in the usualsense of being a specialty field; it is rather an interdisci-plinary combination of many areas of science. The suc-cessful exploration of space, primarily by unmannedspacecraft, has been central. Remote spacecraft have vis-ited all ofthe planets in our solar system now except pluto(-a1d any other more distant objects that might meet myd_efinition of a planet). Several lessons have emerged fromthe paturing of planetary science.a

Common processes are at work. When remotespacecraft visit other planets for the first time, researchersare almost always initially surprised by what they find. Ex-amples include the magnetic field of Mercury; the highargon-36 content (relative to Eadh) of the Venus atmos-phere; Mars features created by water flow; the remarkabledifference in appearance of Ganymede and Callisto, two ofJupiter's moons; and the strength of the winds on Saturn.Yet the underlying physical and chemical processes oftheseplaces are not bizarre; their terrestrial analogs are rightunder our noses. Ice caps form, winds blow, volcanoes erupt,and magnetic fields are produced here on Earth and else-where in the Solar System. Far-flung explorations to otherplanets and moons test our imagination and challenge basicscientific understanding, but they ultimately confirm ourgrasp of the basic physics and chemistry and expand the

46 April 2004 Physics Today

knowledge base about the physical universe. Mars has amass that is Vro that of Earth, but it has volcanic structuressimilar to oceanic island volcanoes on Earth-in Hawaii. forexample. Mars also has sand dunes, valley structures sim-ilar to those found in Earth's arid polar regions, and water-ice polar caps. Bruce Jakosky and Michael Mellon's articleon page 71 discusses some current understanding of Mars,especially the role of water.

Common processes yield diverse outcomes.Mass, compositional class (rock, ice, or gas), and distancefrom the Sun are not sufficient to characterize planetarybehavior. There are too many degrees of freedom, some ofwhich seem minor yet prove to be major. Consider the roleof water in terrestrial planets. Earth's water profoundlyaffects global dynamics. For one, it softens mantle rocks,thereby preparing the way for an asthenosphere-the softlayer underlying the plates. Its presence is also quite likelya pivotal condition for plate tectonics, which in turn partlydetermines the cycle ofwater: When plates subduct, wateiis canied into Earth's interior. Had Earth started out dif-ferently-with a modest amount of more or less water.say-the planet might have evolved quite differently. Andperhaps the major reason Venus is unlike Earth is becauseit lacks water in its upper mantle. That could at leastpartly account for the absence ofplate tectonics there.

Consider also the role of sulfur, another.minor con-stituent of the Earthlike planets (Mercury, Venus, Earth,and Mars). Sulfur is iron-loving, so it likes to reside in theiron core ofa planet. Itt also an antifreeze, so a planetarycore with lots ofsulfur is less likely to fully solidify. Acorethat only partly solidifres yields a buoyantfluid and keepsenerry available to sustain the internal motions requiredto gene,rate a magnetic field. T\vo otherwise identical plan-ets with only modest differences in the sulfur concentra-tions of their cores, therefore, could differ dramatically:One might have a magnetic freld and the other might noi.Gan5rmede and Callisto have ended up remarkabty aiffer-ent, even though they are similar in size and bulk compo-sition. The article by Tbrrence Johnson on page 7Z dis-cusses the remarkable diversity of the Galilein satellites,the first planetary system discovered other than oor own.

Planets must be understood in context. It is notenough to think about planets in isolation. The influenceof the Sun and Moon on Earth is as clear as daylight andtides, but subtler external factors also influenie ihe his-tory and evolution of a planet. Amassive impact at the endof the Cretaceous period on Earth-an event once contro-versial but now widely accepted-is a likely cause of theextinction of many species, dinosaurs among them. Almostcertainly, impacts were important in establishingthe earlyenvironments on Earth and Mars and thereby significantin abetting or hindering the conditions necessary for life.Jupiter, too, may have been an unwitting nurtuier of lifeon Earth by restricting the number of impacting bodies

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that reached it. Small disturbances ofEarth's orbit and ori-entation, for example, help determine fluctuations inEarth's climate, including the coming and going of ice ages.The same probably holds true for Mars.

Ahistorical perspective is important. A major goalof planetary science is to understand how things came tobe. Astronomers may look at large redshifts for clues to an-cient events that occurred far away, but planetary scien-tists trying to understand the Solar System look to clues inthe rocks and morphological forms on solid planets. Thisgeological approach embraces the idea that we can read his-tory in the solid bodies around us because they sometimesretain a memory. The precise dating of meteorites has re-vealed an early, relatively briefperiod ofactivity, includingmelting, in many of the Solar System's small, solid bodies.The uniformity of those dates is one line of evidence en-abling us to estimate the age of the Solar System (now be-lieved to be about 4.6 billion years). The oldest rocks on theMoon are almosL as old as the Solar System itself, a factthat attests to the rapidity ofsome planetary processes.

Planetary scientists cannot yet operate like freld ge-ologists. Even the Apollo astronauts were greatly limitedin their investigations. The technical challenges ofgettingto and studying Mars mean it may be a long time before atrue stratigraphy can be constructed for the various re-gions ofthat planet. Still, there are ways to tease out his-

tory. On Venus, for example, the paucity of impact craterssuggests that the outermost layers have been recycled overthe past billion years. Ofcourse, the apparent lack ofanycurrent process capable of such recycling will need to bereconciled with the inference. Perhaps Venus once hadplate teetonics but no longer does. On Mars, the younger-appearing northern terrains may indicate an early recy-cling, possibly suggesting a more ancient epoch marked byplate tectonics. On Triton, Neptune's large moon, the lowcrater density indicates that resurfacing is at work. Thatcould mean that Thiton may have an active interior, re-markable for a body so small (see figure 3).

Earth-based data, lab work, and theory are stillessential. It is easy to be impressed by the data returnedfrom spacecraft. It is easier to forget that mueh of what welearn about planets comes from Earth-based activities,either as independent efforts or as complements tospacecraft-based activities. Some of that planetary knowl-edge comes from using large telescopes, but much of it issmall science-bench-top experiments on the properties ofmaterials and computer simulations of the formation ofplanets and moons, for instance. The science return per dol-lar invested in ground-based work is very high. The re-search has led to key discoveries and significant informa-tion: the existence of Jupiter's large magnetic field; therotation states of Venus and Mercury; the strong green-

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house effect an{ clouds on Venus; the diversity of shapes,rotation states, and compositions of asteroids; the strangesurface ofTitan; and the persistence and high temperaturesof volcanism on Io, Jupiter's innermost moon. Furthermore,confidently interpreting spectra to learn about the compo-sitions of other atmospheres or interpreting planetary com-positions and behavior from condensed matter physics isonly possible with laboratory data for comparison.

An interdisciplinary approach works best. Outsiderssometimes perceive the resulting science as lacking the de-tail and preiision apparent in each contributing discipline.Consequently, scientists must rely heavily on physical rea-soning, inferential arguments, and modeling to interpretland forms whose natures are particularly diffrcult to dis-cern without more detailed information. But computa-tional studies have emerged over the past few decades asa widely used third branch of scientific investigation thatcomplements the traditional pair of experiment and the-ory. Agreat bonus comes with this broader approach: Plan-etary scientists have become indispensable players in thequest to answer fascinating questions that would fall out-side a more narrowly focused discipline. Research in manyaspects ofEarth's evolution and behavior requires a plan-etary perspective, for example. And one of the grandestscientifrc mysteries of all-the origin of life-is unlikely tobe solved only by biologists, physicists, and chemists. Thatachievement is sure to require the mindset of planetaryscientists as well.

Challenges for the futurePlanetary science is evolving into a field with rich inter-play between the traditional area of geoscientific ap-proaches applied to planets and the new areas of extra-iolar planets and exobiology. Astronomers provide thetechniques to study other planetary systems, but plane-tary science provides the intellectual framework for build-ing a story from what we will learn, especially as we getcompositional spectroscopic data and perhaps detectEarthlike planets5 in the coming decades. In the nearerterm, the exciting investigation of our own Solar Systemcontinues. Figure 4 illustrates one aspect ofa very signif-icant upcoming event, the arrival of the Cassini mission atthe Saturn system in a few months. The greatest scientificquestion of all, the presence and prevalence of life else-where, remains a tremendous motivator despite the un-certainty of success.

Physics students and professionals sometimes expressconcerns about the future of physics. Sidney Nagel's opin-ion piece (see hrvslcs ToDaY, September 2O02, page 55,and reader responses in the January 2003 issue, page 12)exemplifies the feeling amon! some scientists that physicsmight have a problem as a chosen area for exciting re-search. Planetary science is among the frelds of physicsthat do not suffer from a dearth ofchallenges and oppor-tunity. Tbo few highly talented physics-trained people areentering planetary science. Ifyou have read this far, thentake action! Consider this area for yourselfor suggest it totalented students. And enjoy the articles that follow.

References1. For an extrasolar planet catalog, see http//exoplanets.org. See

also J. J. Lissauer, Nature 4L9,355 (2O02).2. See, for example, S. J. Gould, The Structure of Euolutionary

Theory, Belknap Press, Cambridge, MA (2002).3. See, for example, S. C. Morris, Life's Solution: Ineuitable Hu-

rnans in a ltoncly Uniuerse, Cambridge U. Press, New York(2003).

4. For more detail on these lessons, see D. J. Stevenson, Science287,997 (2000).

5. S. Seager,EorthPlanet. Sci. I'ett.208, 113 (2003). I

48 April 2004 Physics Today

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