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| Astronomy & Geophysics 42 (1) 1.8 | | |||||
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David W Hughes Star or planet, or what?Anthony Whitworth (A&G 41 6.18) is worried about differentiating between a star and a planet and suggests that we look inside. He defines a planet as being an object that is “severely fractionated chemically, with the fractionation being caused by gravitational settling rather than nuclear fusion”. Isn’t it dangerous to base such a fundamental definition on something we know so little about? This is not to say that geophysicists haven’t discovered a great deal about the interior of Earth over the last century, but when it comes to other planets our knowledge of what is inside them is skimpy to say the least. And isn’t “gravitational settling” something of a red herring too? Even with stars and nuclear fusion, the end-product is an object with the heaviest element in the centre, where they have fallen, and with the lighter elements progressively floating on top of each other. Maybe the best thing to do is to approach the star/planet boundary problem artistically, zoologically and historically. Artistically we admit defeat before we start. Here a planet is like “good art”. You know good art when you see it, but it is extremely difficult to define. And like poetry and prose, you can easily point to good examples of each, but infrequently one comes across something that is neither one thing nor the other. The zoological approach leans heavily of the work of Martin Harwit, as expounded in his excellent book Cosmic Discovery, the search, scope and heritage of astronomy (The Harvester Press, Brighton, 1981). Harwit talks of an astronomical zoo which gets larger and larger as the subject develops. Each cage in the zoo contains a different kind of celestial animal. Initially the bijou zoo had five cages. The most populous contained the fixed stars; then there was one for wandering stars (planets?), another for hairy stars (comets) and finally two lonely cages for the Sun (which was not yet recognized as a star) and the Moon (which had clear bodily markings). Modern knowledge inflation now decrees that today’s potential occupants of the planetary cage should be dispersed into a row of separate cages, one each for terrestrial planets, gas giant planets, Pluto-like objects in 2, 3 resonances with Neptune, large satellites of gas giant planets, large differentiated asteroids and so on. All these are, to use Whitworth’s words, “severely fractionated chemically, with the fractionation being caused by gravitational settling”. The zoological analogy is easier if we know how an object/animal was formed, where it is found, and its specific characteristics, such as mass, temperature, size, energy output and composition. I was also surprised that Whitworth seemed to give the impression that the problem of defining the term “planet” arose sometime in the mid-1990s. Actually it is as old as astronomical history. A restricted trawl through some handy books revealed the following, which I list in order of increasing age. Planet: An astronomical object which is in orbit around a star, but does not have enough mass to become a star itself, and shines only by reflected light... upper limit mass is 50 times that of Jupiter... at the lower end mass range it is just less than the size of Mercury. (John Gribbin 1996 Companion to the Cosmos Weidenfeld and Nicolson, London.) Planet: A solid (or partially liquid) body orbiting around a star but too small to generate energy by nuclear reaction. (William K Hartmann and Chris Impey 1994 Astronomy the Cosmic Journey 5th edition, Wadsworth Pub. Co., California.) Planet: From the Greek for wanderer; any of the nine (so far known) large bodies that revolve around the sun, traditionally, an heavenly object that moved with respect to the stars (in this sense the sun and moon were also considered planets by ancient astronomers). (Michael Zeilik 1993 Conceptual Astronomy; A Journey of Ideas John Wiley & Sons Inc., New York.) Planet: A body that orbits the Sun or another star and shines only by the light that it reflects. (Valerie Illingworth [ed.] 1979 A Dictionary of Astronomy Macmillan, London.) Planet: Small body having no light or heat of its own which moves round sun. (David S Evans 1946 Frontiers of Astronomy, Sigma Books Ltd, London.) Planets:...dark bodies shining only by reflected sunlight... revolving around the Sun in orbits nearly circular, moving all in the same direction, and nearly in a common plane of the ecliptic and the sun’s equator... there are also at present known nearly 300 little planets, which probably represent a single one, somehow “spoiled in the making” so to speak, or burst into fragments. (Charles A Young 1895 A Textbook of General Astronomy for colleges and scientific schools Ginn & Co., Boston.) Planètes: On donne ce nom aux corps célestes qui n’étant pas lumineux par eux-mêmes, empruntent dans notre systèm éclat du soleil; on les distingue des étoiles en ce qu’ils scintillent moins à vue simple que ces astres, surtout à une certaine hauteur. (1856 Dictionnaire D’Astronomie.) Planet: Heavenly bodies, which move around another, as their centre of motion. Primary planets are such as move around the Sun, as a centre: secondary planets are moon, which move around their primary planet. (Lewis Tomlinson 1840 Recreations in Astronomy Parker, London.) Planet: They are those stars which do not always remain in the same place in the heavens, but move round the sun and receive their light from him. (1828 First Steps to Astronomy and Geography, Hatchard & Sons, London.) Planet: From planhthz, wanderer, in opposition to a star which remains fixed. (Jehoshaphat Aspin 1825 A Familiar Treatise on Astronomy Samuel; Leigh, London.) Planet: A celestial body revolving about the Sun. The planets may be known from the fixed stars by their change of situation in the heavens. (1820 A Popular Grammar of the Elements of Astronomy Thomas Squires.) It is clear from the above that there can be no watertight definition. Astronomy is not like that. There are stars that are obviously stars, and planets that are obviously planets. There is also a small subset of objects that fall between the two extremes. This is often the case. Think of the trouble one has trying to distinguish between asteroids and comets at a time when they are in the outer solar system, and the heated discussion as to whether Pluto is a planet or merely the largest object in the Edgeworth–Kuiper belt. It is difficult to omit extrinsic characteristics. Modern cosmogony indicates that all planets are closely associated with stars. Planets are in orbit around stars and vice-versa. Also there are no known “orphan” planets wandering through the galactic disk. And the formation of stars by the condensation of those fragments of an interstellar gas cloud that happen to obey the Jeans Criterion, differs drastically from the formation of planets, by gentle, slow, accretion of small planetesimals in a flattening circumstellar nebula. Intrinsic characteristics, however, are best. My choice, in order of preference, would be mass, luminosity, composition, energy generation mechanism, and density distribution. What are needed are two punchy definitions that can sit happily in astronomical glossaries. How about: Star: An independent, hot, radiating, near-spherical astronomical body with an initial mass greater than about 0.03 solar masses, and an initial composition close to the universal cosmic composition. The median stellar mass is around 1/7 the solar mass and the majority of stars have, at some time in their lives, generated energy internally by nuclear fusion processes. Planet: A secondary, accreted, cool, near-spherical astronomical body in orbit around a star, having a mass between 30 Jovian masses and 1/6000 Jovian masses, shining in the main by reflecting radiation and having a composition that is metal-rich in comparison to cosmic composition. David W Hughes, Reader in Astronomy in the Dept of Physics and Astronomy, University of Sheffield.  |
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George Cole Size mattersThe article by Anthony Whitworth (A&G 41 6.18) raises the important but neglected question of distinguishing between planets and stars, brought to prominence by the discoveries of bodies orbiting stars other than the Sun. A clear distinction can be made by an approach alternative to his, involving atomic properties. It treats the relative effects, on a constituent atom, of the energy of self-gravity within a body and the energy associated with the atomic structure. The atomic forces are of fixed form and magnitude; the strength of gravity increases with the mass, M, of the body. The changing balance between these two effects over a range of masses leads to hierarchy of bodies (Cole 1984). Gravity has little effect on the internal structure of bodies with the smaller values of M (for instance the smaller asteroids), the dominant internal forces then being chemical. For silicate/ice bodies, the effects of gravity become important at M» 1019 kg. The surface figure becomes spherical, a characteristic feature of the dominance of gravity (Hughes and Cole 1995). The internal structure is chemically differentiated according to the mean density of the material. As the mass of the body increases, so does the strength of gravity inside. The internal equilibrium is now a balance between the packing of the atoms against the pull of gravity. The strength of an atom results from the equilibrium between the attractive internal electric forces (of the nucleus and the orbiting electrons) on the one side and the degenerate force of the electrons resisting compression (a quantum effect) on the other. The regime where the atoms withstand the force of gravity and remain un-ionized is that of planetary bodies (planets and satellites). As M increases there comes a critical value, MC say, when the strength of gravity becomes greater than can be supported by the atoms. From this point onwards atoms become ionized and the body ceases to be a planet. The internal equilibrium begins to change. To see this, notice (Cole 1984) that when M < MC (that is, without internal ionization) the radius increases with the mass according to MR–3 = constant, which is essentially the mean material density: the radius increases with the mass. For M > MC the different relation MR3 = constant applies and the radius decreases with the increasing mass. Free electron degeneracy now opposes gravity. There is, then, a maximum radius Rmax. This is not an ambiguity which requires two different types of body, one a planet and the other not. The magnitude of MC depends on the chemical composition since the atomic strength depends on the particular atom involved. For a hydrogen body it is perhaps about 2 Jupiter masses (2MJ» 4 ×1027 kg) but is greater for silicate bodies (perhaps » 15MJ) and for ferrous bodies (perhaps » 30MJ). The region of deuterium burning (13MJ < M < 80MJ) follows a region (MC < M < 13MJ) where there is ionized matter but not thermonuclear reactions. This region has not been assigned a name but it does not refer to a planet. The situation is very different for silicate or ferrous compositions, which remain un-ionized up to much greater masses. Can silicate or ferrous bodies ever be “brown dwarfs”? As Whitworth concludes, the properties of this region of the mass spectrum are much more interesting now than they used to be. References Cole G H A 1984 Physics of Planetary Interiors Adam Hilger Ltd, Bristol, 208. Cole G H A 2000 Observatory 120 127 –130. Hughes D H & Cole G H A 1995MNRAS 277 99 –105. George Cole, Emeritus Professor of Theoretical Physics, University of Hull, Hull HU6 7RX. |
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Anthony Whitworth In response...Whilst I welcome alternative ideas as to how one might distinguish a planet from a star, I must take issue with some of the suggestions made by David Hughes in the preceding contribution (left). First, the concentration of heavy elements towards the centre of an evolved star is not because “they have fallen” there, but because heavy elements are synthesized much more rapidly in the hotter denser central regions of a star. The main transport mechanisms operating in the interiors of stars are convection and circulation, not gravitational settling, and these mechanisms tend to transport heavy elements towards the surface of the star, thereby reducing chemical fractionation. Second, Hughes cannot feign to be surprised that my earlier article “seemed to give the impression that the problem of defining the term ‘planet’ arose sometime in the mid-1990s”– because there is no way that even a casual reading of the article could give this impression. The only vaguely historical perspective on this problem in my article is a paragraph which starts with, “This is an old question, but it has been brought to the fore by the recent discovery of large numbers of exoplanets”. Third, the various definitions which Hughes has culled from textbooks and encyclopaedias are all now inadequate, for the reasons which I gave in my earlier article; indeed, most were inadequate even before the discovery of exoplanets. With these definitions, internal and orbital evolution routinely convert stars into planets, and vice versa. In fact, Hughes’ final definition is not greatly different from mine, in the sense that he identifies as a discriminating property of planets that they are “metal rich in comparison to cosmic composition”. Where we differ is that I stipulate that this metal richness should be due to gravitational settling rather than nuclear fusion. Hughes instead introduces a critical mass (Mcrit ~ 0.03M My concern with Hughes’ scheme is that implicitly it introduces two further categories: (i) objects which are metal rich but more massive than his Mcrit , and (ii) objects which have cosmic composition but are less massive than his Mcrit . There are at present no obvious candidates to put in either of these two new categories, but – once allowance is made for uncertainty and selection effects – there is also no good reason to believe that they are empty. The central issue here is whether – ignoring secular changes in the cosmic composition of the interstellar matter from which they form – stars and planets form a single continuous family, parametrized by mass and possibly also composition. If this is the case, it is important to identify the critical masses which separate mass-ranges in which the structure and/or evolution are determined by significantly different physics. However, many different mass and composition ranges are required for such a scheme, and the question of distinguishing planets from low-mass stars is then no more fundamental than distinguishing, say, rocky planets from gas giants, or brown-dwarves from core hydrogen-burning stars. Within such a scheme, I strongly recommend the preceding article by George Cole, which identifies the boundary between planets and low-mass stars with the critical density for pressure ionization. This gives a critical mass Mcrit ~ 0.0005M where X is the abundance by mass of hydrogen and Z– is the mean atomic number. In the mass-range directly above Mcrit , the density is high enough for pressure ionization, so at low temperatures non-relativistic electron degeneracy pressure dominates and the mean density approximates to r–µM2. Below Mcrit , the density is low enough for atoms and molecules to form, and the mean density is approximately independent of M (though not of elemental composition). However, the alternative is that planets and stars form two separate families, and then the question of how to distinguish them becomes crucial. Either the two families do not overlap in mass, in which case one must try to establish the range of forbidden masses in between the two separate families. The question of distinguishing individual objects then reduces to determining their masses accurately. Or the two families overlap, in which case distinguishing exoplanets from stars is likely to be very difficult – but we should still aspire to do so. Anthony Whitworth, Department of Physics and Astronomy, Cardiff University, Wales. |
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H J P Arnold Imaginary numbersThe human eye’s capability of discerning patterns in detailed scenes or images (whether or not they exist) is well known. A recent personal instance involving the Moon and a southern hemisphere feature which, while real, took the shape of a readily recognizable Earth-bound object is a case in point. I was testing a newly introduced black and white film to see if it had any qualities that were superior to those of films that I had used for some time. On the night in question the Moon was slightly under two days from full, an extremely bland object bereft of the surface detail seen when the Sun is lower in the lunar sky. I digitized several frames from my test film so that they could be manipulated using computer techniques which are easier to apply and often more successful than their purely photographic equivalents. As I brought one of the scenes up on the monitor, I noticed what appeared to be a very clear Arabic numeral “6” appearing to the east of the dramatic ray crater Tycho. My first thought was there was a film blemish but a check showed that the object appeared on every frame. I then searched negatives and prints that I had shot over the past couple of decades, as well as illustrated books. This revealed that the feature could be seen clearly at full Moon and for a day or so on either side of full. (See for example plates X, XI, XII and CLIV in Photographic Atlas of the Moon, Zdenek Kopal, Academic Press, 1965.) At other times of the lunar cycle it was not visible. Thus, the feature was something that was revealed when the solar phase angle was close to zero – when surface detail is poor but albedo differences are marked. Figure 1 is a print of the southern hemisphere of a full Moon and is representative of many such images. A careful study of the location of the feature, compared with lunar maps and images taken at different solar phase angles, showed that the bright curvilinear object to the west (left) was the eastern rim of the large (114 km diameter) crater Maurolycus, described in one lunar atlas as “a vast walled plain with central peaks”. I then sought higher resolution images. The rich harvest of images obtained by the Clementine spacecraft in the spring of 1994 (2.5 million according to deputy science team leader Paul Spudis of Houston’s Lunar and Planetary Institute) was readily available because after the mission ended the data were placed on the Web at www.nrl.navy.mil/clementine. Lunar Orbiter images were also investigated at cass.jsc.nasa.gov/research/lunar_orbiter/. Figure 2 is a section from a downloaded 415 nm image taken by Clementine’s UV-VIS (Ultraviolet/Visible CCD) camera. This image confirms that Maurolycus and the feature (although the latter is dark) lie on one of the very prominent ejecta rays emanating from Tycho. Further it appears that it is ejecta from Tycho that has rendered the eastern rim of Maurolycus so bright in most of the images. The dark “6” is readily visible in this frame. Finally, presented as Figure 3 is an extreme enlargement from the Clementine image which has also been “contrast stretched”. This technique emphasizes differences in albedo to the east of Maurolycus. An approximate calculation indicates the feature’s north–south length to be somewhat greater than the diameter of Maurolycus – perhaps about 120 km. The surface elements resulting in the formation of the dark figure “6” are readily seen in this last print. The area generally is a somewhat chaotic mixture of dark maria material and much lighter material presumably ejected when Tycho was formed. While there are numerous dark patches (for example, the crater Barocius and its surroundings), the figure runs in an approximate north–south direction across a wide area of ejecta ray which gives it its visibility. Further, by chance two small craters in close proximity have a higher albedo and create the appearance of the centre of the lower “o” of the figure. Off to the north, light material extending from the east runs westwards just far enough to create the upper part of the figure but not so far as to breach the dark regolith which appears to form the upper left curve of the figure. Its upper right curve is formed partly by the fortuitous distribution of darker and lighter albedo material, but also by what may well be a ridge or old crater rim – which is, incidentally, shown in map XVII in Edmund Neison’s classic book The Moon published over 120 years ago! At a time when many seek extra-terrestrial explanations for apparent anomalies on Earth and elsewhere, I must emphasize that at no time did I think that the figure was other than a chance feature. The puzzle for me personally is why I didn’t notice the “6” on numerous other full Moon images taken over the years. I am even now looking again at the dust jacket of Paul Spudis’s book The Once and Future Moon: there is the full Moon in colour and, despite the slight loss of quality resulting from reproduction techniques, the “6” east of Tycho is clearly visible. Readers are invited to make the same test of any reasonable quality picture of a full Moon. H J P Arnold. I am grateful to Patrick Moore, Paul Lowman (NASA Goddard Space Flight Centre) and Paul Spudis (Lunar and Planetary Institute) for information and discussion.  1: Under certain conditions the numeral “6” appears to be imprinted on the Moon.  2: The feature lies on an ejecta ray from Tycho.  3: From this enlargement it can be calculated that the “6”is about 120 km high. |
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) which ensures that his planets (with M < Mcrit) are incapable of significant nuclear fusion – because they become sufficiently dense to be supported by degeneracy pressure before they ever become hot enough for nuclear fusion.