Analog SFF, December 2007
Page 6
Because the planemo twins were found in the constellation Ophiuchus, their system has been named Ophi1622. Based on the observed infrared emission of the planemos and numerical models of their evolution, the discoverers conclude that one of the planemos is about 14 times as massive as Jupiter, the other about seven, and that they are very young, only about a million years old. Strictly speaking, the larger of the two might be either a small brown dwarf or a large planemo, but the smaller one is certainly a planemo, one of many more waiting to be found wandering between the stars.
It's been obvious for some time that there are more objects outside of the traditional solar system of major planets and asteroids than had been previously believed. The Kuiper belt, just beyond the orbit of Neptune, contains the bits and pieces left over from the formation of the solar system; within it, there are perhaps 80,000 objects with diameters in excess of 50 km. In the more distant Oort cloud, the repository of comets ejected from the solar system during and after its formation, there are likely to be millions of such objects. A possible member of the Oort cloud has been found already: Sedna, a dark red world some 1500 kilometers (900 miles) in diameter. Sedna's orbit is very elliptical, so its distance from the Sun varies greatly, from 76 to 975 a.u. There is considerable argument about how such an unusual orbit could have been established in the normal process of solar system formation. One intriguing possibility is that Sedna was perturbed into this orbit by a large undiscovered planemo circling between 1,000 and 5,000 a.u. from the Sun. In any case, Sedna is hardly likely to be the last body of substantial size to be found outside of the orbit of Neptune.
It has also been known for a long time that there are many small comet-sized objects and larger planetary fragments wandering aimlessly in interstellar space, the rejected offspring of solar system formation, ejected from their parent star by gravitational interaction with larger bodies. Simulations of planetary formation all agree that numerous small asteroids and comets, as well as larger planemos, were thrown out of the newborn solar system by the giant planets, particularly Jupiter. It is possible that several planemos the size of Mars or Earth were ejected from our solar system before the existing planets were fully formed and took up their current orbits.
As usual, speculative fiction has anticipated some of these ideas. The idea of a “rogue planet” traveling through interstellar space unattached to any star has been around since at least 1932, when Philip Gordon Wylie and Edwin Balmer published their novel When Worlds Collide, about the havoc wreaked by a pair of rogue planets entering the solar system. The book was made into a movie of the same name in 1951, winning an Oscar for best special effects. Apparently, Steven Spielberg is preparing a remake to be released in 2008. Several other science fiction stories and novels have set at least part of their plot on interstellar bodies, that is, objects between the stars. But even if a couple of rogue planets were headed our way, they would be quite difficult to detect until they were relatively close to the solar system.
Likewise, interstellar planemos are very difficult to find with current techniques. Direct imaging, the technique used to find Ophi1622, can only see close planemos or those whose temperatures are high enough to emit significant amounts of visible or infrared radiation. Young planemos are hot and bright but then cool rapidly, whereas the more massive brown dwarfs start out hotter and cool slower. Thus bright objects can be of various ages and sizes: planemos with masses only several times that of Jupiter must be less than 100 million years old to still shine strongly in the visible or infrared, whereas brown dwarfs can be much older and still be detectable.
Another technique that can find interstellar planemos is microlensing. Microlensing relies on the deflection of starlight by the gravitational field of objects passing in front of stars. The resulting change in the light output of the star can be compared with theoretical predictions to obtain an estimate of the masses of such objects. Of course, this method can detect an interstellar planemo only once; after the initial contact, the planemo is impossible to find again, as it has traveled off in some unknown direction and cannot be observed by any other technique. This method can find a sample of smaller worlds and thus give us an idea of how many are out there, but it leaves the vast majority of interstellar planemos undiscovered.
So it is not a new idea that there may be some planemos drifting in the space between the stars. What is new is the thought that there may well be a lot more between the stars than circling around them, and that their types may be almost as diverse as those of planemos within the solar system.
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Planemo types
How many interstellar planemos there are is hard to say. Interstellar space is huge: many readers will know that the distance from here to Alpha Centauri is about 8,000 times the distance from here to Pluto. That's an awful lot of room, more than enough to hide numerous Earth-size planets.
Nor would these planets necessarily be uninteresting frozen iceballs. For a start, some interstellar planemos about the size of Earth would be able to maintain surface temperatures well above absolute zero, perhaps even above the freezing point of water. In 1999, David Stephenson suggested that Earth-size planets ejected from the solar system during their formation might retain a considerable portion of their primordial thick hydrogen atmosphere. Earth had such an atmosphere very early in its evolution, before the Sun started to shine vigorously, emitting energetic radiation that then stripped away the light hydrogen molecules, leaving only denser gases behind. A thick blanket of hydrogen retained by a planemo after its ejection from a stellar system might be dense enough to trap the heat emitted from its interior, a process driven by radioactive decay and responsible for Earth's molten core. This might be enough to maintain a surface temperature above freezing. For a planemo with a more active internal heat source or a thicker atmosphere, surface temperatures would be higher. Worlds with significant active volcanism would also have numerous local regions where surface temperatures would be well above the freezing point of water, in hot springs, for instance. Such planemos might be reasonable places for primitive life to form.
Note that these planets would need to have very thick atmospheres to trap enough of the internal heat to ensure that the surface temperatures stayed warm. An Earthlike planet doesn't emit much internal heat. For example, the surface of the Earth receives an energy flux of about 300 watts per square meter on average from the Sun, but only about 0.1 watts per square meter from its interior. For an internal heat source of this magnitude, the planemo's hydrogen atmosphere would need to be thick enough to exert a surface pressure of about one kilobar, a pressure a thousand times that experienced at sea level on Earth and about as crushing as that at the bottom of our deepest oceanic trench. So even if these planets had life, they would still be very inhospitable to human beings. Manned exploration of the surface of such a world would be difficult: its atmosphere would become violently explosive when put into contact with oxygen, and its surface pressures would require spacecraft built like deep-sea diving vessels. It would be easier to set up a base on a barren piece of rock in the same interstellar neighborhood.
A more hospitable place, at least for some forms of life, might be in the ocean of a water subgiant planemo. Water subgiants are a class of planet predicted by simulations of planetary formation but not yet detected. They are objects a few times the mass of Earth, covered in water oceans of variable depth, typically about 100 kilometers. Many such planets are likely to have active internal heat sources and thick atmospheres. These ocean worlds would be good locations for life, for the same reasons that the iceball satellites of Jupiter and Saturn, namely Europa and Enceladus, are high priority targets in the search for life in our solar system due to the presence of their subsurface oceans.
Even so, the life that could form on interstellar worlds would have considerably smaller energy resources available to it than on Earth. Since the average amount of energy received by the Earth from the Sun is about three thousand times as much as from internal h
eat, this means that life on interstellar worlds would be generally low energy. On Earth, plants are a relatively low-energy lifeform, and as a result, in their current state will never develop technology or launch spaceships. Human beings, by ingesting the energy content painstakingly gathered up by plants over a large area of the planet's surface, are able to live more active lifestyles and do creative things like build bridges and start wars. In contrast, life on a low-energy world faces an uphill battle to modify its environment in a significant way. Thus any such life will likely be immobile or slow moving.
Whether they host lifeforms or not, interstellar planemos may be almost as varied as the planetary types within the solar system. At the top of the planemo family tree will be behemoths more massive than Jupiter, older versions of the Ophi1622 system, swirling gas giants with brutal atmospheres, still emitting some infrared radiation even after billions of years, like Jupiter itself. Also like Jupiter, they will be accompanied by a retinue of close satellites, formed by the collision of planetary “embryos” agglomerated from smaller particles orbiting in the dusty disc that enveloped such giants during their birth. These satellites could themselves be quite large, just like the major moons of Jupiter. They could be as diverse as volcanic, sulfur-spewing Io, iceball Europa and cratered Callisto. Farther away from the giant planemo, there will be a collection of circling asteroids and comets, a mini-Kuiper belt of its own. Farther out still, there may be a loose and variable number of satellites of opportunity, passing comets that have been temporarily or even permanently snared by the gravity of the giant. There may be more of these big planemos in interstellar space than there are visible stars, perhaps considerably more. Certainly, current work suggests that there are at least as many brown dwarfs as visible stars, and the same may be true of large planemos.
Descending the family tree, there might be numerous interstellar gas giants of varying sizes, from Jupiterlike bodies to worlds smaller than Uranus. How many there are depends on how they originate. It is generally believed that planemos are formed by two processes: by gravitational collapse of a cloud of gas and dust, or by agglomeration from a dust disc surrounding a larger world. The lower mass limit of objects that can form in interstellar space from gravitational collapse has not been firmly established, but it is probably less than seven Jupiter masses, since this is the estimated mass of the smaller component of Ophi1622, designated Ophi1622B. Ophi1622B most probably formed by collapse, as it was likely too distant from the dust disk of its larger companion to have formed from it. It is possible that smaller interstellar objects formed by collapse may exist: a few dozen free-floating interstellar planemos have been discovered, but their masses, being estimated from spectra alone, are not precise. It is also not clear whether they originated by themselves in interstellar space from collapse or by agglomeration in a large stellar system and were then ejected. The Ophi1622 system itself was probably not ejected from another stellar system, as the two planemos are separated by such a large distance that they would have been too easily detached from each other during their violent exit from a previous system. If objects smaller than Ophi1622B can form directly in interstellar space by collapse, this would boost their potential numbers.
Clearly, though, objects of Earth size or somewhat larger should be relatively plentiful, since they would have been routinely ejected during the formation of stellar or large planemo systems. Those Earth-size planemos having thick primordial hydrogen atmospheres will enjoy fairly hospitable surface conditions for some kinds of life. But other worlds may have been ejected into interstellar space without their original atmospheres if they were tossed out from regions close to the star after the star began to emit significant amounts of ultraviolet radiation, thus burning off their hydrogen atmospheres. The temperature of these planemos will be much lower, approaching the surface temperature that could be maintained by internal heating alone, namely only 30 degrees Kelvin or so for an object with an internal heat source similar to Earth's.
If some of these smaller planemos were ejected as close binaries and manage to stay together during ejection, they might be close enough to each other to generate large tides. This would lead to internal heating caused by the friction generated by the stretching and straining of the planets due to their mutual tidal attraction. Although the amount of internal heat created would depend greatly on the separation, mass and orbital characteristics of the planemos, this process can provide considerable energy, as the volcanoes of Io and the geysers of Enceladus demonstrate. In addition, some Earth-size objects orbiting close to large, Jupiter-size planemos may also be tidally heated. On worlds where this process was important, geysers, hot springs, and volcanoes would be standard features of the landscape, as familiar as rolling hills or grassy meadows on Earth.
As for planemos smaller than Earth, there will be large numbers of Moon- and Mars-size bodies between the stars, and they will also have varying atmospheric and surface compositions. Some will be dry, desiccated desert worlds, vast frigid starlit Saharas with less water than Mars and with land surfaces the size of Asia; others will be covered from pole to pole in snow and ice. Some may have cooled so much that their initial hydrogen atmosphere has collapsed into a liquid hydrogen ocean surrounded by a remnant thin hydrogen atmosphere. But most will have one thing in common: they will be dark. Planemos within a few thousand a.u. of a star will have reasonable illumination: at the relatively close distance of 5,000 a.u., the Sun would be starlike but rather brighter than Venus ever gets and would cast noticeable shadows. Farther out, though, even interstellar worlds with clear atmospheres will usually only be lit by the sum total of all available starlight. This gives a reasonable but not ideal amount of light, as anyone who has ever stumbled along a country lane on a crisp, clear moonless night will attest. Planets with thick, cloudy atmospheres will be almost as dark as an underground cavern. The only light will come from lightning flashes, volcanoes, or maybe the dim phosphorescence of organisms floating in an endless sea.
There will also be an assortment of even smaller bodies between the stars: billions and billions of comets, asteroids, boulders, and general stellar system construction debris. The distances between the stars are so enormous, though, that these numerous small objects will typically be separated by millions of kilometers. Nevertheless, the gravitational influence of the large interstellar planemos and stars may sometimes force these smaller objects to cluster in preferred locations, creating belts of rock and ice that might become interstellar navigation hazards. Nor would such hazards be very easy to find with current techniques until a spacecraft was quite close to them. This poses a particular problem for high-speed interstellar craft, no matter what their source of propulsion or where they might be headed.
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Destinations
Apart from providing another set of astronomical objects to explore, there are other implications if planemos are present in large numbers between the stars. If they are—and that is still a big if—they will provide new destinations for exploration. The distance to Alpha Centauri, about 270,000 a.u., is daunting for a practical exploration program using present-day technology or any technology that is likely to become practical any time soon. On the other hand, a large planemo that is 5,000 a.u. away is reachable by a journey of perhaps ten to twenty years using foreseeable technology, namely a fusion-powered rocket with a high exhaust velocity. Thus the technologies that are required to reach the stars can be tested, refined, and developed by deep-space missions to planemos. As scientific and technological breakthroughs are made, more distant planemos will be contacted, until perhaps the day comes when 270,000 a.u. doesn't seem quite so far after all.
Nevertheless, in order to launch a mission to a hypothetical planemo 5,000 a.u. distant, a method of finding such an object has to be found. Naturally, the larger the object is, the easier it will be to find. Many brown dwarfs are relatively easy to detect at stellar distances, provided that an appropriate telescope is searching the right part of the sky. To da
te, though, astronomical surveys have been unable to detect brown dwarfs with temperatures less than about 750 degrees Kelvin. Even a large planemo like Jupiter has a lower temperature than this: if Jupiter were located in interstellar space, it would have an “effective” temperature[2] of only about 100 degrees Kelvin, despite being much warmer deeper within its atmosphere.
[FOOTNOTE 2: The temperature that a perfectly black object must have in order to emit the same amount of radiation as actually emitted by the planemo.]
A new space mission will solve this problem. The Wide-Field Infrared Survey Explorer, an orbital infrared telescope scheduled for launch in 2009, will be able to detect brown dwarfs with effective temperatures of 150K out to 10 light years, so large planemos and brown dwarfs should be well observed by this instrument. A number may be detected that are closer than the Alpha Centauri system. This mission will give us a very good idea of how many interstellar Jupiter-size planemos there really are.
Detection of an Earth-size body at even the relatively close distance of 5,000 a.u. will be difficult, however. These bodies will have rather low effective temperatures, about 30 degrees Kelvin or so, and correspondingly low infrared fluxes. NASA's proposed orbiting Microlensing Planet Finder will be able to conduct a census of interstellar planemos down to objects as small as 0.1 Earth masses, but again microlensing can only give a statistical distribution rather than enabling specific objects to be explored. If it is ever built, the proposed Overwhelmingly Large Telescope, a visible-light instrument with a mirror diameter of 100 meters, may be able to detect Earth-size planemos in visible light out to several thousand a.u., but not much farther away, and only if the telescope were pointed in exactly the right direction. Finding more distant objects of this size would need even larger instruments.