Analog SFF, April 2009

by Dell Magazine Authors

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  Science Fact: RIBBONLAND

  by Kevin Walsh

  Are there worlds somewhere out there in the galaxy that have eternal spring mornings and endless summer days? The prime candidates are ribbon worlds, planets that always keep the same face towards the star that they revolve around. This means that their periods of rotation on their axes are the same as their periods of revolution about their stars. The sun-facing side is impossibly hot, the dark-facing side incredibly cold, and in between is a narrow band of more tolerable temperatures, winding like a ribbon around the entire planet.

  Such worlds are probably common in the galaxy. They are most likely to occur in systems with small stars, because such stars are more likely to have planets forming close to them where they experience strong gravitational forces, similar to the tides that the Moon and the Sun exert on the Earth but much larger. These tidal forces can slowly stretch and strain the crust of a planet over the eons, thus dissipating much of its rotational energy, slowing it down until it finally becomes a ribbon world.

  In the days before space exploration, it was believed that Mercury was a ribbon world. But in 1965, radar observations showed that Mercury did not keep the same face towards the Sun all the time, so sunrises and sunsets do occur on all parts of the planet except deeply shadowed polar craters. Despite this, there is actually a strong relationship between the rotation and revolution periods of Mercury: it rotates three times for every two times it orbits the Sun. This is not coincidence, as studies of orbital dynamics suggest that Mercury's oval-shaped orbit could cause a 3:2 ratio to occur as one possible outcome of strong tidal forces.

  This ratio was the reason why early observers believed that Mercury was a ribbon world. Mercury's rotation rate is 58.6 days, which is just about half the time between successive closest approaches of Mercury to Earth, the so-called synodic period. Now Mercury is close to the Sun and is easily lost in the solar glare; its effective observation period from our planet is limited to those parts of Mercury's orbit when it is furthest away from a line joining the Earth to the Sun. Obviously, this occurs twice every synodic period and is known as a maximum elongation. During one of these maximum elongations, Mercury is on one side of this line, in the northern half of our sky, and during the other it is on the other side of the line, in the southern half of the sky. So Northern Hemisphere astronomers naturally chose to observe Mercury only when it was in the northern sky. Thus the fact that the synodic period is close to twice the rotation rate meant that during the effective observation period astronomers always saw the same part of the planet. Therefore these observations strongly suggested a 1:1 ratio between rotation and orbital periods, so pre-1965 astronomers can hardly be blamed for assuming that Mercury was a ribbon world.

  There are no such worlds in our solar system. But three planets have been recently found circling the small, nearby red-white star Gl 581, and at least one of them may be a ribbon world.

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  Endless Spring

  While no ribbon worlds have yet been identified with certainty, they have been a part of speculative fiction for some time. For instance, some of the action in Isaac Asimov's Foundation series takes place on the ribbon world Radole, where the capital is said to bask in the “eternal morning of an eternal June,” while only thirty kilometers away liquid oxygen flowed like water. Based on our knowledge of climate science, this is impossible. A substantial atmosphere of the kind that would be certain to exist on a habitable world is able to transport large amounts of heat, so an imbalance of temperature of this magnitude over such a short distance would be swiftly corrected by the movement of warm and cold air, along with accompanying storms. Other fictional ribbon worlds include Harlan Ellison's Medeaand, more recently, Aurelia on the National Geographic TV special Extraterrestrial.

  Ribbon worlds have also been constructed in numerical form, as a way of determining their weather and climate. Numerical simulation of the weather and climate of the Earth has become routine, but it is only relatively recently that it has been attempted for extrasolar planetary environments. These simulations are now being used to make inferences regarding the habitability or otherwise of worlds outside the solar system, including ribbon worlds.

  Ribbon worlds have often been assumed by planetary scientists to be essentially uninhabitable, due to serious questions regarding the stability of their climates. Would the dayside temperatures become so hot that the oceans in these regions would start to boil away? This would be fatal to the habitability of such a world, as the evaporation of significant fractions of an ocean would place large amounts of water vapor into the atmosphere, thus strongly increasing the greenhouse effect and leading to a vicious warming cycle ending in a so-called “runaway greenhouse.” Would the night side temperatures be so cold that the atmosphere would start to freeze out on the surface? This would also cause a serious problem, as eventually it would lead to the condensation of most of the atmosphere and to the end of habitable conditions on the planet.

  But in the past ten years, numerical climate simulations have challenged these ideas. Manoj Joshi of the University of Reading has used such simulations to investigate the climate of ribbon worlds. Since this type of planet is most likely to occur around a small star, he assumed that the ribbon planet circled a star with 20% of the mass of the Sun and only 1% of its total brightness. For simplicity, he also assumed that the planet received exactly the same amount of radiation as the Earth receives, and so therefore it would need to orbit its small star much closer than the Earth orbits the Sun, at only one-tenth of the distance. He found that for a planet entirely covered by oceans, the hottest part of the planet, where the star is directly overhead (known as the sub-stellar point), reaches an average temperature of only a little over 30oC (about 85oF).

  This surprising result is due to the simulated temperatures being kept under control by the very strong effect of cooling caused by evaporation of water from the ocean surface. These hot, humid conditions are accompanied by torrential rainfall, with amounts greater than the wettest places on Earth. At the coldest place on the planet, in the center of the dark side, average temperatures are about -30oC (-22oF), or about as cold as an Arctic winter and warmer than typical winter temperatures on the Antarctic plateau.

  The best climate on the planet is found in the narrow ribbon area, in this case a strip of the surface located somewhat sunward of the terminator, the line separating the sunlit side from the dark side. This region of the planet experiences a tolerable climate with pleasant temperatures between 15 and 25oC (59 and 77oF), accompanied by reasonable rainfall amounts. Let's call this optimally habitable area “Ribbonland.”

  Despite its spring-like climate, the day-to-day weather of Ribbonland would be less than idyllic: the horizontal temperature gradients are quite steep and on Earth this usually implies changeable, stormy weather. Certainly these regions are windy, as the planetary atmosphere is heated strongly on the sunlit side and therefore rises rapidly, like a hot air balloon. This would force air to flow in from the sides to replace the rising gases, causing persistent, strong winds to blow along the surface from the dark side to the sunny side.

  A similar wind occurs on Earth: it is called the monsoon and it arises from the strong heating of the Asian continent during summer, drawing in warm, moist air from the tropical Indian Ocean. As the air travels over land, clouds form and the resulting monsoon rains bring life back to the parched landscape. If this monsoonal air is pushed up over mountains, rain clouds are generated more rapidly due to stronger cooling. Such mountainous monsoonal regions are among the rainiest places on Earth: Cherrapunji in India receives a June average precipitation of 107 inches, more than double what New York
City receives in an average year.

  The terrestrial monsoon is a persistent wind, but nothing like the Ribbonland monsoon. Typical simulated average monsoon wind speeds in Ribbonland are around 10-15 ms-1, or about 20-30 knots, comparable to or larger than the average wind speed over the windiest sea-level regions on Earth, including even the monstrous seas of the roaring forties and furious fifties around Kerguelen Island in the south Indian Ocean. On Earth, this average wind speed would be enough to set large branches on trees in motion and would be noticeably difficult to walk against. And that's just on an average day. So Ribbonland would be a stormy, windy place with big day-to-day changes in temperature.

  Now let's see what happens when some land is placed on the surface of the ribbon planet. If land is present at the sub-stellar point, average temperatures in this region exceed 70oC (158oF), making it uninhabitable by human beings. In regions of Ribbonland that are largely dry land, horizontal temperature gradients would therefore become extreme, larger than anything experienced on Earth, with corresponding effects on the weather. Ribbonland would alternately bake or freeze depending on which direction the wind blew—not a nice place to live. So it is clear that Ribbonland would be optimally habitable if it were predominantly ocean and dotted with a few reasonable-sized islands. If these islands were mountainous and faced the monsoon, rainfall over them would be torrential. Alternatively, a reasonably habitable climate could be achieved if the sunlit hemisphere were entirely ocean while Ribbonland and the dark hemisphere were largely land. In that case, though, Ribbonland would be mostly desert, as the monsoon would then be blowing from the dry, cool land regions of the dark side.

  The implication of this and other work is that the climate of ribbon worlds is not necessarily as extreme as we once thought, so they can be hospitable places for life. They are most likely to orbit red-white M-class stars, the smallest and most numerous stellar classification, comprising more than 70% of all known stars. The star Gl 581 is an M-star, and its recently discovered planets, in addition to being possible ribbon worlds, are also possible candidates for life.

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  Super-Earths

  The star Gl 581 is quite close, only about 6.3 parsecs away in the constellation Libra, in pretty much the same direction as the much more distant blue star Beta Librae, also known as Zubeneschamali. Gl 581 has some strange near neighbors. While the closest object to it is the apparently unremarkable M-star Gl 555, less than 4 parsecs away from Gl 581 is the variable star Xi Bootis A. This star is evidently not satisfied with one kind of variability, as it has two. It is a BY Draconis type variable, a star that brightens and dims by about 50% over a period of about ten days due to the presence of large star spots. It is also a UV Ceti type flare star, indulging in irregular minor explosions, much like the flares of our own Sun but considerably bigger. In this regard Xi Bootis A is unusual, as it is about half the luminosity of the Sun and so is more massive than most flare stars. The star is part of a binary system: roughly 30 AU[1] away on average is the smaller orange-white star Xi Bootis B. In addition, the system is listed in the CCDM double star catalogue as having four stars, but in the Gliese catalog of nearby stars there are only two, so we may assume that the other two just happen to be visible in the same line of sight and are actually much further away.

  [FOOTNOTE 1: An astronomical unit (AU) is the average distance from the Earth to the Sun]

  A genuine multiple star system is located less than 3 parsecs away from Gl 581, the triple system GL 644. In this system, two average-sized M stars orbit each other with a period of 1.7 years, which means that they are on average about 1 AU apart. In the same system, about 1,000 AU away, is a very dim M-star, over 100,000 times less bright at visible wavelengths than the Sun. This seems tiny until you realize that such objects far outnumber stars like the Sun and that they are in turn likely to be considerably less numerous than even dimmer objects that have yet to be detected. This star also has periodic flare-ups, like so many smaller M-stars. Also known as VB8, it had a brief claim to fame some twenty years ago when a brown dwarf was reportedly discovered orbiting around it, one of the first to be found. A brown dwarf is an object that is too small to undergo fusion of hydrogen within its core, but nevertheless emits considerable radiation from various processes, including slow contraction. Unfortunately, this detection proved false, and no smaller objects are known to orbit VB8 at this time.

  The most unusual system in the neighborhood is probably that of Gl 570, with four known components. Gl 570A is another BY Draconis type white-orange star. About 190 AU away from it are Gl 570B and Gl 570C, a pair of red-white M-stars separated by only about 0.8 AU. But about 1,500 AU distant is Gl 570D, a T-type brown dwarf only about fifty times the mass of Jupiter. This is a star that never quite made it. It has one of the lowest temperatures of any known brown dwarf, at only 500oC (900oF), compared with 2,500oC or so for a typical M-star. M-stars are often themselves described as appearing red, but close up they would certainly be bright enough to appear mostly white, just as the Sun, officially designated as a so-called “yellow” G-star, in reality appears a kind of very bright white-yellow. Not so T-dwarfs: Gl 570D is actually cool enough to glow dull red, like the embers of a dying fire. Moreover, once our observational techniques improve, we will find a great many more brown dwarfs; our understanding of stellar formation suggests that there are likely to be as many of them as there are regular stars.

  Gl 581 itself is over two hundred times less luminous than the Sun at visible wavelengths and has a total radiation output at all wavelengths only 0.013 times that of the Sun. Unusually for an M-star, the brightness of Gl 581 does not vary much, which implies that it is at least several billion years old, as models of stellar evolution and observations both indicate that M-stars are much more variable when they are young and can take a billion years or more to settle into a more typical pattern of behavior. Some statistics of its known planets are given in the table below. Their orbital parameters are not established with full confidence, as the deduced values are the ones that fit best the observed pattern of the detected movements of the star, caused by the planets tugging on the star as they circle in their orbits. The biggest planet in the system, Gl 581b, has a mass about the same as that of Uranus or Neptune. GL581 has a lower percentage of heavy elements than does the Sun, and this, combined with its low mass, suggests that the formation of large, Jupiter-sized planets in this system would have been unlikely, so they probably do not exist.

  A quick check on the potential habitability of the planets can be made by comparing the amount of radiation that they receive from their star to that received by various planets of our solar system. This shows that both Gl 581b and Gl 581c receive way too much radiation. The cooler of the two, Gl 581c, gets about 30% more total radiation at all wavelengths than that received by Venus. It is hard to envisage under what scenario this planet could remain habitable. Venus was victim of a runaway greenhouse effect a long time ago. Since Gl 581c is larger than Venus, it is likely that its original atmosphere was more massive also, giving even more potential for greenhouse warming. So we can safely scratch Gl 581b and Gl 581c from the habitability list. If Gl 581c started out with lots of water, it could now have a very hot steamy atmosphere with a temperature greater than 700K, a so-called “steam ocean” with no real boundary between the atmosphere and the ocean, rather like the one that exists on Neptune. Alternatively, if it started out with less water, it could have lost it and so would have ended up like Venus is today.

  Gl 581d is a different story. It receives only about 20% of the total radiation received by Earth, less than does Mars, but this turns out to be less of a problem than receiving too much. Once again, the key is the magnitude of the greenhouse effect. If the atmosphere of the planet contains a lot of greenhouse gases like carbon dioxide, perhaps several times as much as all of the gases in Earth's atmosphere combined, then surface temperatures could be kept above the freezing point of water, at least on part of the planet. In that ca
se, the optimally habitable Ribbonland would not be located near the terminator but near the sub-stellar point, where temperatures would be warm enough.

  Of course, such a planet would not really be habitable by human beings. Even if it hosted organisms that were producing oxygen in large quantities, the atmosphere would still be toxic, as the high concentration of carbon dioxide necessary to keep the planet warm enough would be harmful to breathe. Also, estimates suggest that the surface gravity of Gl 581d is approaching twice that of the Earth, far too much for sustained human habitation, although not too much for land plants or for organisms living in the ocean. Oceanic organisms would be neutrally buoyant and thus protected to a large degree from the damage that could be caused by high gravity. Even so, the planet may not have sufficient greenhouse gases to maintain liquid oceans, in which case it would be a giant, frozen wasteland, with no Ribbonland at all.

  Also, for Ribbonland to exist, Gl 581d would need to be in synchronous rotation, the technical term for always keeping the same face towards its star. If its orbit is slightly oval-shaped, as the analysis suggests, then just like Mercury it could have non-synchronous rotation. If it had a 3:2 ratio of rotation to orbital period, like Mercury, then it would have a rotation period of 55.7 days and a “solar day,” the time between sunrise and sunset, of 177 days—again, by strange coincidence, almost exactly like Mercury. This would also mean that there would be no Ribbonland, as there would be sunrises and sunsets and extreme temperature variations from day to night as the Sun crawled across the sky. In this scenario, a completely oceanic world would have smaller daily temperature variations than a planet with large continents, but even on an ocean world nighttime temperatures at equatorial locations on Gl 581d would still likely be well below freezing unless the atmosphere itself were massive enough to prevent large daily variations in temperature. This is what happens on Venus, where despite its slow rotation its thick atmosphere transports heat so effectively that the surface temperature is about the same on the night side as it is in the day side. But a massive atmosphere also means crushing surface pressure, another factor that would limit habitability.