SIMSETI : THE WORLD as created by Karen Anderson, Poul Anderson, Paula Butler and Ctein and visualized by Joel Hagan (summary written by Ctein) The Solar System The solar system we chose for the 1992 Contact Bateson Project was that old stand-by "Alpha Centauri". ØCen is actually a trinary system. The three components, in order of size and brightness are called Alpha Centauri A (aCenA or just "A"), Alpha Centauri B (aCenB or "B") and Proxima Centauri (aCenC). The aCentauri system is 4.3 light years from earth, and is visible to the naked eye as a 0th magnitude star. The celestial coordinates are RA : 14 hr 33', Decl.: -60.4-, so Centauri can be seen only from Hawaii and the southernmost points of the continental US. Just as the Centauri system is the closest known stellar system to Sol, Sol is the closest known star to aCentauri. There is some disagreement between sources as to the precise characteristics of the Centauri stars, as described below. We've chosen a seemingly reasonable set of values from the published data; that doesn't make them authoritative in any way. aCenA is a close match to Sol. It is a type G2 star with a mass of 1.1 solar masses and an absolute visual magnitude of 4.5. It is about 20% larger in radius than the sun, and about 50% brighter. It is very slightly 'bluer' than Sol, with a surface temperature of 5800-K and a peak emission wavelength of 500 nm. aCenB is a smaller, cooler star. It is a type K5 star, with a mass 0.85 of Sol, and an absolute visual magnitude of 5.9. Its luminosity is only 50% of Sol's, and its radius is 87% of Sols giving it a surface temperature of about 5260-K, and a peak emission wavelength of 550 nm. B is definitely yellower than Sol, but to the eye, B would still appear 'white', being comparable in color to an old-style flashbulb. [Note: as of 2014, B may have an Earth-sized planet in a very close orbit, but this is disputed.] A and B orbit each other in a highly eccentric (e = 0.52) ellipse. The minimum separation of A and B is 10.8 astronomical units (A.U.), comparable to the distance from Sol to Saturn. The maximum separation is 35.5 A.U.-- the distance from Sol to Pluto. The orbital period is 82 earth years. Promixa Centauri is in an entirely different class. It is in a large orbit about both A and B. The radius of that orbit is not precisely known, but it is 10,000 A.U. or more, and the orbital period is 300,000-1,000,000 years. pCen is a dim, red star, about 1/20,000 as bright as A. "Red" is a relative term-- the surface temperature of Proxima is over 3000-K, making it whiter-looking than a 100 watt incandescant light bulb. Proxima is an unusual object in that it is also a flare star: it brightens severalfold for a short period of time. The flares happen irregularly, with days to weeks between flareups. As seen from the vicinity of A or B, Proxima would appear as a dim star of visual magnitude 4.5. When it flares, it might sometimes get as bright as 3.5 or so. The question of planets at Centauri (let alone habitable ones) is controversial. The concern is that the orbits of any planets about one star will be perturbed by the periodic close passes of the other major star. The further from a primary a planet is, the more its orbit will be altered by each pass of the other star. Planetary orbits bigger than Jupiter's (5-6 A.U.in radius) are not stable; planets that far from a primary will be ejected from the star system in a short time, geologically speaking. Planets in orbits as small as Mercury's (0.4 A.U.) are 'stable'; the orbit will undergo slight changes in radius and eccentricity (as does earth's, for that matter) but it will basically stay the same for billions of years. As a side note, we consider an orbit to be 'stable' even if it exhibits some chaotic variation in radius and eccentricity, so long as the range of variation remains small. The issue comes up for planets precisely where we would want them, 1 A.U. or so from either major star. So far as we can tell, there are still two schools of thought about such planets: either their orbits are stable or they ain't! The question has not yet been resolved. We chose to take the position that orbits out to 3-4 A.U. would be stable, but readers should be aware this is assumption, not proven fact. [Note: As of 2014, a number of extrasolar planets have been found in similar orbital circumstances.] Of course, one could have planets in stable orbits which circle both A and B at a great distance, but these don't interest us. In our model, the Centauri system does have an Oort cloud, although no one knows if this is true in fact, nor what its characteristics would be. We placed our habitable planet about A; from here on, we speak only of the "A" system, and we think of aCenB as a body which passes through every 80-odd years. We do not know nor care whether B has planets. Because of the perturbing effect of B on orbits in system A, the solar system is fairly clean of planetary debris. There is no major asteroid belt. Most such material was scoured out of the system in a short time, astronomically. On the other hand, the combined effects of the moving gravitational fields of the three stars should perturb the cometary orbits in the Oort cloud more than in our solar system's cloud, so the A system is subject to a much higher rate of comet fly-through and impacts! There are also many more streams of meteors left as the remnants of some of those comets. In our model, we assume the Oort cloud has not been scoured so rapidly that it is gone by the present time. Similarly, we assume the rate of planetary bombardment is not so high as to preclude a thriving ecosystem on a planet of A, but it is high enough to make life "interesting". The Planet Our planet does not have a name in English, because the humans haven't discovered it yet. The planet has the following characteristics: Orbital Mean Radius:1.19 A.U. (somewhat more eccentric than earth's orbit) Irradiation from A:1.03 times what earth gets from Sol Orbital Period:1.22 earth years Length of Day:19.1 hours Length of Year:560 planetary days Axial Inclination:29- (1.23 times earth's tilt) Planetary Mass:1.5 earths Planetary Density:6.4 gm/cc (1.16 times earth) Planetary Diameter:14,000 km (1.09 times earth) Surface Gravity:1.26 times earth Sea Level Air Pressure:2.09 times earth The planet has two moons, called Aleph and Beth for convenience. They have about the same density as Luna ( ~ 3.3. gm/cc). They orbit in approximately (but not precisely) the equatorial plane of the planet; they orbit in slightly different orbital planes. The orbits are rather eccentric, so they precess rapidly. The moons have the following characteristics: Aleph Beth Mean Distance from planetcenter 55,000 km 190,000 km Orbital Period 1.26 planet days 9.92 planet days Diameter 580 km 430 km Mass .003 times Luna. .002 times Luna Angular Diameter 0.7 times Luna 0.14 times Luna (as seen from planet surface) Tidal Effect 1.6 times Luna 0.02 times Luna The stellar tides are about 0.6 times Sol's tidal effects on earth, so spring tides would be about twice earth's while neap tides would be about 1.4 times earth's. Because Aleph and Beth have such short orbital periods, their apparent "months", as seen from the planet's surface, are only slightly longer than their orbital periods. Aleph completes a "month" of phases, as seen from a spot on the planet's surface, in slightly over a day and a quarter. Since the planet rotates almost as fast as Aleph moves in the heavens, Aleph's apparent motion through the sky is slow, taking about five days to make a complete circuit from moonrise to moonrise. At a point directly under Aleph's orbit, when the moon would pass through the zenith, it would take over one planetary day for the moon to "move" from moonrise to zenith and another day to move from zenith to moonset; during that time, Aleph would go through 1.5 full cycles of phases. Another interesting characteristic of Aleph is that it produces large numbers of total "solar" eclipses. Aleph has a visual diameter about 30% greater than of ØCenA. Twice a year, for a period of a few weeks, the orbit of Aleph will be aligned with the planet-to-A line-of-sight such that the shadow of Aleph can cross the planet. Because Aleph orbits so close to the surface of the planet, and because it is visually much larger than A, every one of those shadow crossings produces a total eclipse. During that biannual "eclipse season", there would be 12-15 total eclipses. The umbral track of each eclipse (where a total eclipse would be seen) would be over 150 km in width, and the penumbra would spread for 500 km on either side of that. Aleph has an orbital velocity of about 4 km/sec, so the total eclipse seen at any particular spot would last well under a minute. But, because the track is so wide, every spot on the planet would experience a total eclipse once every couple of years. Most spots on the planet would experience a partial eclipse during every eclipse season. Beth is not large enough to produce a total eclipse, with only about 1/3 the apparent diameter of A. It would produce annular eclipses more frequently than Luna does for earth, but the eclipses would not be very spectacular by human standards. Because of the planet's larger diameter and the two moons' closer orbits, lunar-type eclipses would be common. In fact, lunar eclipses of Aleph would be as common as the Aleph's eclipses of A. Planetary Science Because our planet is larger than earth, it has a smaller surface-to-volume ratio, and so should retain internal heat better. Accordingly, it should have a hotter and more active interior. All other things being equal, internal convection will be more fluid and rapid than earth's, and so the planet will have more rapid crustal plate motion, more vulcanism and more (and possibly larger) earthquakes. Our planet has convection-cell tectonics, like earth, but the upper mantle convective layer should be thinner than earth's, so the average size of the convection cells would be smaller than on earth. Instead of having large plate tectonics, our world has "saucer tectonics". Instead of having 8-10 major plates like earth, our planet has 24-30 major plates, with the average plate being only 40% of the size of the average earth plate. The greater tectonic and plate-spreading activity means that oceans on this world are younger than earth's on the average. As a result, they will tend to be shallower, so there is much shallow flooding of continental crust. The plates are of various sizes, of course, with the largest supporting an Australia-sized continent. The extensive flooding produces highly irregular coastlines, with many shallows and bays, and many chains of islands of widely varying size. Joel Hagan has produced a map of the world as well as some landscape views of the coastline (illustrations enclosed). Compositionally, this planet is not much different from earth; the differences in geography, geology and weather arise from differences in the size of the planet and its relation to ØCenA and B. Climate and Weather B does not have much of a direct effect on the planet's weather. Even at closest approach, B's contribution to the planet's insolation is under 1%. In terms of weather, that's not a lot of light, but visually, it's plenty. "Night" when B is in the sky and at perihelion, would be as bright as dusk on earth. Even at aphelion, B-light would be hundreds of times as bright as our full-moonlight. Of course, B is in the night sky for only half the year; during the other half of the year, the planet has normal nights, lit by nothing more than the stars and the moonlight from Aleph and Beth. While B does not directly affect weather, it does affect climate. Although the orbit of the planet is stable, the uneven gravitational tug of B perturbs the planet's orbit and axis of rotation in a complex cycle which is more irregular and extreme than the variations we see in the earth's orbit and rotation axis. On earth, these cycles are believed to be the primary instigators of ice ages and interglacial warm spells. Accordingly, we expect our planet to undergo even more irregular variations in climate. In contrast, the denser, thicker, and wetter atmosphere (remember all those oceans) is an effective heat transport medium, which does much to ameliorate extremes in temperature. The lack of really large continents helps, too; most of the planet has a coastal kind of climate. One can visual the planet as being pretty much one large semi-tropical shallow sea. It's a pleasant enough place with frequent rainfall, lots of clouds, and gorgeous sunsets-- a product of the extra-thick atmosphere and plenty of high altitude volcanic dust. The flip side of that coin is that the combination of rapid planetary rotation, plenty of open water and lots of atmospheric water vapor makes a prime breeding ground for tropical storms. Such storms may travel a considerable ways before they cross a large enough land mass, or are swept far enough north, to disrupt their massive heat engines. The weather outlook on our planet is sunny and semi-tropical, with frequent earthquakes, volcanoes, typhoons and a good chance of meteor showers. *********************************** June 8, 1991 SAUCTECT.TXT 'SAUCER' TECTONICS ON ALIENS' PLANET In the last meeting we decided that the aliens' planet should have many more, smaller, and more active plates than the Earth. I have looked at what this means for plate tectonics ('saucer' tectonics) on the aliens' planet, and have concluded that this is easiest to achieve if the planet is substantially denser and more massive than Earth. Extrapolating from the densities of Earth and Mars, I modeled a planet with mass = 1.5 Earth's and density = 6.4 gm/cubic cm = 1.16 Earth's density. This gives it a radius = 1.09 Earth's and surface gravity = 1.26 Earth's. Its surface area is 1.19 times Earth's, and its volume is 1.30 times Earth's. With a surface-to-volume ratio smaller than Earth's, the aliens' planet is better able to hold onto internal heat. Other things being equal, the planet will have more rapid convection than Earth, and will therefore have more rapid plate motion, more volcanism, more and possibly larger earthquakes, etc. Smaller lithospheric plates may result if the lithosphere and the convective zone of the upper mantle are both thinner than on Earth. (The lower mantle is also thought to convect, but in a manner largely decoupled from the upper mantle, and therefore also decoupled from the lithospheric plates.) On Earth the base of the lithosphere is about 50 km deep, and the seismic discontinuity dividing the upper and lower mantles is 670 km deep, for a convective zone 620 km thick. I can't calculate the thickness of the lithosphere of the aliens' planet, but I will assume it is about 40 km thick. The reason for the seismic discontinuity at 670 km depth is still actively debated. Some argue for a difference in composition of the upper and lower mantle. Others argue that the composition is the same, but the phase changes from spinel-type structure in the upper mantle to denser, perovskite-type structure in the lower mantle. I have arbitrarily chosen the latter model because I can use it to estimate the depth at which such a density discontinuity would occur on the aliens' planet, assuming its mantle composition is not too different from Earth's. The pressure, and consequently depth, at which the structure changes from spinel-type to perovskite-type, is largely dependent on pressure, not composition. To first order (merely scaling by surface gravity), the upper/lower mantle boundary on the aliens' planet should be about 530 km deep, giving a convective zone only 490 km thick, or about 80% the thickness of Earth's. I don't really know the effect of thinner plates and thinner upper mantle, but assuming that each has a linear effect on the diameter of a plate, major lithospheric plates on the aliens' planet should be about 60% the diameter, or 40% the area of Earth's major plates. Instead of 8-10 major plates, the aliens' planet should have about 24-30 major plates covering the 1.19-times- larger surface area. Smaller plates may or may not differ much from Earth's in size distribution. I suspect that they do not, and there is just more continuity from large to small plate sizes on the aliens' planet. I have not tried to map an internally-consistent set of plate motions for 24- 30 major plates. With that number of plates, I think a largely random distribution of the different types of plate boundaries may suffice for our purposes. At the last meeting we decided to have a greater proportion of the aliens' planet covered by water than on Earth. This can be achieved either of two ways, by having proportionately less area of continental crust, or by simply flooding more continental crust. I favor the latter method because, other things being equal, the smaller surface-to-volume ratio of the aliens' planet would tend to give it a greater (not lesser) proportion of continental crust to surface area. Flooding of more continental crust would tend to occur with the more active tectonics of the aliens' planet. Its oceanic crust will tend to be younger than Earth's, and the younger the oceanic crust, the less its density. The younger oceanic crust floats higher and displaces more ocean water from the ocean basins onto the continental crust.