Currently, the most widely-held hypothesis of the Solar System’s formation is a version of the nebula hypothesis, albeit with some important differences. 7 Today, the Solar System is thought to have formed through the collapse of a small region of one of the giant molecular clouds that are scattered throughout the Galaxy. Just as in the model put forward by the Eighteenth Century pioneers, this region of the cloud collapsed under the influence of gravity, with the bulk of the mass contracting into the central regions to become the Sun. Planets formed from the surrounding cloud through the accretion of solid grains, first into pebbles, then boulders, then larger mini-worlds known as planetesimals and finally into the planets we see today. Nevertheless, the Solar System is a different and more complex place than Laplace, or even Jeans, imagined or even could have imagined given the data available at these earlier times. Partly through the discovery of other Solar Systems—most of which are a lot less sedate than our own—and partly through the continued study of our own system at both observational and theoretical levels, we now believe that the Solar System has passed through eras of turbulence beyond anything that Laplace and his contemporaries could have contemplated.
Between Mars and Jupiter there is no planet but instead we find the asteroid belt. This is because Jupiter perturbed the nascent planets that formed in that region, causing them to collide rather than coalesce. The result is a ring of asteroids, rather than a planet, circling the sun. 9 In the outer regions of the solar system, where the temperature is lower, icy dust collects to form small planetesimals that later attract the hydrogen and helium gases. Left over planetesimals may be captured as moons or are ejected to the outer reaches of the solar system to become comets. Hence the composition of comets and meteorites should represent the early solar nebula. Later, the sun’s radiation and solar wind drive any remaining gas out of the solar system, and the sun’s rotation is dramatically slowed by magnetic braking. This is the rendition of Laplace’s Nebular Hypothesis from recent years, but there remain several anomalies to explain.
For instance, Venus and Uranus have anomalous spin characteristics. Our neighboring planet Venus is an oddball in many ways. For starters, it spins in the opposite direction from most other planets, including Earth, so that on Venus the sun rises in the west. 10
About a third of the more than one hundred moons in the solar system have irregular orbits, revolving about their host planet in the wrong direction. There is no general explanation for these many anomalies. It could be that huge impacts reversed the spin of Venus and tipped Uranus on its side. Perhaps moons that revolve too fast have dropped from a higher orbit, and thus increased their rate of rotation. Or they may have been captured by rather than formed with the planet.
As for Pluto, one idea is that a large planetesimal passed near Neptune, lost some energy and fell down near Jupiter which ejected it to beyond Pluto. In the process the orbits of Jupiter, Saturn, Uranus and Neptune are all perturbed and Neptune, in turn, perturbs Pluto into its highly eccentric and inclined orbit we observe today.
Another difficulty with today’s theory of the solar system origin is the great size of the outer gaseous planets. In order to accumulate so much light gas they must have formed very quickly because early on the sun’s solar wind would have blown the gas out of the solar system altogether.
One explanation for this is that these planets formed via a faster acting mechanism known as disk instability. But if this works for Jupiter and Saturn, it leaves open the question of why Uranus and Neptune are not so large. If the disk instability mechanism gave Jupiter and Saturn their thick atmospheres, why didn’t it give thick atmospheres to Uranus and Neptune?
One answer is that our solar system formed in a cluster of stars. Perhaps the neighboring stars were so close that radiation heated the gases in the outer reaches of our solar system, making them more difficult for Uranus and Neptune to capture
A new flip?
In recent years the Nebular Hypothesis has met with even more failures. For instance, discoveries of distant planets have revealed star systems that make no sense on the Nebular Hypothesis. As one researcher commented, “These discoveries are making it very difficult to stick to the party line endorsing the so-called standard model.”
Planet formation is a paradox: According to standard theory, dust grains orbiting newborn stars should spiral into those stars rather than accrete to form planets. 8
Solar System stability 6
The problem of the Solar System stability dates back to Newton’s statement concerning the law of gravitation. If we consider a unique planet around the Sun, we retrieve the elliptic motion of Kepler, but as soon as several planets orbit around the Sun, they are subjected to their mutual attraction which disrupts their Kleperian motion. At the end of the volume of Opticks, Newton himself expresses his doubts on this stability which he believes can be compromised by the perturbations of other planets and also of the comets, as it was not known at the time that their masses were very small. These planetary perturbations are weak because the masses of the planets in the Solar System are much smaller than the mass of the Sun (Jupiter’s mass is about 1/1000 of the mass of the Sun). Nevertheless, one may wonder as Newton whether their perturbations could accumulate over very long periods of time and destroy the system. Indeed, one of the fundamental scientific questions of the 18th century was to first determine if Newton’s law does account in totality for the motion of celestial bodies and then to know if the stability of the Solar System was granted in spite of the mutual perturbations of planets resulting from this gravitation law. This problem was even more important as observations actually showed that Jupiter was getting closer to the Sun while Saturn was receding from it.
The earth's habitable zone 1
The temperature of a planet is dependent on the orbital distance of the planet from the central warming star. In our own Solar System, the planets Mercury and Venus orbit too near the Sun to support liquid water: water could only exist as vapour, whereas Mars is now too cold for liquid water. The Earth lies within those two orbits, with surface temperatures allowing the continual presence of liquid water. There is a similar orbital range around all stars within which liquid water could exist on a planet. This band is called the habitable zone. Combining the two habitable zone requirements — around a star and also around the galactic center — places severe restrictions on the number of habitable planets in a galaxy. mass-extinction of complex life.
The size of the habitable zone clearly depends on the luminosity of the star, which determines the equilibrium temperature of the planet. However, modern models for the range of the habitable zone take into account more subtle effects, such as the effect of the carbonate-silicate cycle in regulating carbon dioxide in a planet's atmosphere.
The mere fact that 95% of all stars are less massive than the sun makes our planetary system quite rare. Less massive stars are important because they are much more common than more massive ones. For stars less massive than the sun, the habitable zones are located farther inward. The most common stars in our galaxy are classified as M stars; they have only 10% of the mass of the sun. Such stars are far less luminous than our sun, and any planets orbiting them would have to be very close to stay warm enough to allow the existence of liquid water on the surface. However, there is danger in orbiting too close to any celestial body. As planets get closer to a star (or moons to a planet), the gravitational tidal effects from the star induce synchronous rotation, wherein the planet spins on its axis only once each time it orbits the star. Thus the same side of the planet always faces the star. (Such tidal locking keeps one side of the Moon facing Earth at all times.) This synchronous rotation leads to extreme cold on the dark side of a planet and freezes out the atmosphere. It is possible that with a very thick atmosphere, and with little day/night variation, a planet might escape this fate, but unless their atmospheres are exceedingly rich in CO2, planets close to low-mass stars are not likely to be habitable because of atmospheric freeze-out. We can thus look at various stars in our Milky Way galaxy and ask whether they are appropriate places for life or, indeed, have habitable zones at all. For example, could there be habitable planets orbiting binary stars or multiple star systems, places where two or more stars are locked in a complex orbital dance? Can planets with stable orbits and relatively constant regimes of temperature be found in such settings? Can planets even form in such settings? These questions are highly relevant to understanding the frequency of life beyond Earth because approximately two-thirds of solar-type stars in the solar neighborhood are members of binary or multiple star systems. Astrobiologist Alan Hale, who has written on the problems of habitability in binary or multiple star systems, notes, “The effects of nearby stellar companions on the habitability of planetary environments must be considered in estimating the number of potentially life-bearing planets within the Galaxy.”
Like a planet revolving around a star, we maintain roughly the same distance from the galactic center, and this is fortunate. Our star—by chance—is located in the “habitable zone” of the galaxy. We suspect that the inner margins of this galactic habitable zone (GHZ) are defined by the high density of stars, the dangerous supernovae, and the energy sources found in the central region of our galaxy, whereas the outer regions of habitability are dictated by something quite different: not the flux of energy, but the type of matter to found.
The most important factor affecting the surface temperature of the earth is obviously the distance from the sun. 4 If the earth were moved a few million miles closer to the sun, the surface of the earth would become warmer causing our glaciers to melt. With a decrease in the area of ice the total reflectivity of our planet's surface would thereby decrease and more of the sun's heat would be absorbed. The melting of glaciers would produce a rise of sea level, and, apart from flooding most of our modern cities, would create a larger total ocean surface area. Since seawater absorbs larger amounts of solar radiation than equal area land masses, heating of the earth would again be promoted. Furthermore, after increasing the temperature of the oceans, much of the ocean's dissolved carbon dioxide would be added to the atmosphere along with large amounts of water due to increased evaporation. The increased carbon dioxide and water vapor level of the atmosphere would again bring about a significant temperature rise. All things considered, a minor decrease in the sun's distance would have a drastic heating effect on the earth's surface.
What would happen if the earth were a few million miles farther from the sun? The reverse of the previous situation applies. We would have more of our planet covered by ice, with associated increased reflectivity of the sun's heat. The ocean would cover less of the earth's surface and the important process of absorption of heat by seawater would be decreased. Since the ocean would be colder, evaporation would be less with less heat-trapping water vapor in the atmosphere. Much of the carbon dioxide from the atmosphere would become dissolved in the colder ocean. Calculations show that a decrease of carbon dioxide in the air to just one-half of its present level would lower the average temperature of the earth's surface by about 7.0 degrees Fahrenheit! Thus, increasing the sun's distance would have a profound cooling effect on our planet.
From this discussion, we see that the earth is just the proper distance from the sun to maintain the right surface temperature suitable for life and the many important geologic processes!
A Just-Right Sister Planet 2
Jupiter is the just-right size, the just-right distance from Earth, the just-right mass and is in the just-right location to shield Earth from being regularly bombarded with comets and asteroids that would be deadly to life.
Jupiter is 40 light minutes away from Earth (i.e., it takes 40 minutes for light emanating from Jupiter to reach Earth) and Jupiter is so massive it could hold 100 planets the size of Earth. It also outweighs the combined total all of the other planets in the entire solar system by 2 ½ times. Scientists estimate that without Jupiter being the size it is and positioned where it is, comets and asteroids would strike Earth a thousand times more often than they do. One large impact or several smaller impacts would kill off all advanced life on Earth because the dust and debris ejected into the atmosphere by the impacts would prevent too much of the sun’s light and heat from reaching the surface of the earth. Without sufficient light and heat, Earth would get too cold to support life. However, if Jupiter were any larger or closer to Earth than it is, Jupiter’s gravitational force would pull Earth out of its stable orbit around the sun which would subject Earth to deadly temperature variations, unstable atmospheres, high velocity winds and a host of other problems associated with unstable orbits, all of which would be deadly to life on Earth.
Stephen Hawking has said that a collision with a comet or asteroid greater than twenty kilometers in diameter would result in the mass-extinction of complex life. It is now believed that the gravity of massive Jupiter and Saturn located outside the orbit of the Earth acts to catch many asteroids and comets entering the Solar System before they can collide with the Earth. As an example, the comet Shoemaker-Levy 9 was observed colliding with Jupiter in 1994, leaving a scar on its surface. 1 The Earth is completely and perfectly middling: it lies in the narrow habitable zone. The Earth is not the biggest, it's not the smallest. It's not the hottest, it's not the coldest. It lies in a relatively safe planetary system where nothing very exciting ever happens, no planetary collisions or extinction-level asteroid strikes. At first glance, nothing about the Earth appears to be remarkable — apart from the fact that it is the only place in the universe that we know of where life exists. Ironically, it is the perfect mediocrity of the Earth which makes it special.
1. HIDDEN IN PLAIN SIGHT - The Fine-Tuned Universe, page 47
3. RARE EARTH Why Complex Life Is Uncommon in the Universe, Peter D. Ward - Donald Brownlee, page 53
7. Weird Astronomical Theories of the Solar System and Beyond, page 60