Between 1930 and 1992, our solar system seemed fairly straightforward: a star around which 9 planets and an asteroid belt orbited, and a number of comets with long orbits that periodically travelled close enough to the Sun put on a show for us as they flew past Earth. This beautiful contemporary orrery shows the Sun plated in gold, Earth as a blue glass sphere and our moon as a pearl. These devices, run with a clock-like mechanism, were all the rage in the 1700's and 1800's. This one, a replica, can be purchased new today.
Our universe seemed to fit quite nicely into a planetarium and it was easy to conclude that nothing but a vacuum existed past Pluto, except for populations of impossibly distant stars that seemed of no consequence to us here.
Then a lot of things changed, beginning with our realization that space was not just vast nothingness. Instead, it wielded threats such as extinction-level comets and meteors
The image above is an artist's conception of a large meteor that struck North America approximately 65 million years ago and triggered the Cretaceous-Tertiary extinction.
and gamma ray bursts from supernovae.
Above is an artist's illustration of the gamma ray burst GRB 080319B, detected in 2008. It took place 7.5 billion light years away and even from that incredible distance it was so bright it could be seen by the naked eye.
We learned that our solar system began with chaos and destruction.
This is an artist's conception of the solar nebula, from which the protoplanetary disc would emerge and the planets and other objects would form. The Sun is igniting into a star as the temperature and pressure within its core become sufficient to fuse hydrogen into helium.
Orbiting bodies other than planets and moons were observed - grey areas began to erupt, culminating with the controversial decision to demote Pluto from a planet in 2006. The last century and this one signify a period of rapid evolution of knowledge about our solar system, and we are living right in the fascinating thick of it.
The Search for Planet X
Things began to unravel in the early 1900's when astronomer Percival Lowell found discrepancies in his measurements of the orbits of the outer planets. This led him to believe that an unknown planet must orbit beyond Neptune. and he spent his last years trying to find it. He didn't succeed but after his death his observatory continued his search and eventually found a body orbiting where it was predicted to be, based on the calculated gravitational displacement acting on Uranus and Neptune. In 1930, they named it Pluto, after the Greek god of the underworld. However, astronomers grew increasingly skeptical that this small planet had enough mass to affect the orbits of two giant planets. At the time they thought Pluto was Earth-sized and even that mass would not be enough to have any effect. In 1978, a satellite was discovered orbiting Pluto (its moon, Charon, both of which are visible in the following 1990 photo taken by the Hubble telescope) and this discovery made it possible to accurately measure Pluto's mass.
As suspected, Pluto was found to be inconsequentially small, in fact far smaller than even their predictions. Something else was tugging at the two giant ice worlds. In the meantime, soon after Pluto was discovered, astronomers began to speculate that this planet might not be alone. A search led to the discovery of an entire population of comet-like bodies orbiting between Jupiter and Neptune. However, rather than settling the question about the perturbation of the ice giant's orbits, the discovery of these bodies raised even more questions. These icy bodies, called centaurs, were found to have unstable orbits and they were calculated to have lifetimes of only a few million years.
A New Object: Centaur
Centaurs are unusual objects. They are technically minor planets that share characteristics with both comets and asteroids. A minor planet is an object in direct orbit around the Sun that is neither a classical planet (like Jupiter) nor a comet. Minor planets include dwarf planets, all of which I will define in more detail shortly. Centaurs, named after mythical half horse/half human creatures, are known to cross the orbits of the giant planets, and there are many of them. Our solar system is filled with more than 44,000 of these objects, each of which is larger than 1 km across (the largest being a massive 260 km in diameter). The first centaur was discovered in 1920 and they were first classified as a group in 1977. In this image, centaurs are labeled orange, while all Kuiper Belt objects are bright green (we will be investigating the Kuiper Belt shortly as well).
Perhaps even more strange is that centaurs come in a whole range of colours. This challenges any model of surface composition, but it could be attributed to either composition, origin and/or space weathering. Some of these objects demonstrate comet-like behavior such as a comet-like tail as they near the Sun during their orbit. It should be kept in mind that these are examples of grey-area objects - there is no clear line of distinction between centaurs, comets or asteroids. Much more data on these puzzling objects is needed.
As suggested earlier, the discovery of centaurs raised an obvious question: How could they still exist billions of years after the solar system's (and therefore their) formation? Some outer reservoir of ice must be regularly replenishing them, but what?
In the meantime, in the 1950's, astronomers began to also wonder why comets are still so plentiful in the solar system. Like centaurs, they were well known to have finite lifespans because their surfaces sublimate* into space every time they draw near the Sun. Comets are small icy bodies, usually less than 50 km across, that display a visible coma (a thin fuzzy temporary atmosphere), the result of sublimation of their icy surfaces into gas when they are struck by sufficient solar radiation. They may also sport a tail visible from Earth as they near the Sun, which is composed of dust that reflects sunlight and gases that glow because they are ionized by solar radiation.
*Comets loose a great deal of mass when they pass near the Sun! For example, Halley's Comet lost 5 x 1011 kg of mass last time it made a close pass. Extrapolated data, assuming the comet's orbit doesn't change, means that Halley will be gone in 170,000 years. Halley's Comet, its bright tail visible to the naked eye, has streamed past Earth once every 75-76 years ever since it was first recorded in 240 BC. This is what it looked like when we last saw it in 1986.
Comets have been of particular interest to scientists ever since a theory was proposed that Earth's vast reservoir of water might have come from multiple comet impacts during a period early in Earth's history called the Late Heavy Bombardment, and that organic compounds within comets might also have seeded young Earth with the building blocks of life. NASA launched a robotic probe to sample the coma of a comet in 1999. It met up one called Wild 2 in 2004 and collected dust grain samples from the comet's coma. Scientists found a wide range of organic compounds, including two that contain biologically usable forms of nitrogen. In 2005, NASA launched another comet probe, the Deep Impact Spacecraft. This one impacted a comet, creating a large crater in it so that it could remove samples from the comet's interior to send back to Earth to analyze.
The image above is an artist's conception of the Deep Impact Spacecraft landing on the surface of the 370 kg comet called Tempel 1.
They were surprised by the amount of dust inside the comet, which was fine like talcum powder rather than coarse as expected, like sand. They were puzzled to find clays and carbonates, which usually need liquid water to form as well as sodium, which is very rare in space. This dust is debris from the solar system's most distant and coldest regions that formed 4.5 billion years ago. Scientists suspect that Tempel 1 came from a region far past Neptune, based on the amount of low temperature ices, such as ethane, that it contained. This puts it in a group of comets called long-period comets.
There must be a large reservoir of icy bodies very far away from the Sun to explain the origin of these long-period comets, comets that have orbits lasting thousands of years. This suspected reservoir, named the Oort Cloud and first hypothesized by Jan Oort in 1950, is believed to extend into the extreme far reaches of the solar system. This belt is the home of long-period comets but it cannot account for the orbits of short-period comets, of which Halley's Comet is one. In 1988, a Canadian team of astrophysicists ran computer simulations of all observed comets. A group of short-period comets came from an area that was consistently in the same plane as the solar system, whereas Oort Cloud comets came from any point in the sky. This meant that a belt of comets must exist in the same plane as our system, and this has come to be known as the Kuiper Belt. This artist's rendering of the Oort cloud and the Kuiper Belt might help you visualize where these two areas are located in our solar system, and how large they are.
Deciphering the Asteroid Belt, Kuiper Belt and Oort Cloud
All three of these zones contain small bodies, or remnants, from the birth of our solar system.
The Asteroid Belt, by far the innermost belt of objects, orbiting between Mars and Jupiter, is illustrated in the following diagram.
It contains many asteroids, ranging in size from 950 km in diameter to the size of a dust grain, which are so thinly distributed that several unmanned spacecraft have so far traversed it without incident. Asteroids, also sometimes called planetoids especially the larger ones, are generally what we refer to as the bodies that make up the Asteroid Belt.
This is a composite image of asteroids that have been imaged at high resolution.
Vesta, with a diameter of 530 km, dwarfs the other asteroids and is sometimes called a protoplanet. It may be just massive enough to form a sphere and its interior is thought to be differentiated. Its shape is being investigated and if it is determined that this body maintains hydrostatic equilibrium, it will be reclassified as a dwarf planet. The Dawn Mission sent by NASA, is currently investigating Vesta and afterward it will investigate the dwarf planet, Ceres, shown below with both Vesta (left) and Ceres (right) in this artist's concept).
The asteroids, composed mostly of rock and metal with some sporting an icy mantle, are made up of the same stuff as planets and might have accreted into a planet long ago if they had not been so energized by the gravitational field around Jupiter. Collisions between these asteroids were so violent that they shattered on impact instead of sticking together. They were much more numerous and disordered when the solar system was very young. Many collisions occurred and, over time, most asteroids shot off in different directions, decreasing the mass of the belt to what we see today. Most of these asteroids orbit around 2.7 AU (Earth-Sun distance) from the Sun. This is an interesting distance because it is here where the Sun formed a "snow line" when the belt was forming, the same period during which planets were forming. Chunks of rock that formed beyond this radius were able to accumulate water ice. This belt contains short-period comets within its outer regions. The difference between comets and asteroids is that asteroids come from the warmer inner solar system and are composed mostly of rock and metal. Comets come from the colder outer solar system and are composed of water and other ices, rock and organic compounds. Short-period comets, much more plentiful and volatile in this belt's youth, may have supplied Earth with its water (adding significantly to the outgassing of water vapour from Earth's interior).
The Kuiper Belt is much farther out than the Asteroid Belt, extending from Neptune's orbit (30 AU) to about 55 AU from the Sun. Objects here are similar to those of the Asteroid Belt except that rather than being made up mostly of rock and metal these contain mostly frozen methane, ammonia and water. This belt also contains objects large enough to be classified as dwarf planets. Recall that the Kuiper Belt was first hypothesized as recently as 1988. No one is yet sure how it came to be. Astrophysicists are attempting to answer this question by using new wide-field survey telescopes to find more Kuiper Belt objects (KBO's). So far, they believe that these objects, like those of the Asteroid Belt, are remnants from the original protoplanetary disc around the just-formed Sun and that they failed to coalesce into planets. The largest KBO is less than 3000 km in diameter, too small to meet the requirements of a planet. And, like the Asteroid Belt, gravitation from Jupiter is the most likely culprit. Interestingly, recent computer simulations suggest that neither Neptune nor Uranus formed in situ out here. There wasn't enough raw material even when this belt was young to form them. The Nice Model is the model most often cited to explain the migration of these giant planets from more inner orbits close to Jupiter to their present locations. The repositioning of Neptune and Uranus is thought to have occurred when Jupiter shifted into a powerful 2:1 resonance with Saturn, destabilizing the orbits of the two ice giants. The planetary jostling probably threw many early KBO's into disorder, shooting them off in different directions and depleting the population. Keep in mind that the Nice Model is still a work in progress. It is not without its problems in terms of explaining the location and motion of all the current objects in the solar system – our solar system is an extremely complicated system that challenges even the latest modeling hardware and software.
Even though scientists have suspected its existence for over 60 years now, there has yet been no confirmed evidence for the Oort Cloud. One reason for this is that this cloud is very very far away, 50,000 AU. That is about a thousand times further from the Sun than the Kuiper Belt, or almost ¼ of the way to the next nearest star, Proxima Centauri. The Oort cloud defines the limit of our solar system, where the Sun's gravitational force is gradually overcome by the tug of passing stars and the tug of the Milky Way itself, forces at play in local interstellar space. Like the Kuiper Belt, objects that comprise this zone are primarily composed of water, ammonia and methane ices, and they are suspected to have been remnants scattered far out into space by the gravitational forces of the giant planets in the early solar system. This is the hypothetical home of long-period comets, as well as many centaurs. Only four objects, all with highly eccentric orbits have so far been considered as possible members of the Oort group.
If you do some quick internet research you will find other structures of the solar system such as the Scattered Disc and Detached Objects, but the above three main zones of objects hopefully give you a general idea of how the system is set up, keeping in mind that our system did not form itself into discrete objects and zones for our categorical ease. It is our challenge to describe and understand its various indistinct and often puzzling attributes in order to build a picture of how what we see today came to be and what it's future might look like. The following image attempts to put these structures of our solar system into perspective.
Sedna, a dwarf planet past Pluto, is the most distant known object in our solar system, with a highly elliptical orbit ranging between 76 AU and 937 AU, and is used for reference in the above diagram.
Pluto is an object of the Kuiper Belt. Lowell's "Planet X," and its four moons, along with many other recently discovered bodies of similar mass as Pluto such as Chiron, Eris, and Ceres, constitute a group of at least 50 and perhaps more than 200 dwarf planets orbiting our solar system within the Kuiper belt. Either all of these would have to be classified as planets or Pluto would have to be reclassified, and so these objects are now known as dwarf planets. The new term, "dwarf planet" refers to any body orbiting the Sun that has enough mass to form a spherical shape under its own gravity (called hydrostatic equilibrium) but not enough gravitational force to clear the neighbourhood around its orbit of debris, and is not a satellite (a satellite is any body that orbits a planet or other body more massive than itself; moons are also called satellites and the distinction between the two terms is often unclear). The second criterion, clearing its orbit, might seem to be a bit subjective and it is. A more massive classical planet has enough mass to gravitationally interact with other bodies within its orbit and eventually cause these smaller bodies to accrete with it, be distributed to another orbit or be captured as a satellite (a moon) or into a resonant orbit (an example of resonant orbit is a special 1:1 orbit. A number of asteroids have the same orbital period as Neptune and follow the same orbital path. This helps define Neptune as a planet - none of the dwarf planets has enough mass to attract any other bodies into resonant orbit).
Pluto is tiny, only about one fifth the mass of our moon, and, along with other dwarf planets, is likely made up of about 60% rock and 40% ice. It may have a differentiated core because it has a large enough portion of rock, and therefore radioactive material, to heat the ice enough to allow the ice and rock to separate from each other. The ice mantle may still be warm enough to allow a subsurface liquid ocean up to 180 km thick to encircle the dwarf planet. It may have a thin exosphere of nitrogen, methane and carbon monoxide. This is an artist's impression of what Pluto's (frozen nitrogen) surface might look like, with the Sun and one of its moons, Charon, in the sky:
Credit: ESO/L. Calçada
Pluto circles the Sun only once every 248 Earth Years, in a highly inclined eccentric orbit that places it between 30 AU, as close as Neptune, and 49 AU.
If you traveled out into the most distant reaches of our solar system, you would eventually run into the interstellar medium. Notice that I did not say empty space. Our solar wind travels outward from the Sun in all directions at about 400 km/s (that's 1,440,000 km/h!) until it collides with interstellar wind. Interstellar wind is the outward flow of gas, dust and radiation from all the other stars in the Milky Way neighbourhood. The interstellar medium is all that exists in the space in between stars, such as gases in ionic, atomic and molecular forms (99% of matter), as well as dust (1% of matter) and cosmic radiation (it is comprised of both matter and energy). Interstellar space is not a perfect vacuum, but it is extremely dilute, making it a much better vacuum in any practical sense than we can make in a laboratory. Within the densest regions of interstellar medium, inside molecular gas clouds, stars form. Where it is most dilute, few or no stars exist. The collision between solar wind and the interstellar wind is called termination shock, about 90 AU from the Sun upwind and 200 AU from the Sun downwind. The solar wind at this juncture slows, condenses and grows turbulent. Particles become highly energetic in this region. Both Voyager 1 and 2 have just recently passed termination shock. Past termination shock, the pressures of the solar wind and opposing interstellar wind are in balance and this marks the heliopause and the beginning of interstellar space. You can think of the heliosphere as a gigantic bubble surrounding all the planets inflated by solar wind (which itself is powered by the Sun's vast and powerful magnetic field that protects us from deadly galactic radiation). As the heliosphere with the solar system encapsulated inside it plows through space, a bow shock forms in front of it, just like water bunching up in front of a big rock in a stream. This artist's view depicts several structures associated with our heliosphere.
In the center you can see the Sun and the planetary orbits. The edge of the bubble encasing them is termination shock. Voyagers 1 and 2 have just crossed this boundary. The region outside it is called the heliosheath and it is bound by the heliopause. Beyond this region to the left are orange bunched up molecular gas clouds. This marks the bow shock, where galactic radiation collides with our Sun's heliosphere. Right now our Sun is travelling through the Local Interstellar Cloud, a gas/dust cloud about 30 light years across that flows outward from a star-forming region called the Scorpius-Centaurus Association within the Milky Way.
Voyager 1, launched in 1977, is now about 94 AU from the Sun. Having successfully traversed the Asteroid Belt and flown past Jupiter and Saturn, it took the first ever "family portrait" of our solar system as seen from outside (at about 40 AU from Earth) before it embarked on its current Interstellar Mission. The solar system's family portrait is shown below.
This picture is a mosaic of 60 frames. Six planets are visible in the mosaic and labeled. An image of Earth in this mosaic, famously called "The Pale Blue Dot" by Carl Sagan, was captured from about 6 billion km away. It is the very tiny speck just visible halfway down the brown band to the right within the darkness of space in the image below.
Observing our planet, I leave it to you to feel either uncomfortably inconsequential or, borrowing from the movie, "Contact," rare and precious.
Voyager 1 is now the most distant man-made object in the universe. It is now within the heliosheath and sending back data daily thanks to its long–lived nuclear batteries, which are expected to last until around 2020, at which point it should have entered interstellar space.
The Interstellar Boundary Explorer, A NASA satellite, was launched in 2008 to explore the boundary between the solar system and interstellar space. So far, data has revealed a completely unexpected result: a very narrow very bright ribbon of energetic neutral atoms created by interactions between the solar wind and the galactic wind, which seems to be in continuous flux. These findings will certainly help us refine our concepts about the outer solar system boundary.
A Link Between the Oort Cloud and Extinction Events
The Oort cloud is thought to extend far beyond the heliosphere, from about 5000 AU to about 50,000 AU from the Sun. The generally slowly moving objects that make up this cloud are still weakly bound by the Sun's gravity. However, comets in the outer regions of the Oort cloud are also influenced by galactic tidal forces, and these often complex forces may have a significant effect on comet activity. As many as 90% of all comets originating from the Oort cloud may be the result of perturbations caused by the galactic tide. The galactic tide is a tidal force acting on all objects within the gravitational field of the Milky Way. Passing stars and molecular clouds are also sources of comet perturbations. There may be a link between high comet activity and the location of the Sun within the Milky Way. The Sun orbits the galaxy center, revolving on the outskirts within the Orion spiral arm as shown below.
Computer models show that the Sun bobs up and down through the plane of the Milky Way as it revolves around the galaxy. As we pass through the densest part of the plane, once every 35 to 45 million years, increased gravitational forces from surrounding giant gas and dust clouds tend to dislodge more comets from their paths, increasing the probability that one or more will be sent hurtling toward Earth. Evidence from craters on Earth and the record of past extinction events, the latest of which was the Cretaceous-Tertiary Event 65.5 million years ago, wiping out the dinosaurs, lends support to this theory. Our present position in the galaxy suggests that we are due for another "active" period. Another related but opposing theory suggests that the solar system is barraged by as much as 24 times more interstellar radiation every 60 million years or so, when it bobs up out of the plane of the Milky Way and exposes its "head," putting greater stress on the biosphere and possibly leading to mass extinctions. In this case, while we are on the upswing now, we are about 10 million years off of any significant increase in cosmic ray exposure. It's entirely possible that both of these long-term variations in the solar system environment have and will continue to contribute to our environment on Earth.
A Brief Look Into the Future
I think the traditional view of the solar system is that it is steadfast and ever unchanging and in an effort to challenge this view I may have left you with the impression that it is dangerously unstable and chaotic. Despite its early chaos it is in fact a very stable system. The positions of the planets have settled into resonant orbits that keep them in place quite nicely and are expected to continue to do so for a very long time, until our Sun begins to expand into a Red Giant. The Sun is growing brighter as it evolves, at a rate of increase of 10% every billion years. One billion years from now, Earth will be too hot as a result of this process to sustain liquid water on its surface and it will no longer be considered habitable. By that time, numerous comet and asteroid impacts will have left heir marks on various planets and Earth will have evolved through several long-term and dramatic changes in climate and habitability much as it has undergone in the past. The orbits of the major bodies will have shifted as well. In fact, our extremely precise long-term computer models of orbital rotation in the solar system will be valid for only about 10 million years before small but consistent chaotic changes will be sufficient to throw them off. Any immediate threats to human life from within and from outside our solar system are very remote. I urge you to relax and enjoy our lovely backyard and explore the neighbourhood.