WHY EARTH HAS LIFE
Earth is one special planet. It has liquid water, plate tectonics, and an atmosphere that shelters it from the worst of the sun’s rays. But many scientists agree our planet’s most special feature might just be us.
It’s the only planet we know of that has life. Though other bodies in our solar system, such as Saturn’s moon Titan, seem like they could have once been hospitable to some form of life, and scientists still have hope of eventually digging up microbes beneath the surface of Mars, Earth is still the only world known to support life. Having the right ingredients coalesce in just the right zone around a calm, warm star seems to be crucial for creating a life-sustaining world.
The closest star system to our own made headlines with the announcement that it hosts a planet about the mass of Earth — a tantalizing discovery so close, astronomically speaking, to us. While the newfound planet may be Earth-sized, researchers say it is almost certainly barren. Astronomers detected the alien world around the sunlike star Alpha Centauri B, which is a member of a three-star system only 4.3 light-years away from our solar system. This planet, known as Alpha Centauri Bb, is about as massive as Earth, but its hot surface may be covered with molten rock — its orbit takes it about 25 times closer to its star than Earth is from the sun.
None of this is a revelation, but understanding what’s special about Earth is crucial for finding other planets out there and predicting what they might be like.
The fact that Earth hosts not just life, but intelligent life, makes it doubly unique. From our anthropocentric viewpoint, we naturally separate ourselves from the planet that we live on, but if one adopts the point of view of an external observer, it is the ‘planet’ (taken as a whole) that has done these remarkable things.
Earth has not been chosen. Our existence is the result of countless random occurrences that combined to make what we are today. Though Earth has the necessary ingredients for life, it’s unclear whether the development of life here might have been a one-time fluke, or if it’s something that happens pretty much everywhere the conditions are right. Earth is unbelievably well equipped as a planet to support and nurture life. The product of some 4.6 billion years of cosmic construction, our planet is flush with life thanks to a fortuitous set of conditions, from the optimal chemical makeup of our planetary core to our safe distance from the hidden black hole at the heart of our galaxy.
After 11,700 years of relative stability, humans are loading the environment with carbon, rapidly tipping Earth into a new climactic age.
So what makes a world such as ours able to host life? Why is Earth so special?
There are a few key ingredients that scientists often agree are needed for life to exist — but much debate remains as to what limits there actually might be on life. Even Earth hosts some strange creatures that live in extreme environments.
1. Water
First, you’d need some kind of liquid, any place where molecules can go react. In such a soup, the ingredients for life as we know it, such as DNA and proteins, can swim around and interact with each other to carry out the reactions needed for life to happen. The most common contender brought up for this solvent is the one life uses on Earth: water. Water is an excellent solvent, capable of dissolving many substances.
Water contains oxygen. It exists in all three states.
Water, as almost all other substances, contracts when cooled, but in contrast to virtually all other materials (there are very few exceptions, such as rubber and antimony), it contracts when cooled only until it reaches 4° Centigrade, then it amazingly, expands until it freezes (ice is 9% less dense than water and thus floats). If water continued to contract when cooled, it would become heavier and thus sink to the bottom of the ocean. One result of this is that the ocean bottom would be extremely cold—and many fish would die. In time, more and more of the ocean would become ice as more froze on the surface, sank, and accumulated at the bottom.
Thus, for much of the Earth, the ice that forms in seas, oceans, and lakes stays near the surface where the sun and the warm water below melts it in the summer. Ice acts as insulation making water just under it warmer. Water that is warmer than 4°, being heavier, sinks to the bottom and warms the depth of the oceans. This process of surface water warming and sinking to the bottom, plus the Coriolis effect produces ocean currents. These currents, among other things, ensure that most of the ocean stays in a liquid form.
No one knows why Earth has the exact amount of water it does, which is relatively small considering that water molecules outnumber silicate molecules in the galaxy. The Earth is remarkable for its precisely-tuned amount of water, not too much to cover the mountains, and not so little that it’s a dry desert, as are Mars and Venus, our ‘sister’ planets. Having the right amount of water produces relatively stable weather.
Water is unique in that it absorbs large amounts of heat without much alteration in its temperature. Its absorption speed is extremely rapid—about ten times as fast as steel. During the day, the seas rapidly soak up a great deal of heat, thus the Earth stays fairly cool. At night, the oceans release the vast amounts of heat that they soaked up during the day, which combined with atmospheric effects, keeps the surface from getting too cold at night. If it were not for the tremendous amount of water on the Earth, there would be far greater day and night temperature variations. Many parts of the surface would be hot enough to boil water in the day and the same part would be cold enough to freeze water at night. Water is an excellent temperature stabilizer. The large oceans on Earth are a vital part of our survival.
Of course, alien life may not play by the rules we’re used to on Earth.
Astrobiologists increasingly suggest looking beyond conventional habitable zones. For instance, while liquid water might not currently persist on the surface of Mars or Venus, there may have been a time when it did. Life might have evolved on their surfaces in that time, and then either fled to safer locales on those planets, such as underground, or adapted to the environment when it became harsh, much as so-called extremophile organisms have on Earth, or both.
2. Climate
Astronomers looking for extraterrestrial life most often focus on planets in the so-called habitable zones of their stars — orbits that are neither too hot nor too cold for liquid water to persist on the surfaces of those worlds. Earth happened to hit the Goldilocks mark, forming within the sun’s habitable zone. Mars and Venus lie outside it; if Earth’s orbit had been just a bit further inside or outside of where it is, life may likely never have arisen and the planet would be a cold desert-like Mars or a cloudy furnace-like Venus. Earth’s water is also special in that it has remained liquid for so long.
The extremely fine line between an environment where life can and cannot exist is illustrated by the fact that it is estimated that a one-degree temperature change in the average worldwide temperature would, in time, seriously affect life on the Earth, and a two-degree temperature change could be disastrous to life. The tolerances are extremely small, and if there are any other planets in the universe, it is unlikely that any of them could have life, due to the extremely rigid conditions necessary for life to exist. The right temperature to grow plants is necessary for photosynthesis, producing oxygen, and using carbon dioxide.
The Earth’s warmth spreads out from the equator by winds and ocean currents thus distributing the warmth around the globe. Weather is the result of two opposing forces, one that tends to even out the temperature and one that tends to spin it around in one direction.
It is important that the temperature does not go from one extreme to the other. Mercury can be anything from -200 C to +375 C when water would be only gas and the planet would be completely dry. Venus has a surface temperature of +480 C. Venus has a molten core and a robust atmosphere, but it’s likely too near the sun, and it lacks plate tectonics, crucial for regulating climate. Mars, although it can reach +25 C, it can be as cold as -140 C.
Our day length is important for photosynthesis, taking just 24 hours and each side receives light regularly. Venus takes 243 days to spin once on its axis
If evolution works to evolve life to fit the existing environments, why has it not equally conquered all of the environments here and elsewhere? Earth is far better suited for life than any other planet, yet most of the environments even here, are either too hot or too cold, too far underground or too far above ground to support much life. In the several thousands of miles of changing environments from the centre of the Earth to the edge of its atmosphere, there are only a few feet of habitable environment, and therefore almost all creatures are forced to live there. Although only the Earth is inhabited in our solar system, even on the Earth only a thin slice is ideally suited for life.
This thin section, though, is teeming with life. It is estimated that an acre of typical farm soil, six inches deep, has several tons of living bacteria, almost a ton of fungi, two hundred pounds of one-cell protozoan animals, about one hundred pounds of yeast and the same amount of algae.
Some type of life is found in every niche on the Earth. From the top of the atmosphere to the bottom of the oceans, from the coldest part of the poles to the warmest part of the equator, life persists here.
If the Earth travelled much faster in its 292-million-mile-long orbit around the sun, centrifugal force would pull it away from the sun, and if it were too far, all life would cease to exist. If it travelled slightly slower, the Earth would move closer to the sun, and if it moved too close, all life would likewise perish. The Earth’s 365-day, 5-hour, 48-minute and 45.51-second round trip is accurate to a thousandth of a second! If the yearly average temperature on Earth rose or fell only a few degrees, most life on it would soon roast or freeze. This change would upset the water-ice and other balances, with disastrous results. If it rotated on its axis slower, all life would die in time, either by freezing at night because of lack of heat from the sun, or by burning during the day from too much sun.
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3. Atmosphere – the miracle of air. It keeps us warm at night, and the barrier of ozone stops UV light. It is thick enough to produce enough atmospheric pressure to keep the water liquid. It ensures that most small objects (asteroids) burn up.
Ancient plant-like organisms in the oceans added oxygen to the atmosphere. Earth has a breathable atmosphere that contains the right amount of oxygen and carbon dioxide. Carbon dioxide makes up most of the atmosphere of Venus and Mars making them unable to support life. All planets but Mercury have an atmosphere. As the atmosphere is kept on the planet by gravity, Mars is too small to keep an atmosphere – Venus has no atmosphere, and Mars is very thin.
The greenhouse gases – water vapour, carbon dioxide, methane, and nitrous dioxide allow the sun to come through but prevent some of its heat from escaping into space. Air-born protocols that block out solar radiation are essential to cloud formation and have a major effect on climate. Humans warm the earth with greenhouse gases but also chill areas where they concentrate.
Without an atmosphere, the Earth’s average temperature would be 30º C colder.
Ozone: These early plants also created the high-altitude layer of ozone that shielded early land species from lethal radiation. Since then, the atmosphere has been thick enough, and with the help of ozone, to prevent ionizing radiation from penetrating.
If our atmosphere were thinner, many of the millions of meteors that now are burned up would reach the Earth’s surface, causing death, destruction, and fires everywhere.
On the land, the opposite of what happens in water occurs. Air, after it is warmed, rises—and the air close to the surface of the Earth is heated via light energy from the sun. The air near the surface then rises upward. The result is that the air near the Earth’s surface maintains a temperature in which life can exist. If air acted the same way that water did, the temperature on the Earth’s surface would be unbearable—and life could not survive for very long. The temperature a few hundred feet above the surface, on the other hand, would be quite cold and, likewise, life could also not exist there. The only habitable region would be a thin slice of air, but even here life could not exist for long. Plants and trees, necessary to support life in the atmosphere, could not survive as they would be in the cold zone. Thus birds would have no resting place, food, water or oxygen. But air rises when heated and thus life can exist on the Earth.
The movement of warm air from the surface rising upward creates air currents (wind) which are an important part of the Earth’s ecological system. They carry away carbon dioxide from areas which overproduce, such as cities, and move oxygen to areas in need of it, such as large urban population centres.
The mixture of gases usually found in the atmosphere, without man’s pollution, is perfect for life. If it were much different (more oxygen, less carbon dioxide, etc., or the atmospheric pressure was much lighter or heavier), life would cease to exist on Earth.
Mercury is too small to hold onto a protective atmosphere and too near the sun for liquid water to persist on its surface.
4. The Sun and energy
Life needs energy. Without energy, virtually nothing would happen. The sun has 99.8% of the solar system’s mass. Energy is produced in the 15 million degrees core by nuclear fusion that converts hydrogen into helium. Its visible surface is 5,500 C. The dark sunspots are cooler regions where the magnetic field on the inside prevents hot gases from escaping from below. The outer atmosphere is the corona that flows outward as the Solar wind is composed of electrically charged particles.
The most obvious source of energy is a planet or moon’s host star, as is the case on Earth, where sunlight drives photosynthesis in plants. The nutrients created by photosynthesis in turn are what the bulk of life on Earth directly or indirectly relies on for fuel.
It is important how little variation there is in our sun’s radiation compared with more volatile stars. As stars go, our sun is relatively weak and cool. Compared to every other star that is visible, the Earth’s sun is the weakest.
Of all the energy the sun gives off, only one billionth of its daily output is picked up by the Earth. The sun does provide the Earth with more than 130 trillion horsepower each day, about fifty thousand horsepower for each current resident. Even though there are likely several hundred billion galaxies in the universe, there is only one atom for every 88 gallons of space, which means most of the universe (the vast majority, actually) is empty space!
If the Earth was not tilted 23° on its axis but was at a 90° angle in reference to the sun, we would not have four seasons. Without seasons, life would soon not be able to exist here—the poles would lie in eternal twilight, and water vapour from the oceans would be carried by the wind towards both the north and south, and would freeze when close enough to the poles. In time, huge continents of snow and ice would pile up in the polar regions, leaving most of the Earth a dry desert. Eventually, the oceans would disappear and rainfall would cease. The accumulated weight of ice at the poles would cause the equator to bulge and, as a result, the rotation of the Earth would drastically change
Still, countless organisms on Earth subsist on other sources of energy as well, such as the chemicals from deep water vents. There may be no shortage of energy sources for life to live off.
Scientists have argued that habitable worlds need stars that can live at least several billion years, long enough for life to evolve, as was the case on Earth.
Some stars only live a few million years before dying. Still, life might originate very fast, so age is not that important. For instance, the Earth is about 4.6 billion years old. The oldest known organism first appeared on Earth about 3.5 billion years ago, meaning that life might conceivably evolve in 1.1 billion years or less. However, more complex forms of life did take longer to evolve — the first multicellular animals did not appear on Earth until about 600 million years ago. Because our sun is so long-lived, comparatively, higher orders of life, including humans, had time to evolve.
The sun is a stable, long-lasting star. A supergiant can explode producing a Supernovae. Its core collapses to form a dense neutron star or even a black hole.
Red Dwarf stars. Are very cool so any planet would have to orbit very close to have liquid water. But getting too close causes the planet to be tidally locked with its star. One side will always face the star with constant daytime and the dark side is not good for life. Our solar system has a Goldilocks zone 150 million km out, a red drawf 9 million km out and a blue supergiant 45 billion km out. The more massive a star is, the faster it consumes its hydrogen. Stars more massive than the sun burn hotter and usually don’t live long enough for planets to develop life. Less massive, younger stars are often unstable and are prone to blasting their planets with bursts of radiation.
Our sun offers protection from galactic debris. The sun engulfs its planets in a bubble of charged particles that repel dangerous radiation and harmful materials coming from interstellar space.
5. Plate tectonics recycle water and life-friendly carbon
Researchers have suggested that plate tectonics is vital for a world to host life — that is, a planet whose shell is broken up into plates that constantly move around.
Plate tectonics as essential in recycling the molecules life needs. For instance, carbon dioxide helps trap heat from the sun to keep Earth warm. Chemical processes that dissolve minerals in rocks draw carbon out of the atmosphere and eventually incorporates it into Earth’s crust. The carbon gets compacted in the crust over millions of years and eventually dives toward Earth’s centre in the zones where tectonic plates collide. As the crust dips into the hot mantle below, it reaches a melting point, and rises to the surface through volcanoes, sending carbon back into the atmosphere. The fact that Earth has plate tectonics allows for the carbon-silicate cycle to operate over geological timescales. With the carbon-silicate cycle, the levels of carbon in the atmosphere get regulated to keep the surface temperature around that of liquid water.
The slip-sliding movements of Earth’s crust have created the planet’s towering mountain ranges and plummeting ocean depths.
Plate tectonics is useful but probably not imperative. Volcanism might very well provide enough fresh supplies of whatever life might need.
Plate tectonics and water are inextricably linked. Not only does plate tectonics enable liquid water to exist by way of regulating the temperature, but many scientists have argued water enables plate tectonics to happen.
Without water, the planet would be geologically dead. Water is what lubricates plate tectonics, which is what leads to the extreme difference between continents and sea floors, the large number of earthquakes and volcanoes, fresh mountain-building. Venus has a molten core and a robust atmosphere, but no water, no plate tectonics, no deep sea floor, no steep mountains, no continents, and probably few earthquakes or volcanoes. A much less geologically interesting place! Venus is likely too near the sun, and it lacks plate tectonics, crucial for regulating climate.
The dramatic shifts of plate tectonics formed many different surface habitats and terrains. This spurred adaption helping life diversify and survive several mass extinctions.
At first, our planet was an incandescent mass, its elements at the mercy of gravity’s tug and pull. The heavier elements sank to Earth’s center, forming a metallic core. Then a long cooling process produced two key ingredients for life: the Earth’s crust, and water vapour that condensed and fell—the first rain. Meteorites and asteroids bombarded the planet, and constant earthquakes and eruptions released vast amounts of magma and gas from fissures and volcanoes.
At some point, the planet formed distinct tectonic plates that moved and ground together, forcing some of the rock back into the interior. Volcanoes, most of which formed near the edges of tectonic plates, provided an ongoing outlet for the planet’s internal heat. Fortunately for us, Earth’s interior continues to generate heat from the radioactive decay of uranium and other elements left over from the formation of the planet. Along with the sun, this process keeps the planet at a comfortable temperature for life.
6. Size of Earth
Another “just-right” aspect of Earth is its size: If it was much smaller, it wouldn’t be able to hold on to our precious atmosphere, but much larger and it might be a gas giant too hot for life. If the earth were larger than 5 times its present size, the weight of the atmosphere would crush us.
Large planets made mostly of gas like Jupiter, have crushing atmospheres swirling with powerful storms.
Mars is about half the size of Earth and a tenth of its mass. With a patchy magnetic field and weaker gravity, it holds onto just a thin atmosphere and little to no liquid water on its surface.
7. Presence of Jupiter and Saturn
The presence of our big brother planet, Jupiter, farther out in the solar system blocking Earth from much of the incoming debris, has also helped Earth become a safe haven for life. Jupiter acts like a giant broom, sweeping the solar system of debris – rocks as small as cars and as huge as moons that could snuff out life in one fatal blow. This protective effect was particularly helpful in the solar system’s early years when Earth still got pummelled but, scientists say, not nearly as bad as would have been the case without Jupiter.
Even though they have been protective, the Earth is situated safely far enough away from the gas giants so that their powerful gravity does not cause a disastrous fluctuation in Earth’s distance from the sun.
8. A friendly moon
Life on Earth may also owe a debt to our nearest celestial neighbour, the moon. The moon is just the right size and distance from Earth. Our moon is the result of a collision 4.5 billion years ago between our planet and a planetoid the size of Mars. The earth was totally destroyed and that’s how we ended up with the moon and a big iron core at the centre of the earth. The moon both rotates and orbits in one month so the dark side is never seen.
The tilt of the Earth at 23.5 degrees is caused by the moon. The tilt causes the four seasons. If the tilt increases, then the seasons and weather will be more extreme. If there was no tilt, then no seasons and a limit on biodiversity.
The Earth teeters as it spins. This tiny wobble shifts the climate from hot to icy every 41,000 years – this might vary more without the moon’s stabilizing pull. This ultimately prevents drastic movements of the poles that could cause massive changes in climate that some scientists think could have doomed any chance for budding life to form or evolve.
The pull of the moon helped slow young Earth’s rotation rate, giving us roughly 24-hour days and the ebb and flow of tides, which scientists suggest might have been the perfect place for early life to begin evolving to survive on land.
9. A magnetic field. The interstellar cloud of gas and dust that gave rise to Earth contained enough radioactive elements to power a churning core for billions of years.
The Earth’s Core: Core. Solid nickel/iron at 5500 C. Inner Core. Liquid nickel/iron at 2500 C. Outer Core. 3000 km thick. Currents in the molten outer core generate a magnetic field that extends far beyond the atmosphere and acts as a protective shield. The iron core is big enough to produce this magnetic field strong enough to protect us from the sun’s radiation and solar storms of charged particles. The solar wind and violent bursts of radiation would rip off our atmosphere and could have scoured life from Earth in its early, fragile stages. The magnetic field continues to deflect the bulk of our star’s damaging radiation and solar outbursts. Mantle. 2900 km thick. Crust. 30 km thick. The deepest ocean is only 5 km deep.
Even though Venus has a molten core, it does not have plate tectonics and has only a weak magnetic field.
10. A Safe Location in our Galaxy.
The Milky Way is a spiral galaxy 100,000 light years in diameter. with gracefully curving arms and a bright, central bar of stars passing through its core. The Solar System is halfway out in the galactic disc.
To sustain life, planets embedded within the galaxy must avoid catastrophic threats such as close supernovae, gamma-ray bursts, and active black holes. They also can’t be crowded in star clusters that would jostle them around too much. Luckily Earth is in an ideal place for its inhabitants to thrive.
The Milky Way’s arms are filled with hazards to habitability including radioactive clouds, areas of active star formation, and sterilizing blasts from dying stars.
Galaxy Halo. Loose and some 150 dense stellar clusters orbit within the Milky Way’s halo. Life-sustaining planets are unlikely here because heavy elements are too sparse to build Earth-like worlds. Small, rocky planets like ours can’t form without elements heavier than hydrogen and helium which become less common at the far edges of the galaxy.
Chaotic Core: A hidden black hole four million times the mass of the sun makes the galaxy’s heart a dangerous place, with intense bursts of radiation hostile to life. A 10,000 light-year-wide bulge of dust, gas, and old stars surrounds the core. It is doubtful that this area could support life.
Our galactic path steers us clear of hazards. The solar system is comfortably nestled in a safe harbour between major spiral arms, and its nearly circular orbit helps it avoid the galaxy’s perilous inner regions. There are relatively few stars near the sun, reducing risks to Earth from gravitational tugs, gamma-ray bursts, or collapsing stars called supernovae.
11. Rare Earth
All of these features make Earth special among known planets near and far.
You hear all the time how Earth-like Mars is, but if you were taken to Mars you wouldn’t feel happy there at all. It’s not Earth-like. And Titan, when the [Huygens] probe landed, there was all this stuff in the media about how Earth-like it is. Earth-like? It is completely different. It has all this methane on the surface. Venus has about the same mass [as Earth], almost the same distance from the sun. But it’s a totally different place – no oceans, no plate tectonics and it’s not a place you would want to be.
So far, we haven’t seen any planet outside the solar system come very close to Earth either. Of the nearly 300 new worlds glimpsed elsewhere in the galaxy, most are “hot Jupiters” – large planets that orbit close to their stars, on which life and liquid water are unlikely to exist.
It is doubtful that in our galaxy typical stars have planets just like Earth around them. There are lots of planets in the galaxy that are somewhat similar to Earth, but the idea that this is a typical planet is nonsensical.
Conclusion
The extremely fine line between an environment where life can and cannot exist is illustrated by the fact that it is estimated that a one-degree temperature change in the average worldwide temperature would, in time, seriously affect life on the Earth, and a two-degree temperature change could be disastrous to life. The tolerances are extremely small, and if there are any other planets in the universe, it is unlikely that any of them could have life, due to the extremely rigid conditions necessary for life to exist.
As our planet-hunting technology improves, many planet hunters expect to find Earth’s twin. The search has led scientists to debate whether Earth is really as special as we think it is. There are two points of view.
The chances of a planet being just the right size, the proper distance away from the right star, etc., are extremely minute, even if many stars have planets circling them, as some speculate. The mathematical odds that all of these and other essential conditions happened by chance are astronomical—something like billions to one!
The optimists believe otherwise. In the past 10 years, everything has been pointing in the direction of the solar system, which we thought was unique, is not unique at all. Many scientists think it’s likely that some form of life exists on some of those countless other planets out there. It is almost certain that life is actually quite common. There may be literally billions of them in the galaxy.
PLANETS
Mercury. Has no atmosphere so has had many meteorites producing many craters (the largest one is 100 km across and resulted in most of the original crust being stripped away). Temperature ranges from 180 C to 430 C. Its orbit takes 88 days and it rotates once every 59 days and 176 Earth days elapse between sunrises.
Venus. Almost as big as Earth. The surface is hidden by sulfuric acid clouds in a dense CO2 atmosphere that produces a runaway Greenhouse Effect and a surface temperature of 460 C day and night as one side never sees the sun. It rotates backward and takes longer to rotate (243 days) than to orbit the sun (226 days). The surface is vast plains of solidified lava and large impact craters and volcanoes.
Mars. Half the size of Earth. Lifeless, cold and dry with an iron-rich soil. There is evidence of past water flow when it was warmer and wetter billions of years ago. The Olympus Mos is three times the height of Everest and 400 km in diameter. The Valles Mariner is a canyon 4000 km long. It has two moons: Phobos 22.2 km diameter and Eimos 12.4 km diameter. It has 635,000 impact craters larger than 1 km in diameter.
Asteroid Belt. A remnant from an earlier Solar system. Its combined mass is only 4% of the total mass of the moon. Ceres is the largest at 1000 km in diameter and is round because of its size.
Jupiter. Diameter 11 times Earths. Mainly made up of hydrogen and helium. The great sunspot is a constant storm.
Saturn. Its density is so low, it would float in an ocean large enough to hold it. Nine times further away from the Sun than Earth. Its rings are ice particles and dust.
Uranus. Featureless with 900 km/hour winds. Its small rocky core is surrounded by a massive icy layer and hydrogen/helium outer layer. It orbits on its side.
Neptune. Five times Earth’s diameter and orbit 30 times further out than Earth. Internally like Uranus. Methane gives it its blue colour.
Pluto. Dwarf planet.
Kuiper Belt. Scattered dust.
Oort Cloud. Contains trillions of comets.