The solar system also includes the Kuiper Belt that lies past Neptune's orbit. This is a sparsely occupied ring of icy bodies, almost all smaller than the most popular Kuiper Belt Object — dwarf planet Pluto.
Beyond the fringes of the Kuiper Belt is the Oort Cloud. This giant spherical shell surrounds our solar system. It has never been directly observed, but its existence is predicted based on mathematical models and observations of comets that likely originate there.
The Oort Cloud is made of icy pieces of space debris - some bigger than mountains — orbiting our Sun as far as 1. This shell of material is thick, extending from 5, astronomical units to , astronomical units. One astronomical unit or AU is the distance from the Sun to Earth, or about 93 million miles million kilometers. The Oort Cloud is the boundary of the Sun's gravitational influence, where orbiting objects can turn around and return closer to our Sun.
The Sun's heliosphere doesn't extend quite as far. The heliosphere is the bubble created by the solar wind — a stream of electrically charged gas blowing outward from the Sun in all directions.
The boundary where the solar wind is abruptly slowed by pressure from interstellar gases is called the termination shock. This edge occurs between astronomical units.
Voyager 1 went interstellar in and Voyager 2 joined it in But it will be many thousands of years before the two Voyagers exit the Oort Cloud. There are more than known moons in our solar system and several more awaiting confirmation of discovery. Of the eight planets, Mercury and Venus are the only ones with no moons.
In some ways, the swarms of moons around these worlds resemble mini versions of our solar system. Below is a brief overview of the eight true planets in our solar system , moving from that closest to the sun to farthest from the sun:.
Zipping around the sun in only 88 days, Mercury is the closest planet to the sun, and it's also the smallest, only a little bit larger than Earth's moon.
Because its so close to the sun about two-fifths the distance between Earth and the sun , Mercury experiences dramatic changes in its day and night temperatures: Day temperatures can reach a scorching F C , which is hot enough to melt lead. Meanwhile on the night side, temperatures drop to minus F minus C. Mercury has a very thin atmosphere of oxygen, sodium, hydrogen, helium and potassium and can't break-up incoming meteors, so its surface is pockmarked with craters, just like the moon.
Among those findings was the discovery of water ice and frozen organic compounds at Mercury's north pole and that volcanism played a major role in shaping the planet's surface. The second planet from the sun, Venus is Earth's twin in size. Radar images beneath its atmosphere reveal that its surface has various mountains and volcanoes. But beyond that, the two planets couldn't be more different. Because of its thick, toxic atmosphere that's made of sulfuric acid clouds, Venus is an extreme example of the greenhouse effect.
It's scorching-hot, even hotter than Mercury. The average temperature on Venus' surface is F C. At 92 bar, the pressure at the surface would crush and kill you.
And oddly, Venus spins slowly from east to west, the opposite direction of most of the other planets. The Greeks believed Venus was two different objects — one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky, Venus has generated many UFO reports. The third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean.
It's the only world known to harbor life. Earth's atmosphere is rich in nitrogen and oxygen. Earth's surface rotates about its axis at 1, feet per second meters per second — slightly more than 1, mph 1, kph — at the equator. The planet zips around the sun at more than 18 miles per second 29 km per second.
The fourth planet from the sun is Mars, and it's a cold, desert-like place covered in dust. This dust is made of iron oxides, giving the planet its iconic red hue. Mars shares similarities with Earth: It is rocky, has mountains, valleys and canyons, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. Substantial scientific evidence suggests that Mars at one point billions of years ago was a much warmer, wetter world.
Rivers and maybe even oceans existed. It took 40 years for the spectra of the inclusions to be discovered and then extrapolates to very old asteroids still in orbit around the Sun. Additionally, the Universe is thought to have been created about Measuring two long-lived radioactive elements in meteorites, uranium and thorium, has placed the age of the Milky Way at in the same time frame. From these measurements, it appears that large scale structures like galaxies formed relatively quickly after the Big Bang.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. HD b is one of the most eccentric planets ever discovered — the planet is notable for having and extremely weird and extremely elongated orbit. The eccentric orbit of HD b is similar to that of a comets, the planet passes very close to its star before moving very far out from it every days.
Due to its elongated orbit, one side of HD b gets dramatically hotter than the other side when it passes close to its sun. According to NASA, the side facing the sun quickly heats up to more than 2, degrees Fahrenheit 1, degrees Celcius. Scientists believe that HD b is currently in the process of migrating from its elongated orbit to a more regular one.
They believe this is typical of all hot Jupiter planets, which start out with eccentric orbits and eventually settle into a more regular, circular orbit over hundreds of millions of years.
However, this idea was challenged in and scientists now believe that 55 Cancri e is not as carbon-rich as the previous study suggested. The planet is also so close to its star that 55 Cancri e has a permanent day and night side. This makes Kepler the oldest known system of terrestrial-size planets. The five rocky planets in the Kepler system range in size, with the smallest similar to Mercury and the largest to Venus.
All of the planets only take about ten days to orbit their sun-like star, making them all extremely hot. The exoplanet known as PSR B b is the oldest known planet in the universe, with an estimated age of about 13 billion years.
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