What is planets

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As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.

In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune.

Some scientists are calling this object a planet. Others think that it can only be a planet if it formed around a star. Science is full of arguments like this. What do you think?

Do all planets, even exoplanets, need to form around stars? Learn about a possible planet that may have formed in a completely different way! Check out:. The Lone Planet. What Is a Planet? The Short Answer:. A planet must do three things: it must orbit a star, it must be big enough to have enough gravity to force a spherical shape, and it must be big enough that its gravity cleared away any objects of a similar size near its orbit. 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. Although Mars' atmosphere is too thin for liquid water to exist on the surface for any length of time, remnants of that wetter Mars still exist today. Sheets of water ice the size of California lie beneath Mars' surface, and at both poles are ice caps made in part of frozen water.

In July , scientists revealed that they had found evidence of a liquid lake beneath the surface of the southern pole's ice cap.