Planets are among the most diverse of all space objects and phenomena. Each one is unique and has a variety of characteristics that define it. Some are larger and made of gas, some are smaller and extremely hot. Some are ice cold. And some are capable of hosting beautiful life.

Scientists are discovering new planets around other stars in our galaxy. With a nearly infinite number of stars in our universe, the possibilities of what might be found on the planets around them are limited only by the imagination.

Facts about Planets

  • Planets form during the solar system formation process.
  • Planets composed of more heavier elements tend to form closer to their parent star, and planets composed of lighter elements like hydrogen and helium form away from the star.
  • There are eight planets in our solar system. The largest is Jupiter and the smallest is Mercury.
  • Planets can have as little as no moons, or as many as 70 or more moons.
  • Astronomers have discovered over 3,700 exoplanets orbiting around stars in other solar systems!

What are planets?

Planets are large bodies in space that orbit around a star or pair of stars, and are composed of a variety of elements. Planets closer to the star are mostly composed of heavier elements such as iron, oxygen and nitrogen. Planets further from the star are mostly composed of lighter elements, such as hydrogen and helium.

Orbits and Rotation

Orbit and Rotation

The following are two key terms that are often used when discussing the formation and characteristics of planets.

What is an orbit?

An orbit occurs when an object remains in constant freefall around a second object in space, due to gravity. The path of the orbit is slightly elliptical to circular.

An orbital period is the amount of time it takes for the first object to complete one full trip around the second object.

What is rotation?

Rotation, in space terminology, is the term for an object in space spinning on its own axis.

A rotational period is the amount of time it takes for an object in space to turn a complete 360 degrees on its axis.

How are planets formed?

Protoplanetary Disk

The science of planet formation is theory. However, a large amount of evidence points to a number of events that must take place in order for planets to form.

  1. Dust, gas and matter gather in a Nebula. This matter may exist from a past supernova or other cosmic event. This region is called a nebula.
  2. Matter becomes denser in areas. Gravity causes matter in less dense regions to accumulate around areas of denser matter within the nebula. These areas then become even more dense and attract even more material.
  3. The region within the nebula collapses. This denser part of the nebula begins to collapse due to gravity and the enormous amounts of dust and matter that have accumulated in the region. The majority of the dust and gas collects towards the center of a new, smaller sphere of dust.
  4. A protostar forms. The temperature and density at the center of the local collapse increases substantially. A protostar is a new star that hasn’t yet begun to burn hydrogen. A number of stars often begin collapsing in a relatively close region. These regions are referred to as star nurseries.
  5. A protoplanetary disk forms. As the core collapses into a protostar, angular momentum causes the surrounding material to flatten into a disk. Because of the momentum, the flattening disk begins to ‘spin’ around the new protostar. Heavier elements within the disk move towards the center and lighter elements tend to remain towards the outside of the disk.
  6. Planetesimals begin to form. Large amounts of dust may begin to slow the rotation of the disk. Dust particles collide with other dust and rock and begin to accrete into small planetesimals. Over thousands of years, these planetesimals continue to accumulate dust and gas. As their mass grows, their gravitational pulls increase immensely.
  7. Excess matter goes away. The dust and gas within the flattened disk that does not merge into the star or the planetesimals is likely blown away by the solar wind.
  8. A solar system and planets form. As the disk of matter dissipates, the planetesimals become planets orbiting the protostar. Once the protostar begins to burn hydrogen, it moves on to the main sequence of stars. A new solar system is formed.

What are the planets in our solar system?

Our solar system formed as eight planets orbiting around the Sun, which is the scientific name of the star in our solar system. These eight planets include the following.

  • Mercury.

    Image Credit: NASA

    Mercury is the smallest planet in our solar system. It is very close to the Sun. Because of its proximity to the Sun, it gets extremely hot in the daytime, up to an astounding 800° Fahrenheit on the daytime side of the planet! On the nighttime side, it can get as low as -290° Fahrenheit.

    Mercury also has very little atmosphere. The Sun’s solar wind blows most of the atmosphere off the tiny planet. What remains is made of small amounts of hydrogen, helium, oxygen and solar wind particles.

    Mercury rotates at a very slow rate with respect to the other planets. It completes a rotational period once every 56.6 Earth days. Its orbital period, however, is very quick. It travels around the Sun once every 88 Earth days.

    Mercury is one of the two planets in our solar system with no moons.

  • Venus.

    Image Credit: NASA

    Venus is the 6th largest planet, just behind Earth at about .8 times the size of Earth.

  • Earth

    Image Credit: NASA

    Earth is the 5th largest planet, and also our home planet. Earth is extraordinarily special – it lies in a special zone around the Sun called the habitable zone. In this zone, the temperature is just right such that our planet can support life.

    On Earth, we also have a wide variety of elements, including carbon, hydrogen, oxygen and nitrogen. These elements are critical to supporting the vast and amazing variety of life found here.

    Earth orbits the Sun once every 365 Earth days. It rotates approximately once every 24 hours.

    Earth has one moon, which may have formed during a collision with another stellar object. The Earth’s moon is the reason for the ocean tides on Earth. Ancient and some modern societies based their calendars and growing seasons on the moon’s movement around the Earth.

  • Mars.

    Image Credit: NASA

    Mars is the second smallest of the planets, next to Mercury. Of the eight planets, it is most like Earth, and may have even supported life at one time. It is known as the “Red Planet”, due to the high amounts of iron rusting on its surface.On July 4, 1997, NASA landed the first of 4 rovers that have set down on the planet, called Sojourner. This and the other rovers have taken beautiful imagery and taken data samples of Mars surface. The next rover is scheduled to land sometime around 2021.

    Mars orbits the Sun once every 1.88 Earth years. It rotates once every 24.67 hours.

    Mars has two moons – Phobos and Deimos. Scientists believe that Phobos will crash into Mars several million years from now, because of its proximity to Mars.

  • Jupiter.

    Image Credit: NASA

    Jupiter is the largest of the eight planets, and the first of the “gas giants”. It is approximately 318 times larger than the Earth. Jupiter is often recognized by its “giant red spot”, a large storm in Jupiter’s atmosphere that doesn’t go away. Jupiter is so large, that this storm by itself can fit three Earths inside of it!

    Jupiter orbits, or travels around the Sun once every 11.9 Earth years.

    It rotates very rapidly, completing a rotation once every 9.9 Earth hours. It rotates the fastest of all the planets!

    Jupiter has at least 67 moons! And 10 new moons have been discovered which would put the total at 77. The largest four of the moons are well known. They are Io, Europa, Ganymede and Callisto. Io is extremely volcanic, and has provided a substantial amount of information to scientists about volcanoes on other planets.

  • Saturn.

    Image Credit: NASA

    Saturn is the second of the gas giants, and the second largest planet in the solar system at approximately 95 times the mass of Earth.

    It is distinguishable from the rest of the planets by its beautiful ring system. These rings are easily seen with most telescopes. At 250,000 km wide, Saturn’s ring system stretches almost 20 times the width of the Earth!

    Saturn orbits the Sun once every 29 Earth Years. Like Jupiter, it also rotates very quickly and makes a full rotation once every 10 hours and 42 minutes.

    Saturn has at least 62 moons. The two most well known of these are Titan and Enceladus. Scientists belive that Enceladus may have the ability to support extremophiles, which are small organisms that can survive in extreme environments.

  • Uranus.

    Image Credit: NASA

    Uranus is the fourth largest planet and gas giant. It is approximately 14.5 times the size of Earth.  Uranus’ most unusual characteristic is that it is flipped on its side with respect to the orbital plane that the planets rotate the Sun on. Like Saturn, it also has a ring system, but its rings are closer to perpendicular with the orbital plane.

    Uranus orbits the Sun once every 84 Earth years, and rotates once every 17 hours and 14 minutes.

    Uranus has at least 27 moons and likely more than that. These include Ariel, Miranda and Umbriel.

  • Neptune.

    Image Credit: NASA

    Neptune is the third largest planet and gas giant, at approximately 17 times the mass of Earth. It is deep blue in color because of the large amounts of methane in its atmosphere, which absorbs red light and reflects blue.

    Neptune orbits the Sun once every 165 Earth years, and rotates once every 16.1 Earth hours.

    Neptune has at least 13 moons. The most well known is Triton. Triton has a significant number of ice volcanoes, also called cryovolcanoes or cryogeysers.

Did you know? For many years astronomers and scientists said that our solar system had nine planets. However, in August of 2006, the ninth ‘planet’ Pluto was demoted to a Kuiper Belt Object, when the International Astronomical Union stated that it did not meet the criteria for a planet.

What are exoplanets?

Exoplanets are simply planets that have formed around stars other than our Sun. They are a relatively new discovery, as the first exoplanet was discovered in 1992. Since then, over 3,700 exoplanets have been discovered orbiting stars in our galaxy, known as the Milky Way.

There are several methods that scientists can use to discover these exoplanets. One such method is called Transit Photometry. With this method, astronomers and scientists will watch for a planet to orbit in front of its star. If the star dims at regular intervals, it is likely that a planet is passing between the star and Earth.