The Size of the Universe
Scientists believe the universe is 13.8 billion years old and telescopes allow them to see 13.8 billion light years in every direction. A light-year is how far a beam of light can travel in one year. One light-year is six trillion miles. Scientists also know the universe is constantly expanding, which makes the physical size infinite. A study in 2013 by the European Space Agency estimated it was 92 billion light years. However, another twist is how a light-year itself is measured. Experts have a few ways to determine the speed of light, and one method puts the size at 100 sextillions. Because of this, scientists know how many planets are visible but not the total number in the universe.
The universe is broken into galaxies. A galaxy is a large group of stars. Scientists estimate that there are hundreds of billions of galaxies in the universe. The Earth is located in the Milky Way galaxy, which has about 100 billion planets and at least 1,500 within 50 light years of Earth.
The three types of planets are terrestrial, Jovian and dwarf. Terrestrials include Mercury, Venus, Earth and Mars, which are the inner planets that are the closest to the sun. These have a hard surface and dense molten cores. Terrestrial planets share several topographical characteristics such as craters, valleys and volcanoes. Venus and Mars are roughly the size of Earth while Mercury as the smallest at one-third the size of these other planets.
Also known as the outer planets, the largest of these, Jupiter and Saturn, were first seen in 1610 by Galileo using a telescope. The other Jovian planets include Jupiter, Saturn, Uranus and Neptune. Jovian planets are either gas or ice giants. Unlike the terrestrial planets, these don't have a hard surface. The surface of Jupiter and Saturn is made of hydrogen and helium while that of ice giants Uranus and Neptune is composed of high amounts of methane on the surface and heavy elements on the inside.
Not big enough to be full-fledged planets, dwarfs meet all of the characteristics of a planet except for size. Pluto is the most well-known dwarf classification after scientists downgraded it from being the ninth major planet in the Milky Way. The experts agree a ninth planet exists in the Kuiper Belt, which is located just beyond Neptune. Known dwarfs in the solar system by size include Eris, Pluto, Haumea, Makemake and Ceres. Slightly bigger than Pluto, Eris' discovery in 2003 prompted the reclassification of Pluto. Eris has one moon and is 68 times farther from our sun than the Earth. Pluto, first discovered in 1930, has five moons and is almost 40 times farther from the sun than Earth. Haumea is more oval than round than the other planets, and Makemake was the newest dwarf, classified in 2005. The oldest known dwarf planet is Ceres, which is also the closest to the Earth and our sun.