By Kevin Beck | Updated Aug 30, 2022
SCIEPRO/Science Photo Library/GettyImages
As of 2018, our solar system is composed of one massive star— the Sun— and eight orbiting planets, five dwarf planets, roughly 150 moons, and a variety of smaller bodies. Earlier classifications listed nine planets, a stance that stood from 1930, when Pluto was first discovered, until 2006, when the International Astronomical Union redefined Pluto as a dwarf planet. This evolution illustrates that astronomy is a dynamic field, constantly updated by new observations from advanced missions such as the Hubble Space Telescope.
While interplanetary travel has long captured the imagination of science‑fiction writers, it is now a tangible goal for humanity. When considering which planet to visit first, the answer is not as straightforward as choosing the one that is physically nearest.
The Sun and everything that orbits it—planets, moons, comets, asteroids, and meteoroids—form the solar system. The eight major planets are divided into four inner, terrestrial worlds and four outer gas giants. From the Sun outward, the sequence is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The dwarf planet Pluto has an orbit that lies well beyond Neptune for most of its trajectory. The asteroid belt, containing over 780,000 rocky bodies, separates the terrestrial planets from the gas giants.
Planetary distances grow dramatically with distance from the Sun. For example, Mars is about 1/20th as far from the Sun as Neptune. Saturn is nearly twice as far from the Sun as Jupiter, and Uranus is almost twice as distant from the Sun as Saturn. Thus, the interval between neighboring planets widens rapidly as one moves outward.
Venus orbits the Sun at an average distance of 67 million miles (107 million kilometers), whereas Earth orbits at about 93 million miles (150 million kilometers). When the two planets are aligned on the same side of the Sun— a configuration that occurs every 584 days—their separation shrinks to roughly 26 million miles (42 million kilometers). On the opposite side of the Sun, the distance expands to about 160 million miles (258 million kilometers). During those times when Venus and Earth are on opposite sides, Mercury, at 33 million miles from the Sun, is actually closer to Earth than Venus is.
Venus shares many dimensions with Earth: its diameter is 95 % of Earth's, its density is 90 %, and its mass is 81 % of Earth's. However, its atmosphere is dominated by carbon dioxide, producing a runaway greenhouse effect that elevates surface temperatures to nearly 900 °F (475 °C). These extreme conditions make Venus a hostile environment for life as we know it.
Mars, the next inner planet after Earth, orbits the Sun at an average of 131 million miles (211 million kilometers). When Earth and Mars are at their closest approach, they can be as near as 36 million miles (58 million kilometers), a proximity last seen in July 2018. Mars has attracted intense scientific interest because it once possessed liquid water and has surface features that hint at the possibility of past microbial life. Current consensus, however, suggests that Mars is no more likely to host life than Venus.
The planet’s harsh climate makes surface landings challenging; most imaging is performed with radar. The Soviet Union’s Venera program first landed a probe on Venus in 1966, becoming the first human‑made object to touch another planet’s surface. Over its course, Venera transmitted valuable data until the program concluded in 1983. The United States’ Mariner series conducted fly‑by missions from 1962 to 1974, but no landings occurred. NASA’s Magellan spacecraft, launched in 1989, used radar to map 98 % of Venus’s surface over five years. In 2006, the European Space Agency’s Venus Express studied the atmosphere and confirmed the presence of an ozone layer, similar to Earth’s.
