Our Solar System is far more than the Sun and eight planets. It also contains hundreds of moons, five officially recognized dwarf planets, countless asteroids and comets, and distant regions that have barely been explored.
From a double sunrise on Mercury to a moon that helps create one of Saturn’s rings, these ten facts reveal just how strange, varied and vast our cosmic neighborhood really is.
For a broader introduction, see The Formation and Structure of Our Solar System.
1. The Sun Contains 99.8% of the Solar System’s Mass—but Is Not Fixed at Its Center
The Sun contains approximately 99.8% of all the mass in the Solar System. Of the small amount left, most is concentrated in Jupiter.
Despite its dominance, the Sun does not remain perfectly stationary at the system’s exact center. Instead, the Sun and planets move around their common center of mass, known as the barycenter. Its position constantly changes as the planets travel around their orbits.
Jupiter exerts the greatest planetary influence because it is by far the most massive planet. Depending on the positions of Jupiter and the other planets, the Solar System’s barycenter can lie inside the Sun or beyond its visible surface. The Sun therefore follows a small, complicated wobble as the entire Solar System travels through the Milky Way.
Learn more about the characteristics and evolution of the Sun.
2. The Solar System Is Only About 20 Galactic Years Old
The Solar System formed approximately 4.6 billion years ago. The Sun and its accompanying planets take roughly 230 million years to complete one orbit around the center of the Milky Way—a period sometimes called a galactic year or cosmic year.
Taken together, these figures mean that the Solar System has completed only about 20 galactic orbits during its entire existence. The earliest dinosaurs were beginning to appear roughly one galactic year ago. Homo sapiens, by comparison, has existed for around 300,000 years—only about 0.13% of a single galactic year.
Estimates of a galactic year vary because the Sun does not follow a perfectly circular orbit and the Milky Way’s structure and mass cannot be measured exactly. The figure of 20 orbits should therefore be understood as an approximate count.
3. Voyager 1 and 2 Are in Interstellar Space—but May Not Have Left the Solar System
Voyager 1 and Voyager 2 have both crossed the heliopause, the boundary where the bubble created by the solar wind gives way to the surrounding interstellar medium. This places both spacecraft in interstellar space.
Even so, they may not have completely left the Solar System. The answer depends on which boundary is used. The heliopause marks the outer edge of the Sun’s heliosphere, but not the outer limit of its gravitational influence.
Far beyond the planets lies the predicted Oort Cloud, an enormous spherical reservoir of icy objects that remain gravitationally bound to the Sun. Voyager 1 could take approximately 300 years to reach its inner edge and perhaps 30,000 years to travel beyond its outer region.
The Voyager probes have therefore left the Sun’s heliosphere. If the Oort Cloud defines the Solar System’s outer boundary, however, they will remain inside the wider Solar System for many thousands of years.
4. On Mercury, the Sun Can Rise, Set and Rise Again
Mercury spins once on its axis every 59 Earth days while completing an orbit around the Sun every 88 Earth days. It also has the most elliptical orbit of any planet in the Solar System.
This unusual combination creates a double sunrise at certain locations on Mercury. The Sun can appear above the horizon, reverse direction and briefly set, before changing direction again and rising for a second time. At other locations, the same effect occurs around sunset.
This happens near Mercury’s closest approach to the Sun, when the planet is moving fastest in its orbit. For a short time, its orbital motion overtakes the apparent movement of the Sun produced by Mercury’s rotation.
Because Mercury rotates while traveling around the Sun, one complete solar day—from one noon to the next—lasts 176 Earth days. That is exactly twice as long as its 88-day year.
5. A Day on Venus Is Longer Than Its Year
Venus takes approximately 225 Earth days to orbit the Sun but about 243 Earth days to rotate once relative to the distant stars. Its rotation period is therefore longer than its year.
Venus also rotates in the opposite direction from most planets. Viewed from above the Sun’s north pole, most planets rotate counterclockwise, while Venus rotates clockwise. This is called retrograde rotation. As a result, the Sun would appear to rise in the west and set in the east beneath Venus’s dense clouds.
A solar day measures how long it takes the Sun to return to the same position in the sky. Because Venus rotates in the opposite direction to its orbit, its rotation and orbital motion work together to shorten this cycle. One solar day on Venus therefore lasts about 117 Earth days, compared with its 243-day rotation period.
Astronomers do not know exactly why Venus rotates backward. Possible explanations include a major early collision and long-term gravitational or atmospheric tidal effects.
See how Venus compares with the eight planets of our Solar System.
6. Jupiter Is More Than Twice as Massive as All the Other Planets Combined
Jupiter is the largest and most massive planet in the Solar System. It is approximately 318 times as massive as Earth and contains more than twice the combined mass of the other seven planets.
Its immense gravity has played an important role in shaping the Solar System. Jupiter can alter the paths of asteroids and comets, sometimes capturing them, redirecting them into new orbits or ejecting them from the Solar System entirely.
Despite its enormous size, Jupiter is nowhere close to becoming a star. It would need to grow to approximately 75–80 times its present mass before its core could sustain ordinary hydrogen fusion.
7. Two Moons Are Larger Than Mercury
Mercury is the smallest planet in the Solar System, but two moons are larger in diameter. Jupiter’s Ganymede measures approximately 5,270 kilometers (3,275 miles) across, while Saturn’s Titan is about 5,150 kilometers (3,200 miles) wide. Mercury’s diameter is approximately 4,880 kilometers (3,030 miles).
Mercury is nevertheless more than twice as massive as either moon because it contains much larger proportions of dense rock and metal. Size and mass are not the same thing.
Ganymede and Titan are also remarkable in other ways. Ganymede is the only moon known to generate its own magnetic field, while Titan possesses a thick, nitrogen-rich atmosphere with greater surface pressure than Earth’s.
8. Enceladus Sprays Its Ocean into Space and Feeds Saturn’s E Ring
Enceladus is a small icy moon of Saturn, measuring only about 500 kilometers (310 miles) across. Yet jets erupt from long fractures near its south pole, spraying water vapor, ice particles and other material into space.
Much of this material escapes the moon and spreads along its orbit, continually replenishing Saturn’s E ring. The E ring is a broad, diffuse ring outside Saturn’s bright main rings and consists mainly of microscopic particles of water ice. Enceladus is considered its principal source.
Measurements by NASA’s Cassini spacecraft indicate that the material comes from a global ocean beneath Enceladus’s icy crust. Cassini also detected salts, organic compounds and evidence of chemical interactions between liquid water and rock.
These discoveries do not demonstrate that life exists on Enceladus, but they show that it possesses several important ingredients associated with a potentially habitable environment.
9. The Entire Main Asteroid Belt Has Only About 3% of the Moon’s Mass
Movies sometimes portray the asteroid belt as a densely packed obstacle course. In reality, its objects are separated by enormous distances, and spacecraft can normally cross the region without having to dodge one asteroid after another.
If every object in the main asteroid belt were combined, the resulting body would contain only about 3% of the Moon’s mass. Ceres alone accounts for roughly one-quarter of the belt’s total mass. It is also the belt’s largest object and the only one classified as a dwarf planet.
Despite another common misconception, the asteroid belt is not the remains of a planet that exploded. Its objects are leftover planet-building materials from the early Solar System. Jupiter’s powerful gravity helped prevent them from combining into a planet and scattered much of the region’s original material.
10. A Comet’s Tails Do Not Simply Trail Behind It
As a comet approaches the Sun, solar heating turns some of its ice directly into gas, releasing dust from its nucleus. Sunlight pushes the dust outward, while the solar wind carries electrically charged particles away from the Sun.
A comet’s tails therefore point generally away from the Sun, not necessarily behind the comet. After a comet passes around the Sun and begins traveling outward, its tails can extend ahead of its direction of motion rather than trailing behind it.
Comets commonly develop two main tails. The dust tail is usually broad and curved, while the ion tail is typically narrower, straighter and more strongly influenced by the solar wind.
A comet’s tail reveals the direction of the Sun—not the direction in which the comet is traveling.
A Solar System Full of Surprises
The same fundamental processes operate throughout the Solar System, yet produce a remarkable variety of outcomes. Gravity, sunlight, rotation, orbital motion and the solar wind shape everything from planetary days to icy moons and comet tails.
Even so, many of the Solar System’s smaller worlds and most distant regions remain only partly explored. Future missions will almost certainly reveal that our cosmic neighborhood is even stranger and more varied than we currently understand.