Solar System

The Sun contains 99.8% of all the mass in the Solar System. Everything else — planets, moons, asteroids and comets — fits in the remaining 0.2%. Its light takes about 8 minutes and 20 seconds to reach Earth.

The four planets closest to the Sun — Mercury, Venus, Earth and Mars — are rocky and small. The four outer planets are giants: Jupiter and Saturn, made mainly of gas, and Uranus and Neptune, the ice giants. They all orbit the Sun in the same direction and nearly in the same plane because they formed from the same disk of gas and dust about 4.6 billion years ago.

Jupiter has more than twice the mass of all the other planets combined. With that gravity, it deflects many comets and asteroids: some are ejected from the Solar System and others fall into it. In 1994, fragments of comet Shoemaker–Levy 9 struck Jupiter and left scars larger than Earth.

For that reason Jupiter is often called Earth’s protector. Astronomers note, however, that the same gravity can also push some bodies toward us. It protects and it perturbs, and the net effect is still under study.

Far beyond Pluto, the Oort Cloud is a vast shell of icy comets surrounding the Solar System, at roughly 1,000 to 100,000 times the Earth–Sun distance. No one has seen it directly; it is inferred from the orbits of long-period comets that take thousands of years to return.

The Voyager 1 and Voyager 2 probes, launched in 1977, have already left the bubble of the solar wind and are in interstellar space. On November 18, 2026, Voyager 1 reaches a one light‑day distance from Earth: a signal takes 24 hours to reach it. Even so, it would still take about 300 years to reach the start of the Oort Cloud, and around 30,000 years to cross it.

Five planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Venus is the brightest point in the sky after the Moon — the “morning star” at dawn or the evening “star” after sunset. With a simple telescope you can already see Saturn’s rings and Jupiter’s four largest moons.

The best observing sites are dark, dry places far from city lights. Chile’s Elqui Valley and the Atacama Desert have observatories open to visitors, and in Hawaii you can climb Mauna Kea, home to some of the world’s largest telescopes.

Stargazing depends on the weather: when it is cloudy, observatories may cancel the session, and what you see through the telescope changes with the time of year and the phase of the Moon.