Solar System

Our Sun and everything gravitationally bound to it make up the Solar System. This includes eight major planets—Earth is one of them—along with countless smaller objects. It is a single-star system, not part of a binary or multiple-star arrangement, located within the Milky Way Galaxy. The whole system began roughly 4.6 billion years ago when a dense patch inside a molecular cloud collapsed, forming the Sun at its center and a surrounding disc of material that later coalesced into the orbiting bodies.

Sun

The Sun holds 99.86% of all mass in the Solar System. Deep inside its core, hydrogen atoms fuse into helium, releasing energy that streams out through the photosphere. This outflow creates the heliosphere and a steady drop in temperature as you move away from the Sun.

The next most massive objects are the eight planets, each of which dominates its own orbital zone. Closest to the Sun, in order, are the four terrestrial planets: Mercury, Venus, Earth, and Mars. These make up the inner Solar System.

Outer planets

Only Earth and Mars lie within the Sun’s habitable zone, where sunlight can keep liquid water stable on a surface under normal atmospheric pressure. Beyond about five astronomical units (AU)—the frost line—lie the outer planets: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). Jupiter and Saturn together contain nearly 90% of all non-stellar mass in the system.

Objects that are massive enough to be round but do not clear their orbits are called dwarf planets. The International Astronomical Union’s Minor Planet Center officially lists five: Ceres, Pluto, Eris, Makemake, and Haumea. Four others—Orcus, Quaoar, Gonggong, and Sedna—are also widely recognized as dwarf planets.

Trans-Neptunian region

Smaller still are the countless small Solar System bodies: asteroids, comets, centaurs, meteoroids, and interplanetary dust. Ceres and many of these smaller bodies reside in the asteroid belt between Mars and Jupiter. The other dwarf planets belong to populations of trans-Neptunian objects, found either in the Kuiper belt just beyond Neptune or farther out in the scattered disc.

Many objects do not orbit the Sun directly but instead circle larger bodies as natural satellites, or moons. These range from planetary-mass moons down to tiny moonlets. The two largest moons—Jupiter’s Ganymede and Saturn’s Titan—are bigger than Mercury (though less massive). The seven most massive moons, including Earth’s Moon, are both more massive and larger than any dwarf planet.

Boundary region and uncertainties

Inside the heliosphere, the solar wind—a constant stream of charged plasma—floods the system. Together with interplanetary dust, gas, and cosmic rays, it forms the interplanetary medium that fills the space between bodies. At roughly 70 to 90 AU from the Sun, the solar wind meets the interstellar medium, creating the heliopause—the boundary between the interplanetary medium and interstellar space. Beyond that, the theorized Oort cloud, source of long-period comets, stretches from somewhere past 2,000 AU to the system’s outermost edge.

That edge reaches the limit of the Sun’s Hill sphere, between 178,000 and 227,000 AU (2.81 to 3.59 light-years), where the Sun’s gravitational pull equals that of the Milky Way. The Solar System currently moves through a patch of interstellar medium called the Local Cloud. The nearest star, Proxima Centauri, lies 269,000 AU (4.25 light-years) away. Both the Solar System and Proxima Centauri sit inside the Local Bubble, a relatively small region of the Milky Way about 1,000 light-years across.