The Sun is a star, a hot ball of glowing gases at the heart of our solar
system. Its influence extends far beyond the orbits of distant Neptune
and Pluto. Without the Sun's intense energy and heat, there would be no
life on Earth. And though it is special to us, there are billions of
stars like our Sun scattered across the Milky Way galaxy.
Names: Sun, Sol /ˈsɒl/, Helios /ˈhiːliəs/
Escape velocity (from the surface): 617.7 km/s; 55 × Earth
Mean radiance (Isol): 2.009×107 W·m−2·sr−1
Equatorial surface gravity:274 m/s2; 28 × Earth
Center density (modeled): 162.2 g/cm3; 12.4 × Earth
Luminosity (Lsol): 3.828×1026 W; ≈ 3.75×1028 lm; ≈ 98 lm/W efficacy
Sun-scorched Mercury
is only slightly larger than Earth's Moon. Like the Moon, Mercury has very
little atmosphere to stop impacts and it is covered with craters. Mercury's
dayside is super heated by the Sun, but at night temperatures drop hundreds
of degrees below freezing. Ice may even exist in craters. Mercury has egg-shaped
orbit.
Orbital period: 88 days
Mass: 3.285 × 10^23 kg (0.055 M⊕)
Radius: 2,439.7 km
Length of day: 58d 15h 30m
Venus is a dim world
of intense heat and volcanic activity. Similar in structure and size to
Earth, Venus' thick, toxic atmosphere traps heat in a runaway 'greenhouse
effect'. The scorched world has temperatures hot enough to melt lead.
Glimpses below the clouds reveal volcanoes and deformed mountains.
Venus spins slowly in the opposite direction of most planets.
Orbital period: 225 days
Length of day: 116d 18h 0m
Surface area: 460.2 million km²
Equatorial rotation velocity: 6.52 km/h (1.81 m/s)
Earth is an ocean planet.
Our home world's abundance of water - and life - makes it unique in our
solar system. Other planets, plus a few moons, have ice, atmospheres, seasons
and even weather, but only on Earth does the whole complicated mix come
together in a way that encourages life - and lots of it.
Radius: 6,371 km
Mass: 5.972 × 10^24 kg
Age: 4.543 billion years
Though details of Mars
surface are difficult to see from Earth, telescope observations show seasonally
changing features and white patches at the poles. For decades, people speculated
that bright and dark areas on Mars were patches of vegetation, that Mars
could be a likely place for life-forms, and that water might exist in the
polar caps. When the Mariner 4 spacecraft flew by Mars in 1965, many were
shocked to see photographs of a bleak, cratered surface. Mars seemed to be
a dead planet. Later missions, however, have shown that Mars is a complex
member of the solar system and holds many mysteries yet to be solved.
Gravity: 3.711 m/s²
Orbital period: 687 days
Surface pressure: 0.636 (0.4–0.87) kPa; 0.00628 atm
The most massive planet in our solar system, with four large moons and many
smaller moons, Jupiter forms a kind of miniature solar system.
Jupiter resembles a star in composition. In fact, if it had been about 80
times more massive, it would have become a star rather than a planet.
Orbital period: 12 years
Surface area: 61.42 billion km²
Mass: 1.898 × 10^27 kg (317.8 M⊕)
Radius: 69,911 km
Saturn was the most distant of the five planets known to the ancients.
Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is
755 times greater than that of Earth. Winds in the upper atmosphere reach
500 meters (1,600 feet) per second in the equatorial region. These super-fast winds,
combined with heat rising from within the planet's interior, cause the yellow
and gold bands visible in the atmosphere.
Orbital period: 29 years
Mass: 5.683 × 10^26 kg (95.16 M⊕)
Radius: 58,232 km
Gravity: 10.44 m/s²
The first planet found with the aid of a telescope, Uranus was discovered
in 1781 by astronomer William Herschel. The seventh planet from the Sun is
so distant that it takes 84 years to complete one orbit.
Orbital period: 84 years
Radius: 25,362 km
Mass: 8.681 × 10^25 kg (14.54 M⊕)
Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun,
Neptune orbits the
Sun once every 165 years. It is invisible to the naked eye because
of its extreme distance from Earth. Interestingly, the unusual elliptical
orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year
period out of every 248 Earth years.
Orbital period: 165 years
Discovered: 23 September 1846
Surface area: 7.618 billion km²