Venus

Sirius B is the white dwarf companion of Sirius A, the brightest star in the night sky. Hubble measurements show it packs about the Sun’s mass into a body smaller than Earth, with gravity so strong that a person would weigh tens of millions of pounds on its surface.

Object class

White dwarf star

Mass

~1.02 solar masses

Host

Sirius system

Estimated age

~228 million years

Composition

Electron-degenerate carbon-oxygen matter

Temperature

~25,000 K surface

Ganymede is Jupiter's largest moon and the largest moon in the Solar System, bigger than Mercury. It is also the only known moon with its own magnetic field, and evidence points to a buried saltwater ocean beneath its icy shell.

Object class

Galilean moon

Mass

~0.025 Earth masses

Host

Jupiter

Estimated age

~4.5 billion years

Composition

Water ice, silicate rock, metallic iron core and probable saltwater ocean

Temperature

~90-160 K surface

Titan is Saturn's largest moon and the only moon in the Solar System with a dense atmosphere. Its methane weather cycle feeds clouds, rain, rivers, lakes, and seas, making it one of the most world-like places beyond Earth.

Object class

Large icy moon

Mass

~0.023 Earth masses

Host

Saturn

Estimated age

~4.5 billion years

Composition

Water ice, silicate rock, nitrogen-methane atmosphere

Temperature

~94 K surface