STUDY / Astronomy
Solar system flashcards
Get to know our corner of space, one question at a time.
10 random cards per round · 50 to explore
Choosing your ten cards…
Ready to put it to the test?
Try the same topic in a ten-question scored round.
All 50 study questions
Open a question to check its answer.
Planets, moons, and the Sun
01 / 50Which planet is closest to the Sun?Reveal answerHide answer
Answer
Mercury. Mercury is the innermost planet. Its short path around the Sun takes about 88 Earth days, the shortest year of any planet.
Sources for this answer
02 / 50Which planet is the largest?Reveal answerHide answer
Answer
Jupiter. Jupiter is the largest planet in both size and mass. It is a gas giant, with a diameter about eleven times Earth’s.
Sources for this answer
03 / 50What is the Sun?Reveal answerHide answer
Answer
A star. The Sun is a star, producing energy through nuclear fusion in its core.
Sources for this answer
04 / 50Which planet is known as the Red Planet?Reveal answerHide answer
Answer
Mars. Iron minerals on the surface oxidize, giving Mars its reddish appearance.
Sources for this answer
05 / 50Which planet is famous for its prominent ring system?Reveal answerHide answer
Answer
Saturn. Saturn has bright, extensive rings made mostly of pieces of ice, with rock and dust.
Sources for this answer
06 / 50Which planet is hottest on average at its surface?Reveal answerHide answer
Answer
Venus. Venus has a thick atmosphere that traps heat through an extreme greenhouse effect.
Sources for this answer
08 / 50Which planet rotates nearly on its side?Reveal answerHide answer
Answer
Uranus. Uranus’s axis is tilted about 98 degrees, so the planet spins almost on its side. This gives it especially extreme seasons.
Sources for this answer
09 / 50How many planets are in our solar system?Reveal answerHide answer
Answer
Eight. There are eight planets. Pluto is classified as a dwarf planet.
Sources for this answer
10 / 50Which planet is farthest from the Sun?Reveal answerHide answer
Answer
Neptune. Neptune is the outermost of the eight planets. Pluto lies in the Kuiper Belt and is classified as a dwarf planet.
Sources for this answer
11 / 50Which is the smallest of the eight planets?Reveal answerHide answer
Answer
Mercury. Mercury is the smallest planet; it is only a little wider than the Moon.
Sources for this answer
12 / 50Which planet is closest to Earth in size?Reveal answerHide answer
Answer
Venus. Venus has a diameter only slightly smaller than Earth’s.
Sources for this answer
13 / 50Which two planets have no natural moons?Reveal answerHide answer
Answer
Mercury and Venus. Mercury and Venus have no natural moons. A nearby asteroid that follows a similar path around the Sun is not necessarily a moon orbiting the planet.
Sources for this answer
14 / 50What are the names of Mars’s two moons?Reveal answerHide answer
Answer
Phobos and Deimos. Phobos and Deimos are small, irregularly shaped moons. Phobos is the larger one and orbits closer to Mars.
Sources for this answer
15 / 50Jupiter’s Great Red Spot is what kind of feature?Reveal answerHide answer
Answer
A giant storm. The Great Red Spot is a long-lived storm in Jupiter’s atmosphere.
Sources for this answer
16 / 50Which moon of Jupiter is the largest moon in the solar system?Reveal answerHide answer
Answer
Ganymede. Ganymede is even larger in diameter than Mercury.
Sources for this answer
17 / 50Which moon of Saturn has a thick nitrogen-rich atmosphere?Reveal answerHide answer
Answer
Titan. Titan’s atmosphere is mostly nitrogen. Its surface lakes and seas contain liquid methane and ethane, rather than the liquid water in Earth’s lakes.
Sources for this answer
18 / 50Which large moon of Neptune orbits opposite to the planet’s rotation?Reveal answerHide answer
Answer
Triton. Triton’s orbit is retrograde: it travels opposite to Neptune’s rotation. This unusual motion is evidence that Neptune may have captured it.
Sources for this answer
21 / 50What force keeps planets in orbit around the Sun?Reveal answerHide answer
Answer
Gravity. Gravity continually bends a moving planet’s path toward the Sun. The planet’s forward motion keeps it orbiting instead of falling straight inward.
Sources for this answer
22 / 50What is the name for Earth’s path around the Sun?Reveal answerHide answer
Answer
An orbit. An orbit is the path one body follows around another under gravity.
Sources for this answer
23 / 50About how long does Earth take to orbit the Sun once?Reveal answerHide answer
Answer
One year. One orbit is one year, roughly 365.25 days. The extra fraction of a day is why our calendar needs leap-day adjustments.
Sources for this answer
24 / 50What mainly causes day and night on Earth?Reveal answerHide answer
Answer
Earth rotating on its axis. Earth’s rotation brings different places into sunlight and then darkness.
Sources for this answer
25 / 50What mainly causes Earth’s seasons?Reveal answerHide answer
Answer
Its tilted axis as it orbits the Sun. Earth’s tilt changes the angle and duration of sunlight over the year.
Sources for this answer
26 / 50Why does the Moon appear bright in our night sky?Reveal answerHide answer
Answer
It reflects sunlight. Moonlight is reflected sunlight. The phases change because we see different amounts of the Moon’s sunlit half as it orbits Earth.
Sources for this answer
27 / 50What is between Earth and the Sun during a solar eclipse?Reveal answerHide answer
Answer
The Moon. The Moon passes between Earth and the Sun, blocking some or all of the Sun from a small area on Earth.
Sources for this answer
28 / 50Which body casts its shadow on the Moon during a lunar eclipse?Reveal answerHide answer
Answer
Earth. Earth lies between the Sun and Moon, and the Moon moves into Earth’s shadow. This arrangement occurs only at full Moon.
Sources for this answer
30 / 50What causes the bright streak commonly called a shooting star?Reveal answerHide answer
Answer
A meteoroid entering the atmosphere. The visible atmospheric streak is called a meteor.
Sources for this answer
33 / 50What is the solar wind?Reveal answerHide answer
Answer
A stream of charged particles from the Sun. The Sun continuously releases charged particles into space.
Sources for this answer
34 / 50What is the Sun’s visible surface called?Reveal answerHide answer
Answer
The photosphere. The photosphere is the layer from which most visible sunlight escapes. It is called a surface even though the Sun has no solid ground.
Sources for this answer
35 / 50What is the Sun’s outer atmosphere called?Reveal answerHide answer
Answer
The corona. The faint corona extends far above the Sun’s visible surface.
Sources for this answer
36 / 50Which process powers the Sun’s core?Reveal answerHide answer
Answer
Nuclear fusion. Hydrogen nuclei combine to form helium, releasing energy.
Sources for this answer
37 / 50About how long does sunlight take to reach Earth?Reveal answerHide answer
Answer
Eight minutes. Sunlight takes about eight minutes and twenty seconds to cross the average Sun–Earth distance. The delay changes slightly as Earth moves along its orbit.
Sources for this answer
38 / 50One astronomical unit is approximately which distance?Reveal answerHide answer
Answer
Earth’s average distance from the Sun. One astronomical unit is about 150 million kilometers, approximately the average Sun–Earth distance. It is a convenient scale for planetary orbits.
Sources for this answer
39 / 50Which planet has the shortest year?Reveal answerHide answer
Answer
Mercury. Mercury completes an orbit in about 88 Earth days.
Sources for this answer
40 / 50Which planet has the longest year?Reveal answerHide answer
Answer
Neptune. Neptune needs about 165 Earth years to orbit the Sun.
Sources for this answer
41 / 50Which planet completes a rotation in roughly ten hours?Reveal answerHide answer
Answer
Jupiter. Jupiter spins once in about 9.9 hours, giving it the shortest rotation period of the eight planets. A rotation is distinct from an orbit.
Sources for this answer
42 / 50Which pair are classified as ice giants?Reveal answerHide answer
Answer
Uranus and Neptune. Uranus and Neptune are ice giants. The name refers to their water-, ammonia-, and methane-rich interiors; it does not mean they are solid balls of ice.
Sources for this answer
43 / 50Olympus Mons, a huge volcano, is on which planet?Reveal answerHide answer
Answer
Mars. Olympus Mons is an enormous shield volcano on Mars.
Sources for this answer
44 / 50The Valles Marineris canyon system is on which planet?Reveal answerHide answer
Answer
Mars. Valles Marineris stretches across thousands of kilometers of Mars.
Sources for this answer
50 / 50Which planet is the only one known to support life?Reveal answerHide answer
Answer
Earth. Earth is the only world with confirmed life. Evidence that another world once had water or could be habitable is not a detection of life.
Sources for this answer
Small bodies and space terminology
07 / 50The main asteroid belt lies between which planets?Reveal answerHide answer
Answer
Mars and Jupiter. Most main-belt asteroids orbit between Mars and Jupiter.
Sources for this answer
19 / 50Which dwarf planet lies in the main asteroid belt?Reveal answerHide answer
Answer
Ceres. Ceres is the largest object in the main asteroid belt.
Sources for this answer
20 / 50Pluto belongs to which region beyond Neptune?Reveal answerHide answer
Answer
The Kuiper Belt. The Kuiper Belt contains many icy bodies beyond Neptune.
Sources for this answer
29 / 50What is a space rock called after it reaches the ground?Reveal answerHide answer
Answer
A meteorite. A meteoroid is the object in space, a meteor is the bright atmospheric streak, and a meteorite is material that survives to reach the ground.
Sources for this answer
31 / 50What are comet nuclei mainly made of?Reveal answerHide answer
Answer
Ice, dust, and rock. Comets contain frozen materials mixed with dust and rocky material.
Sources for this answer
32 / 50Which way does a comet’s ion tail point?Reveal answerHide answer
Answer
Away from the Sun. The solar wind carries ionized gas away from the Sun.
Sources for this answer
49 / 50What is Pluto’s largest moon called?Reveal answerHide answer
Answer
Charon. Charon is about half Pluto’s diameter. That unusually large relative size makes Pluto and Charon a distinctive pair.
Sources for this answer
Missions and observation
45 / 50Which spacecraft flew past Pluto in 2015?Reveal answerHide answer
Answer
New Horizons. New Horizons flew past Pluto on July 14, 2015. It sent back the first close views of the dwarf planet and its moons.
Sources for this answer
46 / 50Which mission first landed humans on the Moon?Reveal answerHide answer
Answer
Apollo 11. Apollo 11 landed Neil Armstrong and Buzz Aldrin in July 1969. Michael Collins remained in lunar orbit in the command module.
Sources for this answer
47 / 50Which spacecraft orbited Saturn from 2004 to 2017?Reveal answerHide answer
Answer
Cassini. Cassini studied Saturn, its rings, and its moons before its mission ended.
Sources for this answer
48 / 50Which NASA spacecraft entered Jupiter’s orbit in 2016?Reveal answerHide answer
Answer
Juno. Juno was sent to investigate Jupiter’s interior, atmosphere, and magnetic field.