Space trivia questions and answers
Ten questions about the Solar System, the nearest stars and the planets beyond, each answer traced to NASA, JPL, the IAU or the NASA Exoplanet Archive.
Updated on October 7, 2026
The questions
Two of these answers are numbers that genuinely move — the moon counts and the exoplanet total — so they carry the date we checked them. A space quiz written five years ago would get both wrong today.
1. How many planets are there in the Solar System?
Answer: Eight, since the IAU definition of 2006 moved Pluto to the new category of dwarf planet.
Source: União Astronômica Internacional (IAU) — Resolução B5 (2006) sobre a definição de planeta
2. Which planet has the most confirmed moons?
Answer: Saturn, with 274 confirmed moons against 97 for Jupiter. The count changes as new ones are confirmed.
Source: NASA/JPL — Solar System Dynamics, Planetary Satellite Discovery Circumstances
3. Which star is closest to the Sun?
Answer: Proxima Centauri, a red dwarf 4.2465 light years away, with two confirmed planets.
4. How long does sunlight take to reach Earth?
Answer: About 499 seconds, that is 8 minutes and 19 seconds: one astronomical unit (149,597,870,700 m) divided by the speed of light.
Source: Cálculo próprio a partir dos dados citados nesta página
5. Which planet spins backwards compared with the others?
Answer: Venus. Its rotation is retrograde and takes 5,832 hours, longer than its 225-day year.
Source: NASA — Planetary Fact Sheet (NSSDCA) e NASA Science: Solar System
6. Which is the largest planet in the Solar System?
Answer: Jupiter, with an equatorial radius of 71,492 km, more than 11 times that of Earth.
Source: NASA — Planetary Fact Sheet (NSSDCA) e NASA Science: Solar System
7. What is Proxima Centauri b?
Answer: The nearest known exoplanet: at least 1.07 Earth masses, a year of 11.19 days, found by radial velocity in 2016.
8. When will Halley's Comet come back?
Answer: Its next perihelion is expected on 28 July 2061. It last passed in February 1986 and its orbit takes about 76 years.
9. Which is the hottest planet in the Solar System?
Answer: Venus, with a mean surface temperature of 464 °C, hotter than Mercury because of its dense carbon dioxide atmosphere.
Source: NASA — Planetary Fact Sheet (NSSDCA) e NASA Science: Solar System
10. How many exoplanets have been confirmed?
Answer: 6,372 in the NASA Exoplanet Archive when we last read it, on 1 October 2026. The first confirmation around a Sun-like star dates from 1995.
Source: Cálculo próprio a partir dos dados citados nesta página
The answers that go out of date
Saturn passed Jupiter in confirmed moons in 2023 and then jumped again in 2025, when 128 more were announced at once. The number of confirmed exoplanets has gone from under 2,000 in 2015 to over 6,000. Quiz questions about either are really questions about a particular date, and we print it.
Source: NASA/JPL — Solar System Dynamics, Planetary Satellite Discovery Circumstances
How to read this page
How to read the numbers here
Every value on "Space trivia questions and answers" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
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Linked to the rest of the module
"Space trivia questions and answers" is one of this module’s reference pages; the glossary below defines the technical terms used here.
More about “Space trivia questions and answers”
How many answers our own code checks
Of 10 questions on "Space trivia questions and answers", 1 have their answer tied to a value in one of our own datasets, checked automatically by our tests. The rest name their source alongside, for you to verify yourself.
Words on this page
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Isotope
Atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass. Carbon-12 and carbon-14 are isotopes of carbon; only the second one is radioactive.
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit
Exoplanet
A planet orbiting a star other than the Sun. Most were found indirectly, by the star’s faint dimming as the planet crosses in front of it (transit) or by the gravitational wobble it causes in the star (radial velocity), not by direct image.
Standard definition by NASA Exoplanet Archive · Exoplanet