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All StarQuest: Daily Cosmos Facts guides
There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. At its 2006 General Assembly the International Astronomical Union adopted Resolution B5, which defines a planet as a body that orbits the Sun, has enough gravity to pull itself into a round shape, and has cleared the neighbourhood around its orbit. Pluto meets the first two tests but shares its orbital zone with thousands of other Kuiper Belt objects, so it was reclassified as a dwarf planet. The reclassification changed Pluto's label, not Pluto itself.
A light-year is a distance, not a time: it is how far light travels in one Julian year in a vacuum, at exactly 299,792,458 metres per second. That works out to about 9.46 trillion kilometres, or roughly 5.88 trillion miles. Astronomers use it because interstellar distances are absurd in miles - Proxima Centauri is 4.24 light-years away, which reads far better than 25 trillion miles. Professional papers more often use parsecs, where one parsec equals about 3.26 light-years.
Venus rotates on its axis once every 243 Earth days, but completes an orbit around the Sun in 224.7 Earth days, so one sidereal rotation outlasts one full year. It also rotates retrograde, backwards compared with almost every other planet, which means the Sun rises in the west. Day lengths vary enormously across the solar system: Jupiter spins once in under 10 hours, while Mars is close to Earth at 24 hours 37 minutes. Year lengths range from 88 Earth days at Mercury to about 165 Earth years at Neptune.
This is Olbers' paradox: in an infinitely old, infinitely large, static universe every line of sight would eventually land on a star and the whole sky would blaze. The resolution is that the observable universe has a finite age of about 13.8 billion years, so light from beyond a certain distance has not had time to reach us. Cosmic expansion also redshifts the light of the most distant sources out of the visible band. The sky is not truly empty, either - it glows uniformly in microwaves as the cosmic microwave background.
The Moon orbits at an average distance of about 384,400 kilometres, or 238,855 miles, varying between roughly 356,500 km at perigee and 406,700 km at apogee. Apollo 11 launched on 16 July 1969 and entered lunar orbit about 76 hours later, with the landing on 20 July, so the outbound trip took a little over three days. Lunar laser ranging using retroreflectors left by Apollo shows the Moon is receding from Earth at about 3.8 centimetres per year.
A nebula is an interstellar cloud of gas and dust, and astronomers sort them by how they are lit. Emission nebulae, like the Orion Nebula, glow because nearby hot stars ionise their hydrogen; reflection nebulae simply scatter starlight and look blue; dark nebulae are dense clouds that block the light behind them, as in the Horsehead. Planetary nebulae are unrelated to planets - they are shells of gas thrown off by dying Sun-like stars, misnamed by William Herschel because they looked round in early telescopes.
A star generates its own light by fusing hydrogen in its core, which requires roughly 80 Jupiter masses. A planet does not fuse anything, orbits a star, is round under its own gravity and has cleared its orbital neighbourhood, per IAU Resolution B5 of 2006. A dwarf planet meets every test except clearing its orbit, which covers Pluto, Eris, Haumea, Makemake and Ceres. An exoplanet is simply any planet orbiting a star other than the Sun, so the term describes location rather than a different class of object.
The NASA Exoplanet Archive has passed 6,000 confirmed exoplanets, and the number climbs continuously as candidates are validated. The transit method, used by Kepler and TESS, watches for the tiny periodic dip in a star's brightness when a planet crosses in front of it, and it accounts for the large majority of confirmations. The radial velocity method detects the wobble a planet induces in its host star through Doppler shifts in the starlight, and it yields a mass estimate. Direct imaging, microlensing and astrometry contribute the rest.
A supernova is the explosive death of a star. Core-collapse supernovae happen to stars above roughly eight solar masses when the iron core can no longer resist gravity, while Type Ia supernovae occur when a white dwarf in a binary system is pushed past its stability limit. Betelgeuse is a red supergiant of about 10 to 20 solar masses in the late stage of its life, so it will certainly explode - but the best models put that anywhere within the next 100,000 years, which makes it very unlikely in any human lifetime. At roughly 500 light-years away it poses no danger to Earth; it would simply be bright enough to see in daylight.
As of now there is no confirmed detection of life anywhere other than Earth. What exists is a set of unresolved hints: seasonal methane at Mars measured by Curiosity, subsurface oceans at Europa and Enceladus with plume chemistry sampled by Cassini, and contested biosignature claims such as phosphine at Venus and dimethyl sulfide at K2-18b, none of which has survived independent confirmation. The Drake equation, written by Frank Drake in 1961, is a way to organise the unknowns rather than a prediction, since most of its terms are unmeasured. The Fermi paradox notes the absence of observed contact and asks why, without asserting that nobody is out there.