50 Fun Facts About Exoplanets

What is an exoplanet?

A planet outside the Solar System is an exoplanet, from the Greek exo, 'outside'; surveys suggest the Milky Way holds at least one for every star, so planets probably outnumber the stars in the night sky.

Around what kind of object were the first confirmed exoplanets found in 1992?

A pulsar, the millisecond pulsar PSR B1257+12, hosts the first confirmed exoplanets: Aleksander Wolszczan and Dale Frail detected two planets in 1992 from tiny timing shifts in its radio pulses.

Which two astronomers announced 51 Pegasi b, the first exoplanet found around a Sun-like star?

Michel Mayor and Didier Queloz caught 51 Pegasi b from a French observatory in 1995 by timing shifts in its star's light; the find set off a planet-hunting boom that has since turned up thousands more worlds.

Which name, Latin for 'half', did the IAU give 51 Pegasi b?

Dimidium, Latin for 'half', points to a mass about half that of Jupiter; it won the IAU's 2015 public naming vote, which also renamed the planet's star Helvetios.

51 Pegasi b is the prototype of which class of exoplanet?

Hot Jupiters are gas giants that whip around their stars in days; 51 Pegasi b upset the old view that giants stay far out, and astronomers still debate whether such planets formed close in or migrated inward.

Which three names did the IAU give the planets of the neutron star PSR B1257+12, itself renamed Lich?

Draugr, Poltergeist and Phobetor, a Norse undead, a noisy ghost and a dream-haunter from Ovid, won the IAU's 2015 public vote because the planets circle the corpse of a star, renamed Lich after an undead sorcerer.

Which French-led space telescope, launched in 2006, began hunting transiting planets over two years before Kepler?

CoRoT was the first spacecraft dedicated to finding planets by their transits; it caught its first within three months of starting observations in 2007, and two years later found the first planet shown to be rocky.

Which two methods have discovered the most exoplanets?

Transit photometry and Doppler spectroscopy account for the great majority of confirmed exoplanets; the transit dip reveals a planet's size, the shifting stellar spectrum its minimum mass, and both favour large planets close to their stars.

An Earth-sized planet crossing a Sun-like star dims its light by roughly what share?

About 0.01% is all the light an Earth-sized planet blocks, because its disc covers roughly one twelve-thousandth of the star's; catching so slight a dip took a space telescope above the blur of the atmosphere.

Which space telescope detected carbon dioxide in the atmosphere of the exoplanet WASP-39 b in 2022?

James Webb Space Telescope caught the gas's fingerprint in starlight filtered through the air of WASP-39 b, a puffy gas giant 700 light-years away, during its very first weeks of science work.

The radial velocity method detects a planet by measuring what?

The star's small orbital wobble shows up as a rhythmic Doppler shift in its light; the planet at Proxima Centauri moves its star at only walking pace, about 5 kilometres an hour, which is why the method needs superb spectrographs.

Which planet-finding method relies on a nearer star briefly magnifying the light of a far more distant one?

Gravitational microlensing happens when two stars line up by chance as seen from Earth: the nearer one bends and magnifies the farther one's light like a natural lens, and a planet circling it adds a brief extra flash.

The IAU's working mass limit for a planet, about 13 Jupiters, marks where an object can fuse what?

Deuterium, a heavy form of hydrogen, fuses at lower temperatures than the ordinary kind, so anything massive enough to burn it, from about 13 Jupiters up, is no longer counted as a planet.

The first exoplanet found in a system is lettered 'b'; what counts as 'a'?

The host star counts as 'a', so the first planet found takes b and later finds take c, d and so on in order of discovery, not distance; a system's b can orbit farther out than its c.

How far away are the nearest known exoplanets, which orbit Proxima Centauri?

Proxima Centauri, 4.2 light-years away, is the Sun's nearest neighbour, close enough that a 2016 plan imagined laser-driven light sails making the trip in about 20 years at a fifth of light speed; by 2025 that project was on hold.

Proxima Centauri b hugs its dim star so closely that one of its years lasts about how long?

About 11.2 days is all Proxima Centauri b needs for one orbit, twenty times closer in than Earth is to the Sun; yet the star is so faint that this tight orbit still sits in the habitable zone.

What term describes a planet that always shows the same face to its star, as the Moon does to Earth?

Tidally locked planets turn exactly once per orbit, which leaves one side in endless day and the other in endless night; worlds that orbit close to small, dim stars are expected to end up this way.

How many known planets orbit the ultra-cool red dwarf TRAPPIST-1?

Seven Earth-sized planets circle TRAPPIST-1 so tightly that all their orbits would fit inside Mercury's; from the surface of one, its neighbours could at times loom larger than the Moon does in our sky.

The Belgian telescope behind TRAPPIST-1 has an acronym chosen to honour which national speciality?

Monastery-brewed beer, in the shape of Belgium's Trappist ales, gave the telescope its name: the Liège astronomers bent 'Transiting Planets and Planetesimals Small Telescope' into an acronym to flag the project's Belgian roots.

HD 209458 b, a planet caught losing its atmosphere to space, is nicknamed after which Egyptian god?

Osiris, in the telling the astronomers cited, was cut to pieces by his brother, a fit for a planet that Hubble found in 2003 to be shedding its hydrogen atmosphere into space.

NASA's Kepler telescope, launched in 2009, honours a 17th-century astronomer famous for his three laws of what?

Planetary motion: Johannes Kepler showed in the early 1600s that planets move in ellipses, not perfect circles, and the telescope named after him leaned on his laws to turn repeating dips in starlight into orbits.

About how many stars did Kepler monitor at once in its fixed field of view?

150,000 stars were watched at once, out of about half a million in view, because a planet crosses its star's face only if its orbit happens to be seen edge-on, so most systems show nothing at all.

Why was Kepler retired in October 2018?

Its fuel ran out: with the last hydrazine spent, Kepler could no longer point precisely, and NASA retired it on 30 October 2018 after nine and a half years and more than 2,600 confirmed planets.

What was the first Earth-radius planet found in another star's habitable zone?

Kepler-186f, found in 2014, gets only a third of the starlight Earth does: on this world, which circles a red dwarf every 130 days, high noon would look about as bright as an hour before sunset here.

Which planet, announced in July 2015, was nicknamed 'Earth 2.0' or 'Earth's Cousin'?

Kepler-452b takes 385 days to circle a Sun-like star and is about 60 per cent wider than Earth, which earned the nickname; a 2018 reanalysis questioned whether its faint transit signal was real at all.

Kepler-16b, nicknamed Tatooine, was the first transiting example of what kind of world?

One orbiting two stars at once: Kepler-16b would see double sunsets like Luke Skywalker's home in Star Wars, but it is a cold, Saturn-sized world of gas rather than a desert.

Kepler-90's eighth planet, tying the Solar System's tally, was found in old data in 2017 by what?

A Google neural network, trained on 15,000 signals that people had already checked, combed 670 star systems and caught the faint transits of Kepler-90i that earlier searches had missed.

After a second reaction wheel failed in 2013, Kepler was kept steady for its K2 mission by the pressure of what?

Sunlight pressing evenly on the spacecraft stood in for the lost wheel: engineers balanced Kepler against it, a trick NASA likened to a kayak steering into the current, and won four more years of planet hunting.

Which satellite, launched on a Falcon 9 in April 2018, surveys 85% of the sky for transiting planets?

TESS, the Transiting Exoplanet Survey Satellite, concentrates on the nearest and brightest stars, so the planets it finds are close enough for other telescopes to weigh them and study their atmospheres.

Which planet is the best-known candidate 'hycean' world, with a possible ocean beneath a hydrogen-rich atmosphere?

K2-18b, a sub-Neptune in its star's habitable zone, became the poster child for 'hycean' worlds when 2023 spectra showed methane and carbon dioxide; a 2025 claim of a gas linked to life there was soon disputed.

Which possible sign of life did astronomers controversially report in the air of a 'hycean' planet in 2025?

Dimethyl sulfide is made on Earth only by living things, chiefly marine plankton, which is why the 2025 report made headlines; the signal reached only modest statistical confidence and has not been widely accepted.

55 Cancri e, a scorching rocky world, belongs to which class of planet?

Super-Earth is the label for 55 Cancri e, about eight times Earth's mass; it orbits so close that its year lasts under 18 hours and its star-facing side is probably an ocean of lava.

Which Italian friar, burned at the stake in Rome in 1600, argued that the stars are suns with planets of their own?

Giordano Bruno wrote in the 1580s that the stars are distant suns circled by worlds of their own; the idea had to wait four centuries, until the 1990s, for the first proof.

What is the true colour of HD 189733 b, whose weather includes molten glass rain?

Deep blue, an azure tint measured by Hubble in 2013 from light reflected as the planet slipped behind its star, probably comes from silicate clouds scattering blue light; the planet also got the first exoplanet weather map.

The reflectivity of WASP-12b has been described how?

Black as asphalt is how NASA described WASP-12b in 2017 after Hubble found it reflects under 6 per cent of incoming light; hot hydrogen in its atmosphere absorbs most of it instead.

What might rain on the night side of the ultra-hot planet WASP-76b?

Liquid iron: day-side temperatures above 2,400 °C vaporise iron, winds carry it across the terminator, and ESPRESSO spectra in 2020 showed the iron vapour missing on the cooler night side, implying iron rain.

PSR B1620-26 b, a planet more than twice as old as Earth, goes by which nickname?

Methuselah, named after the Bible's longest-lived man, formed nearly 13 billion years ago in the star cluster M4, evidence that planets were already taking shape within a billion years of the Big Bang.

Astronomers found the four giant planets of the young star HR 8799 by which method, a first for a multi-planet system?

Direct imaging caught them because the four giants are young, still hot and far from their star's glare; seven years of Keck telescope pictures have even been strung into a movie of them orbiting.

2M1207b, among the first planets ever photographed, circles what kind of object, only about 25 times Jupiter's mass?

A brown dwarf, an object too light to shine like a true star, hosts 2M1207b; the planet first showed up as a faint red speck in VLT images in 2004 and was confirmed a year later to be travelling with it.

What is the range of orbits where liquid water could exist on a planet's surface popularly called?

The Goldilocks zone, or habitable zone, sits where a planet is neither too hot nor too cold for surface water; around small, dim red dwarfs it lies so close in that a year there can last just days.

A 2013 study of Kepler data estimated that roughly what share of Sun-like stars host an Earth-sized habitable-zone planet?

About 1 in 5, or 22 per cent, was the estimate drawn from Kepler data in 2013; it implies that the nearest such planet may circle a star only 12 light-years away, one visible to the naked eye.

What is a 'rogue' planet?

A planet not gravitationally bound to any star, a rogue or free-floating planet, drifts through the galaxy alone, either flung out of the system where it formed or born on its own from a collapsing gas cloud.

Which astronomer claimed in the 1960s that Barnard's Star had planets, based on 'wobbles' from telescope flaws?

Peter van de Kamp, director of Sproul Observatory, announced Barnard's Star planets in 1963; in 1973 others showed that stars on the same plates shifted identically, tracing the signal to adjustments of the telescope's lens.

Helvetios, the IAU's name for the star 51 Pegasi, honours an ancient Celtic tribe from which modern country?

Switzerland was the land of the Helvetii in Roman times, and Swiss astronomers found the star's planet in 1995; twenty years later a proposal from Switzerland won the IAU's public vote to name the star.

The earliest evidence of planets beyond the Sun, recognised only in 2016, is a 1917 spectrum of what kind of dead star?

A white dwarf, van Maanen's Star, had its spectrum photographed at Mount Wilson in 1917; the heavy elements it showed, a puzzle for decades, are now read as rubble from rocky bodies falling onto the dead star.

Which Nobel Prize did the discoverers of 51 Pegasi b share in 2019, along with cosmologist Jim Peebles?

Physics is where astronomers win their Nobels, since no prize exists for astronomy itself; half the 2019 award went to Jim Peebles and half to the two planet-finders, 24 years after their discovery.

Which ESA mission, launched in December 2019, measures the sizes of known transiting exoplanets?

CHEOPS does not hunt for new planets: it measures the exact size of ones already found, which combined with a known mass gives a density that tells a rocky world from a gassy one.

Which nickname was given to Kepler-10c in 2014 for its unexpectedly rocky bulk?

Mega-Earth was coined in 2014 for Kepler-10c, a 2.3-Earth-radius world then estimated at 17 Earth masses, seemingly solid despite a size that should have made it gaseous; later work cut its mass.

The fraction of stars with planets is one term in which 1961 formula for estimating alien civilisations?

The Drake equation multiplies seven factors, from the birth rate of stars to the lifetime of civilisations; when Frank Drake wrote it the planet fraction was a guess, and surveys now put it close to one.

Which NASA space telescope was built to run a microlensing survey of rogue planets?

Nancy Grace Roman Space Telescope, named after NASA's first chief astronomer, the 'mother of Hubble', has a view about 100 times wider than Hubble's, so it can watch vast fields of stars for the brief flicker of a lensing event.