50 Fun Facts About Exoplanets

What is an exoplanet?

A planet outside the Solar System is an exoplanet; the word combines the Greek prefix exo-, 'outside', with planet, and thousands have been confirmed since the first discoveries in the 1990s.

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 the first exoplanet around a Sun-like star, 51 Pegasi b, in October 1995?

Michel Mayor and Didier Queloz of the University of Geneva made the detection with the ELODIE spectrograph at the Observatoire de Haute-Provence and shared the 2019 Nobel Prize for it.

What official proper name did the IAU give 51 Pegasi b?

Dimidium, chosen in the IAU's 2015 NameExoWorlds vote, is Latin for 'half', a nod to the planet's minimum mass of about half that of Jupiter; its star became Helvetios.

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

Hot Jupiters are gas giants orbiting scorchingly close to their stars, typically in under ten days; 51 Pegasi b, which circles its star in about four days, was the first found.

What are the names of the three planets orbiting the neutron star PSR B1257+12?

Draugr, Poltergeist and Phobetor were chosen in the IAU's 2015 naming contest for the three planets, while the star itself became Lich; the names come from undead and nightmare lore.

Which was the first space telescope designed to find exoplanets by the transit method?

CoRoT, a French-led mission launched in December 2006, was the first space telescope designed to hunt transiting exoplanets, staring at star fields for months at a time, years before Kepler.

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 transiting a Sun-like star dims it by roughly how much?

Roughly 80 parts per million: the dip equals the ratio of the planet's disc area to the star's, and Earth's radius is only about 1/109 of the Sun's, so the drop is 0.008 per cent.

What was the first exoplanet discovered by the transit method, in 2002?

OGLE-TR-56b, a giant planet flagged by the OGLE survey in 2002 and confirmed by follow-up spectroscopy, was the first planet found by its transits; HD 209458 b had been seen transiting in 1999 but was discovered by radial velocity.

The radial velocity method detects a planet by measuring what?

The star's small orbital wobble, as it circles the system's centre of mass, shifts its spectral lines back and forth by the Doppler effect; 51 Pegasi's wobble is only about 50 metres per second.

Which detection method uses a star's gravity to magnify a distant background star?

Gravitational microlensing briefly brightens a background star when a foreground star passes in front, and a planet around the lens adds a short extra spike; OGLE-2005-BLG-390Lb was an early find.

What is the IAU's working mass limit separating planets from brown dwarfs?

13 times Jupiter is the mass at which an object of solar composition can fuse deuterium; the IAU's working definition draws the planet boundary there, with 80 Jupiters the hydrogen-burning limit for stars.

By convention, the first planet discovered around a star gets which letter?

The letter b goes to the first planet found, because the star itself is treated as component 'a'; later planets get c, d and so on in order of discovery, not distance.

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

Proxima Centauri, 4.2 light-years away, is the Sun's nearest stellar neighbour, a faint red dwarf in the Alpha Centauri system; its planet Proxima b was announced in August 2016.

What is the orbital period of Proxima Centauri b?

About 11.2 days is the orbital period of Proxima Centauri b, which circles its star at just 0.05 AU, yet the red dwarf is so dim that this still lies in the habitable zone.

Who led the team that announced Proxima Centauri b in 2016?

Guillem Anglada-Escudé of Queen Mary University of London led the Pale Red Dot campaign, which used the HARPS spectrograph to nail down a Doppler signal suspected in earlier data.

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

Seven Earth-sized planets circle TRAPPIST-1 with periods from 1.5 to 19 days, three of them in the habitable zone; the ultracool dwarf lies about 40 light-years away in Aquarius.

TRAPPIST-1 takes its name from a telescope at which observatory?

La Silla in Chile hosts the 60-centimetre TRAPPIST-South telescope, a Belgian-run robotic instrument whose 2016 detection of the first TRAPPIST-1 planets gave the system its name.

What is the informal nickname of HD 209458 b, the first exoplanet seen transiting its star?

Osiris, named for the Egyptian god who was cut into pieces, fits a planet seen shedding hydrogen from its atmosphere in 2003 Hubble observations; it was also the first exoplanet with a detected atmosphere.

In which year did NASA launch the Kepler space telescope?

NASA launched Kepler on 7 March 2009 aboard a Delta II rocket from Cape Canaveral, sending it into an Earth-trailing solar orbit to stare at a single patch of sky in Cygnus and Lyra.

Approximately how many stars did Kepler continually monitor in its fixed field of view?

150,000 stars, selected from the roughly half a million in its 115-square-degree field in Cygnus and Lyra, were monitored by Kepler's single photometer every 30 minutes for brightness dips.

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, announced in April 2014, is about 1.1 times Earth's radius and the outermost of five planets around a red dwarf some 500 light-years away in Cygnus.

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

Kepler-452b orbits a Sun-like star at 1.04 AU every 385 days and is about 1.6 times Earth's radius; a 2018 reanalysis questioned whether its transit signal was real.

Kepler-16b, found in 2011, was the first transiting example of what kind of world?

One orbiting two stars at once: Kepler-16b, announced in September 2011, circles a pair of stars every 229 days, and its transits of both stars gave a radius precise to about 0.3 per cent.

Kepler-90i, found in 2017, made Kepler-90 the first system besides our own with how many known worlds?

Eight planets made Kepler-90 the first system to tie the Solar System's count; Kepler-90i was picked out of archival Kepler data in 2017 by a Google neural network trained to spot weak transits.

In which month and year was Kepler-22b, the first transiting planet in a Sun-like star's habitable zone, found?

December 2011 brought the announcement of Kepler-22b, a 2.4-Earth-radius world 600 light-years away that orbits its Sun-like star every 290 days inside the habitable zone.

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

TESS, the Transiting Exoplanet Survey Satellite, launched on a Falcon 9 on 18 April 2018 into a lunar-resonant orbit and took its first-light image on 7 August 2018; it targets nearby bright stars.

Which planet is the prototype of the 'hycean' class, with water under a hydrogen envelope?

K2-18b, a sub-Neptune 124 light-years away, became the model 'hycean' world after 2023 JWST spectra showed methane and carbon dioxide; a 2025 claim of a biosignature gas there was quickly disputed.

Which possible biosignature chemical was controversially reported on the hycean prototype planet in 2025?

Abiotic chemistry could explain it, and some doubt it was detected at all.

55 Cancri e, officially named Janssen, belongs to which class of planet?

Super-Earth describes 55 Cancri e, with about 8 Earth masses and twice Earth's diameter; it races around its star in under 18 hours and is hot enough for a lava surface.

Super-Earths are generally defined by an upper mass limit of how many Earth masses?

10 Earth masses is the usual ceiling, above which planets are called mini-Neptunes or gas dwarfs; the lower bound varies from about 1 to 5 Earth masses between definitions.

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, in the globular cluster M4, is remarkable for being what?

About 12.7 billion years old, it orbits a neutron star and white dwarf pair in the globular cluster M4, making it among the oldest planets known and the first found in a globular cluster.

Which star system, announced in November 2008, was the first multi-planet system to be directly imaged?

HR 8799, a young star 130 light-years away in Pegasus, showed three giant planets in Keck and Gemini adaptive-optics images by Christian Marois's team in 2008; a fourth was imaged in 2010.

2M1207b, one of the first exoplanet candidates directly imaged, orbits what kind of object?

A brown dwarf, 2M1207, is the host: the planetary-mass companion was imaged with the VLT in 2004 by Gaël Chauvin's team, about 170 light-years away in Centaurus.

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

The Goldilocks zone, or habitable zone, is the band of orbits where a planet could keep liquid water on its surface; the nickname alludes to the porridge that was 'just right'.

Roughly what fraction of Sun-like stars are estimated to have an Earth-sized habitable-zone planet?

That implies some 11 billion potentially habitable Earth-sized planets in the Milky Way, or 40 billion including red dwarfs.

What is a 'rogue' world, in astronomers' terms?

A planet not gravitationally bound to any star, a rogue or free-floating planet, drifts through the galaxy after being ejected from its birth system or forming alone; the Milky Way may hold billions of them.

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.

Which planet, suspected in 1988 and confirmed in 2003, is technically the first exoplanet discovered?

Gamma Cephei Ab was tentatively reported in 1988 by a Canadian team led by Bruce Campbell, who later withdrew the claim; improved radial-velocity data confirmed the giant planet in 2003.

The first possible evidence of an exoplanet, recognised only in 2016, dates from which year?

1917 is the date of a Mount Wilson photographic spectrum of the white dwarf Van Maanen 2 whose calcium and iron lines, reinterpreted in 2016, show pollution by rocky planetary debris.

Which Nobel Prize did Mayor and Queloz share in 2019, along with Jim Peebles?

Physics was the 2019 prize, half going to Jim Peebles for cosmology and half shared by the two Geneva astronomers, one of whom had been the other's doctoral student when they found 51 Pegasi b.

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

CHEOPS, ESA's Characterising Exoplanet Satellite, launched on 18 December 2019 from Kourou; rather than searching, it revisits stars with known planets to measure their transits precisely and so pin down radii.

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.

Beta Pictoris b, a directly imaged super-Jupiter, orbits its young star roughly every how many years?

About 24 years, at roughly 9 to 10 AU from the star, is its orbital period; identified in 2008 from VLT images, it sits in the plane of the famous debris disk 63 light-years away.

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

Nancy Grace Roman Space Telescope, named for NASA's first chief of astronomy, carries a 2.4-metre mirror and will stare at the galactic bulge for the brief brightenings that betray unseen planets, starless ones included.