50 Fun Facts About Exoplanets

What is an exoplanet?

More than 6,300 have been confirmed as of mid-2026, in over 4,700 systems.

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

Aleksander Wolszczan and Dale Frail found two terrestrial-mass planets orbiting PSR B1257+12.

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

They worked at the Observatoire de Haute-Provence and shared a 2019 Nobel Prize.

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

It is Latin for 'half', for the planet's roughly half-Jupiter mass.

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

Gas giants orbiting scorchingly close to their stars turned out to be common among early discoveries.

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

The pulsar itself was named Lich; all are undead creatures from folklore.

What is the least massive exoplanet known, at about twice the mass of the Moon?

It orbits the pulsar Lich.

Which two methods have discovered the most exoplanets?

Both are biased towards finding large planets close to their stars.

An Earth-sized planet transiting a Sun-like star dims it by roughly how much?

That is 0.008 per cent, which is why space telescopes are needed.

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

HD 209458 b had been the first seen transiting in 1999, but was found first by radial velocity.

The radial velocity method detects a planet by measuring what?

A star with a planet moves in its own tiny orbit in response to the planet's gravity.

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

NASA's Roman Space Telescope, due in 2027, will run a microlensing planet survey.

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

It is the threshold for thermonuclear fusion of deuterium in objects of solar metallicity.

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

The star counts as 'a', and later planets get subsequent letters in order of discovery.

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

Proxima b was announced by ESO in August 2016 after the Pale Red Dot campaign.

What is the orbital period of Proxima Centauri b?

It orbits at just 0.048 AU, yet sits in its dim red dwarf's habitable zone.

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

Their Pale Red Dot project confirmed years of Doppler measurements.

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

They take between 1.5 and 19 days to orbit, and the star is about 40 light-years away in Aquarius.

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

The Transiting Planets and Planetesimals Small Telescope found the first two planets in 2016; five more followed in 2017.

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

It was also the first found with an atmosphere, one seen evaporating hydrogen.

In which year did NASA launch the Kepler space telescope?

Ironically, Johannes Kepler was convinced no other stars had planetary systems.

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

Its single instrument was a photometer watching for tiny dips in brightness.

Why was Kepler retired in October 2018?

It had operated for nine and a half years, finding 2,778 confirmed planets by 2023.

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

It is the outermost of five planets around a red dwarf.

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

It orbits a Sun-like star at 1.04 AU every 385 days, though its status is now disputed.

Kepler-16b was the first confirmed example of what kind of world?

The 'Tatooine' comparison was inevitable; its radius was known to within 0.3 per cent.

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

Google's machine-learning method found it in old Kepler data.

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

Liquid water could in principle exist on its surface.

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

Its first light image was taken on 7 August 2018.

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

JWST found water vapour, carbon dioxide and methane; a 2025 biosignature claim 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, was the first what discovered around a main-sequence star?

It has roughly 8 Earth masses and twice Earth's diameter, announced in 2004.

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

The lower bound varies from about 1 to 5 in different definitions.

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

It was the first exoplanet to have its overall colour determined and its thermal map made.

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

Its star is stretching it into an egg shape and stripping its atmosphere at six billion tonnes a second.

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

Neutral iron was found in its atmosphere in 2020, vaporised on the day side.

PSR B1620-26 b, nicknamed Methuselah, is remarkable for being what?

It was also the first planet found in a globular cluster.

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

Christian Marois's team used the Keck and Gemini telescopes with adaptive optics.

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

It lies 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 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?

The Milky Way may hold billions to trillions of them.

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

Hundreds of other stars showed the same wobbles once the lens had been cleaned, exposing the error.

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

A Canadian team led by Bruce Campbell first tentatively identified it.

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

A spectrum of the white dwarf Van Maanen 2 showed pollution from planetary material.

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

Queloz was Mayor's doctoral student when they found 51 Pegasi b.

How many confirmed exoplanets were listed as of 30 July 2026?

They sit in 4,740 planetary systems, 1,056 of which have more than one planet.

What is the most massive exoplanet on NASA's archive, so heavy it may not be a planet at all?

By some definitions it is too heavy to be a planet at all.

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

It sits close to the plane of the star's famous debris disk, 63 light-years away.

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

Roman was van de Kamp's pupil and NASA's first Chief of Astronomy.