Numinous / Thread
Other Earths
Is there anywhere else like here?
For most of history the question was philosophy. Since 1995 it has been observational, and the count of known planets around other stars has gone from zero to thousands.
How it runs
The first confirmed planets around a Sun-like star arrived in 1995, and they looked nothing like ours: gas giants orbiting closer than Mercury.
Transit photometry turned discovery into a survey. Watch enough stars for long enough, and the tiny regular dimming as a planet crosses the face gives you its size and its year.
The habitable zone is a starting point rather than an answer. It marks where liquid water could sit on a surface, which is one requirement among many.
Atmospheres are the current frontier. Starlight filtered through a planet's air carries the fingerprints of its molecules, and reading them is what the newest telescopes were built for.
Astrobiology supplies the harder half of the question: what would actually count as evidence of life, and how would we know we were not fooling ourselves.
The entries
In the order they enter the story. Each opens its full entry in the wiki.
Nicolaus Copernicus
Nicolaus Copernicus replaced the Earth-centered cosmos with a Sun-centered one, worked on the mathematics for roughly thirty years, and let the book be printed only in the year he died.
Johannes Kepler
Johannes Kepler took the heliocentric model from Copernicus and the best observations ever made from Tycho Brahe, and turned them into the first mathematical laws of planetary motion.
Nancy Grace Roman
NASA's first Chief of Astronomy and its first woman executive, who built the constituency and the political case for putting large telescopes above the atmosphere.
Carl Sagan
Carl Sagan did real planetary science and then reached more people with it than anyone before him. Cosmos ran in sixty countries, and he wrote the paragraph about the pale blue dot.
NASA
The civilian space agency of the United States, created in 1958 in answer to Sputnik and still the largest single funder of space science, human spaceflight and aeronautics research.
Astrobiology
The search for life beyond Earth, run as an observing programme rather than a speculation. Habitable zones, subsurface oceans, and atmospheric chemistry that nothing but biology would sustain.
Exoplanetology
Planets around other stars. In one working lifetime the count went from zero to several thousand, and the work moved from finding them to reading the chemistry of their atmospheres.
ALMA
Sixty-six dishes on a Chilean plateau, combined into one telescope for the cold universe: the molecular gas and dust that stars and planets are made from.
James Webb Space Telescope
A 6.5-metre segmented mirror behind a tennis-court sunshield, parked at L2 and cooled to about 40 kelvin so it can read infrared light from the first galaxies and from exoplanet air.
Future Missions
The instruments already being built for the next two decades: three telescopes with mirrors over 24 metres, a survey that photographs the whole southern sky every few nights, and spacecraft aimed at the ocean moons.
The Solar System
Eight planets, hundreds of moons, millions of asteroids, billions of comets, one star and an argument about Pluto. The only planetary system anyone has been able to visit.
Timeline
Krakow, and the case against Ptolemy
Copernicus studied mathematics and astronomy at the University of Krakow, where the Ptolemaic system was taught with its full accumulation of epicycles, equants and ad hoc corrections.
Doctorate in canon law at Ferrara
A decade in Italy at Bologna, Padua and Ferrara ended with a law degree and a return to Warmia as a cathedral canon, a post that paid for the rest of his life and left the evenings free.
De Revolutionibus Orbium Coelestium
The finished heliocentric system was printed at Nuremberg with an unsigned preface, added by Andreas Osiander without permission, calling the moving Earth a calculating device rather than a fact.
Mysterium Cosmographicum
Kepler proposed that the spacing of the six known planets came from nesting the five Platonic solids between their orbits. The answer was wrong. The ambition brought him to Tycho Brahe's attention.
Assistant to Tycho Brahe in Prague
Tycho held the most precise observations in history, accumulated by naked eye over decades. He died in 1601, and Kepler inherited the Mars data that would break the circular orbit.
Astronomia Nova, and the first two laws
Years of failed circular fits ended with an ellipse and the Sun at one focus. The second law followed: a planet sweeps equal areas in equal times, so it moves faster when it is closer.
Four moons of Jupiter
Galileo watched four points of light move around Jupiter across several nights. Something in the sky was orbiting a body that was not the Earth, which the geocentric model had no room for.
Dioptrice
The first systematic treatment of lenses, written after Galileo published his telescopic observations. It described the two convex lens astronomical telescope now called the Keplerian design.
Rome suspends the book
The Church placed De Revolutionibus on the Index seventy-three years after publication, and only once Galileo had turned heliocentrism into a public argument it could not ignore.
Harmonices Mundi, and the third law
The square of a planet's orbital period is proportional to the cube of its semi-major axis. This was the relation Newton used to pull the inverse-square law out of the sky.
Stellar parallax closes the argument
Friedrich Bessel measured the annual shift of 61 Cygni against the background stars. It was the first direct evidence that the Earth moves, almost three centuries after the claim.
Neptune is found by calculation
Perturbations in Uranus's orbit predicted an unseen planet, and it was found within a degree of the predicted place. Newtonian mechanics had passed the hardest test anyone could set it.
Starts an astronomy club at eleven
Living in Reno, Nevada, under a dark sky at the edge of town, Roman organized a club with her classmates and worked through a book on constellations and celestial objects. She never lost the interest, despite two decades of adults telling her that science was not for girls.
Doctorate from the University of Chicago
After a bachelor's degree in astronomy from Swarthmore in 1946, earned at the college's Sproul Observatory, she took her PhD at Chicago, whose graduate astronomy program ran out of Yerkes Observatory in Williams Bay, Wisconsin. She stayed on at Yerkes for six more years.
Links stellar chemistry to stellar motion
Handed the sun-like F5 to G5 stars nobody else wanted, Roman found that those with weaker lines from elements heavier than helium had a far wider spread of velocities and sat further off the plane of the Milky Way. Chemistry records when a star formed; orbit records where. Published in the Astrophysical Journal, received 22 July 1950.
The changed spectrum of AG Draconis
Observing at McDonald Observatory in September 1952, Roman found that the tenth-magnitude object BD+67 922, now known as AG Draconis, had changed almost beyond recognition since the 1940s: an emission continuum swamping all but the heaviest absorption lines. The note raised her profile in the field.
Leaves Yerkes for the Naval Research Laboratory
Told plainly that a woman had no path to tenure, she took a job in the new field of radio astronomy at NRL, mapped the galaxy at 67 centimeters, and rose to head the microwave spectroscopy section. Four years in an engineering shop taught her how flight projects are actually built.
The Space Act creates a civilian agency
Eisenhower signed the National Aeronautics and Space Act on 29 July 1958. NASA opened on 1 October, absorbing the NACA laboratories and their staff. Making it civilian rather than military allowed open publication and foreign partnership.
NASA's first Chief of Astronomy
Approached after a lecture by Harold Urey and asked whether she knew anyone who might build a space astronomy program, she applied herself. She joined six months after the agency opened, becoming its first Chief of Astronomy and the first woman to hold an executive position at NASA. Taking the job ended her own research.
Project Ozma listens to two stars
Frank Drake pointed the Green Bank dish at Tau Ceti and Epsilon Eridani for a few months. Nothing came back, but the search for extraterrestrial intelligence became something a scientist could do at work.
A thesis on the surface of Venus
Sagan's doctorate under Gerard Kuiper argued that the extreme surface temperature came from a runaway greenhouse effect. Mariner 2 confirmed it two years later.
The Drake equation organises the ignorance
Written for a small conference at Green Bank, it multiplies star formation rate, planet fraction, life fraction and civilisation lifetime. Most terms are still unmeasured, which is the point of it.
The Orbiting Solar Observatories begin flying
John Lindsay's spinning-wheel design pointed two instrument boxes at the Sun, the easiest target in the sky to lock onto, so the OSOs flew years ahead of the harder pointed telescopes. Eight launched between March 1962 and June 1975, covering a full eleven-year solar cycle.
Mariner 4 photographs Mars
Twenty-two grainy images showed a cratered, apparently dead surface. Every romantic account of canals and vegetation ended in a single data downlink.
Orbiting Astronomical Observatories, two of four
Four experiments proposed during the International Geophysical Year were flown on a common spacecraft that had to point accurately anywhere in the sky. Four OAOs launched between 1966 and 1972 and only two worked, which was a poor procurement record and a decisive proof that a pointed telescope in orbit could do real astronomy.
Apollo 11 lands on the Moon
Armstrong and Aldrin landed in the Sea of Tranquillity on 20 July 1969, eight years after Kennedy set the deadline. Five more landings followed. Twelve people walked on the surface and brought back 382 kilograms of rock and soil.
Life is found that does not need the Sun
Submersible dives on the Galapagos Rift found whole ecosystems at hydrothermal vents, running on chemical energy from rock and water. Sunlight stopped being a requirement for habitability.
The Voyager Golden Record
Sagan chaired the committee that chose the sounds, images and music sent out on both Voyager spacecraft, a message with no expectation of a reader.
The Voyagers launch for the grand tour
A planetary alignment that comes round every 175 years let two spacecraft reach all four giant planets. They found volcanoes on Io, a cracked ice shell on Europa and an atmosphere on Titan.
The Pulitzer Prize for The Dragons of Eden
A book about the evolution of human intelligence won the general nonfiction prize, which is not a category astronomers usually appear in.
Retires from NASA
She left during an early-out window, partly to care for her mother. Edward Weiler, her successor as Chief of Astronomy, coined the phrase "mother of Hubble." She then worked half-time for nearly two more decades, ending up effectively running the science side of Goddard's Astronomical Data Center.
Cosmos: A Personal Voyage
Thirteen episodes on PBS, seen by an estimated 500 million people in 60 countries. It made science television cinematic and personal instead of instructional.
The nuclear winter paper
Sagan and four co-authors applied planetary atmospheric modelling to nuclear war and argued that soot in the stratosphere would collapse global agriculture. It changed the arms control debate.
Challenger breaks up after launch
Challenger was destroyed 73 seconds into flight on 28 January 1986, killing all seven aboard including the teacher Christa McAuliffe. The Rogers Commission traced it to a failed solid rocket booster seal and to a launch decision made over engineering objections.
The pale blue dot
At Sagan's urging, Voyager 1 turned its camera back from six billion kilometers out. Earth occupies less than a pixel, suspended in a stray beam of scattered sunlight.
The first confirmed planets are around a pulsar
Timing variations in the millisecond pulsar PSR 1257+12 revealed two planets, later three. They probably formed from supernova debris, and they remain among the strangest systems known.
The Demon-Haunted World
Published the year before he died, it is the standing popular defence of scientific thinking, and the book people reach for when the argument is about evidence rather than astronomy.
51 Pegasi b breaks every expectation
Mayor and Queloz announced a Jupiter-mass planet in a four-day orbit around a Sun-like star on 6 October. Nothing in the solar system suggested a gas giant could be there.
A Martian meteorite makes a claim
ALH 84001 was announced as carrying possible fossil traces. Every proposed biosignature in it turned out to have a non-biological explanation, and the episode is why the field is careful about single-line evidence.
Death at sixty-two
Sagan died of pneumonia, a complication of myelodysplastic syndrome. The Carl Sagan Institute at Cornell continues the work on planetary habitability.
The ISS takes on its first crew
Expedition 1 arrived on 2 November 2000. The station has been continuously inhabited ever since, orbiting at roughly 408 kilometres and circling Earth about every 92 minutes. NASA, Roscosmos, ESA, JAXA and CSA run it together.
Columbia is lost on re-entry
Columbia broke apart over Texas on 1 February 2003, killing seven. Foam shed at launch had holed the wing leading edge. The accident board found the same institutional pattern the Rogers Commission had described seventeen years earlier.
Cassini finds geysers at Enceladus
Plumes erupting from fractures near the south pole of a 500-kilometre moon meant liquid water under the ice, and a way to sample an alien ocean without drilling into it.
Huygens lands on Titan
The most distant landing ever made. It came down on a surface shaped by liquid methane and ethane, on the only body besides Earth with stable liquid on the surface.
Pluto is reclassified
The new definition required a planet to have cleared its orbital neighbourhood. Pluto had not, and the Kuiper Belt was already known to contain other objects of comparable size.
A planet hunter carries his name
NASA launched the Kepler telescope, which watched one patch of sky for the tiny regular dimming of transits and turned exoplanet discovery from an event into a survey.
Kepler stares at one patch of sky
Four years of continuous photometry on more than 150,000 stars, yielding 2,662 confirmed planets. It established that small planets outnumber large ones and that multi-planet systems are ordinary.
The Shuttle flies for the last time
Atlantis landed on 21 July 2011, ending 135 missions flown since 1981. The Shuttle never made spaceflight routine or cheap, but nothing else could have assembled the ISS or serviced Hubble in orbit.
ALMA opens its eyes
Early Science began with roughly a third of the array in place. The first released image, the colliding Antennae Galaxies, already showed molecular gas in more detail than every previous submillimetre telescope combined.
The array is inaugurated
All 66 antennas were in place on the plateau at 5,058 metres, funded jointly by ESO, the US National Science Foundation and Japan's National Institutes of Natural Sciences, with Chile guaranteed a share of the time.
HL Tauri resets planet formation
The Long Baseline Campaign imaged a star less than a million years old surrounded by concentric rings and cleared gaps. Theory had put that structuring several million years later.
New Horizons reaches Pluto
A world with nitrogen ice glaciers, water ice mountains and a convecting plain the size of Texas. Nobody had expected geological activity that far out and that long after formation.
A planet around the nearest star
Proxima Centauri b, at least 1.17 Earth masses, orbits in the habitable zone of the closest star to the Sun. Proxima is a flare-prone M dwarf, so whether it kept an atmosphere is another question.
Hydrogen in the Enceladus plume
Cassini flew through the geysers and measured molecular hydrogen, which points to active hydrothermal vents on the ocean floor. That is the same chemistry that supports the Galapagos Rift ecosystems.
Seven Earth-sized planets around TRAPPIST-1
A cool M dwarf 40 light years away turned out to host seven roughly Earth-sized planets, three of them in the habitable zone. It became the most-observed system in the field.
DSHARP finds structure everywhere
Twenty nearby protoplanetary disks imaged at about 5 AU resolution. Essentially all of them showed rings, gaps, spirals or crescents, which made substructure the normal state of a young disk rather than a curiosity.
TESS trades depth for sky coverage
Where Kepler went deep on one field, TESS surveys nearly the whole sky around bright nearby stars. Those are the systems close enough for follow-up spectroscopy to work.
Dies at 93
Roman died on 25 December 2018, having spent her last decades teaching high school science teachers, recording astronomy texts for blind readers, and telling young women that astronomy was a field they could enter.
The M87 shadow is published
ALMA's collecting area exceeds that of every other Event Horizon Telescope station combined, so it anchors every baseline it joins. The result was the first image of the region around a black hole's event horizon.
WFIRST is renamed for her
On 20 May 2020 NASA announced that the Wide Field Infrared Survey Telescope would become the Nancy Grace Roman Space Telescope. Release 20-054 quotes Barbara Mikulski, who fought for Hubble in the Senate, calling it well deserved.
Ariane 5 launches Webb from Kourou
Lift-off on 25 December, after twenty-five years of development. The injection was accurate enough to leave far more station-keeping propellant than planned, which is why the mission life estimate doubled.
Every deployment works
Sunshield pallets, five tensioned membranes, secondary mirror tripod and both primary mirror wings unfolded over 29 days, through 344 steps with no backup. The telescope reached L2 on 24 January.
Sagittarius A* gets its portrait
The same network turned on the Milky Way's own supermassive black hole, a far harder target because the gas orbits it in minutes rather than days and the picture will not hold still.
The first full-colour images
Released on 12 July: a lensing cluster deep field from 12.5 hours of exposure, a water detection in the atmosphere of WASP-96b, the Southern Ring Nebula, Stephan's Quintet and the Carina cliffs.
Carbon dioxide, unambiguously
Webb's detection in WASP-39b was the first clear identification of CO2 in an exoplanet atmosphere. Later work on the same planet found sulphur dioxide, a product of starlight-driven chemistry.
Carbon dioxide in an exoplanet atmosphere
The unambiguous CO2 detection in WASP-39b was the first of its kind. Later work on the same planet found sulphur dioxide, which is produced by starlight-driven chemistry rather than by inheritance.
Artemis I flies an uncrewed Orion around the Moon
The Space Launch System flew for the first time on 16 November 2022, sending an uncrewed Orion into distant retrograde orbit around the Moon. Orion splashed down on 11 December after 25 days.
Early galaxies come out too grown up
Spectroscopy confirmed a population of galaxies at 300 to 500 million years after the Big Bang that are more massive and more assembled than pre-launch models allowed for. The models are still being revised.
TRAPPIST-1b turns out to be bare
Thermal emission from the innermost planet of the seven-planet system is consistent with no substantial atmosphere, which is what happens when an M dwarf strips a close-in rocky world.
JUICE leaves for the Jupiter system
ESA's Jupiter Icy Moons Explorer launched on 14 April 2023. It arrives in 2031 and will eventually enter orbit around Ganymede, which would be the first time any spacecraft has orbited a moon other than our own.
Euclid launches to map the dark universe
Launched on 1 July 2023 on a Falcon 9 and stationed at Sun-Earth L2, Euclid is building a three-dimensional map covering a third of the sky and ten billion light-years, using weak lensing and galaxy clustering.
ESA adopts LISA
The Laser Interferometer Space Antenna was formally approved on 25 January 2024: three spacecraft 2.5 million kilometres apart, six laser links, launching on an Ariane 6 in 2035. Construction began in June 2025.
Europa Clipper departs for Jupiter
Launched on 14 October 2024 on a Falcon Heavy for a 2.9 billion kilometre cruise. It reaches Jupiter in April 2030 and will make 49 close flybys of Europa, running all nine instruments on every pass.
Europa Clipper departs for Jupiter
Launched to make dozens of close passes of a moon with a global ocean under its ice. It reaches the Jupiter system in 2030 after gravity assists at Mars and Earth.
Rubin Observatory releases its first imagery
On 23 June 2025, after just over ten hours of test observations, Rubin published images containing millions of galaxies and stars and thousands of asteroids. The ten-year Legacy Survey of Space and Time follows.
TRAPPIST-1 comes back thin or bare
Webb has now reported on four of the seven planets. The innermost shows no sign of a thick atmosphere, the second would have at most a very thin one, and thick hydrogen envelopes are ruled out for the next two.
Artemis II carries a crew around the Moon
Artemis II launched on 1 April 2026 with Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen aboard, and splashed down on 10 April after 9 days and about 1 hour 32 minutes. It was the first crewed flight beyond low Earth orbit since Apollo 17.
The ten-year survey formally begins
NOIRLab announced on 30 June 2026 that the Legacy Survey of Space and Time had officially started. For the next decade Rubin photographs the southern sky every few nights and issues roughly ten million alerts a night for anything that moved or changed.
The Cycle 6 call goes out
Webb is observing Cycle 5 while proposals for Cycle 6 are due at the end of September 2026. Demand for time has run at roughly five times the available hours since the first cycle.
The Roman Space Telescope launches
Roman flew on 30 August 2026 on a Falcon Heavy toward Sun-Earth L2. Its mirror matches Hubble's 2.4 metres but its field of view is about a hundred times wider, which is what makes wide-area dark energy and microlensing surveys possible.
The Roman Space Telescope launches
Liftoff at 7:26 a.m. EDT on 30 August 2026, aboard a SpaceX Falcon Heavy from Launch Complex 39A at the Kennedy Space Center. The observatory separated cleanly, deployed its high-gain antenna and aperture cover, powered on its coronagraph on 1 September, and entered a commissioning period on the way to L2. First science images are expected in early 2027.
The archive passes 6,300 confirmed planets
The NASA Exoplanet Archive listed 6,360 confirmed planets on 7 September 2026. Thirty-one years after the first around a Sun-like star, the count still rises most weeks.