Six Thousand Worlds: The State of Exoplanet Discovery
Three decades after the first confirmed exoplanet, the count of confirmed worlds is past six thousand and climbing. We summarise how they were found, what they look like, and what we still can't measure.
In 1992, two pulsar planets were detected around PSR B1257+12 by precise timing of the pulsar's radio pulses. These were the first confirmed planets outside the solar system, and they were strange: rocky bodies around a neutron star, not a regular companion to a normal star.
In 1995, Michel Mayor and Didier Queloz announced the first exoplanet around a sun-like star: 51 Pegasi b, a Jupiter-mass world in a four-day orbit. This was the moment exoplanet science began as a field. They received the Nobel Prize in Physics in 2019 for the discovery.
Three decades later, the NASA Exoplanet Archive lists more than six thousand confirmed exoplanets, with thousands more candidates awaiting confirmation. This note summarises what the census looks like today, by what means the planets were found, and what remains observationally inaccessible.
How the discoveries break down
The current confirmed-exoplanet count is dominated by two detection methods, each suited to a different range of planet sizes and orbital periods:
| Method | What it measures | Best for | Confirmed planets |
|---|---|---|---|
| Transit | Dip in stellar brightness as planet crosses the disk | Short-period planets, sized from sub-Earth to Jupiter | ≈ 4,500 |
| Radial velocity | Stellar wobble from gravitational pull of the planet | Massive planets at all separations | ≈ 1,100 |
| Direct imaging | Photons from the planet itself | Wide-orbit giants, young systems | ≈ 80 |
| Microlensing | Gravitational lensing brightness curve of background star | Distant, wide-orbit planets including free-floating ones | ≈ 250 |
| Astrometry | Tiny sky-plane wobble of host star | Long-period planets around nearby stars | ≈ 5 |
| Timing variations | Periodic shifts in pulsar pulses or eclipse times | Planets around exotic hosts | ≈ 50 |
(Rounded approximations. The exact counts shift weekly as the NASA Exoplanet Archive publishes new confirmations.)
Why transit dominates
The Kepler space telescope (2009-2018) stared at a single field of about 150,000 stars for four years (extended K2 mission added more), watching for the characteristic brightness dip a transiting planet produces. A Sun-like star dims by about 0.01% when an Earth-sized planet crosses it; about 1% for a Jupiter-sized one. With photometric precision down to a few parts per million, Kepler could detect even small terrestrial planets at orbital periods up to a few hundred days.
Kepler delivered the bulk of the current exoplanet sample. Its successor, TESS (Transiting Exoplanet Survey Satellite), launched in 2018, surveys the whole sky in 27-day chunks, focused on shorter-period planets around brighter nearby stars (which makes follow-up easier). Most of the high-priority targets for atmospheric characterisation, including by JWST, are TESS discoveries.
Why radial velocity is still essential
Transit gives you the planet's size. Radial velocity gives you its mass. Get both, and you get the planet's density, which tells you whether it is rocky, gaseous, or somewhere in between. Most of the rocky-planet candidates with measured densities have been characterised via this dual-method approach.
Radial-velocity facilities like HARPS, ESPRESSO, and the new MAROON-X (commissioned 2020) reach precisions of about 0.3 to 1 metres per second. That is the wobble caused by an Earth-mass planet on a one-year orbit around a Sun-like star, at the edge of detectability. Detecting Earth analogs reliably remains hard. Most confirmed Earth-mass planets are around red dwarfs, where the smaller stellar mass makes the wobble proportionally larger.
What the census tells us
Several genuine astronomical surprises have come out of the census.
Planets are common
The current best estimate, derived from Kepler's statistical sample, is that on average there is roughly one planet per main-sequence star in our galaxy. The galaxy has between 100 and 400 billion stars. Even with conservative assumptions, that puts the number of planets in the Milky Way alone in the hundreds of billions. The exact rate depends on assumed planet size and orbital period.
The "missing class": super-Earths and sub-Neptunes
The most common type of planet in the galaxy is not found in our solar system. Planets with radii between about 1.5 and 4 Earth radii (super-Earths to sub-Neptunes) are abundant in the Kepler sample, especially around red dwarfs. There is also a clear gap (the "radius valley" or Fulton gap) in the population between about 1.5 and 2 Earth radii. The current explanation is photoevaporation: planets in this size range with thin hydrogen-helium envelopes have either kept them (becoming sub-Neptunes) or lost them entirely (becoming super-Earths) over their first few hundred million years.
Hot Jupiters and migration
Jupiter-mass planets in days-long orbits ("hot Jupiters") are the easiest to detect by both transit and radial velocity, but they are not common. About 1% of Sun-like stars host one. Their existence forced the field to abandon the assumption that giant planets always form in their final locations. Models with substantial inward migration during the first few million years of the planetary system's history are now standard.
Free-floating planets
Microlensing surveys (KMTNet, MOA, OGLE) have detected planets that are not bound to any star. The estimated frequency is at least as many free-floating planets as bound ones in the galaxy, possibly several times more. Whether they formed in situ in dense star-forming regions or were ejected from planetary systems is being worked out.
Atmospheric characterisation: what JWST has changed
Before JWST (launched 2021, science operations 2022), exoplanet atmospheric spectra came primarily from Hubble's WFC3 instrument and ground-based facilities. The detections were limited to a handful of strong molecular features in larger, hotter planets. JWST's NIRISS, NIRSpec, and MIRI instruments have, in the years since, produced:
- The first definitive detection of CO₂ in an exoplanet atmosphere (WASP-39b, 2022).
- The first comprehensive transmission spectrum of a temperate sub-Neptune (K2-18b, 2023).
- Constraints on rocky-planet atmospheres including hot terrestrial worlds (LHS 3844b, TRAPPIST-1 system).
- The first detection of a clear sulfur dioxide photochemistry signature.
The TRAPPIST-1 system, with seven Earth-sized planets in resonant orbits 40 light-years away, has been a particular focus. As of 2026, JWST data on the inner TRAPPIST-1 planets has provided strong constraints on the absence of thick hydrogen-helium atmospheres on the inner worlds; the outer planets (e, f, g, in the habitable zone) remain observationally open, with thinner atmospheres still possible.
What we cannot yet do
Three big things remain out of reach with current instrumentation:
Earth analogs around Sun-like stars. No confirmed planet yet meets all four conditions: roughly Earth-sized, roughly one-year orbit, roughly Sun-mass host, with a measured density. The closest matches are in the Kepler sample but lack radial-velocity confirmations of mass.
Direct biosignature detection. Atmospheric oxygen, methane, and water in disequilibrium would suggest a biosphere, but the photon budget required to detect these in transmission spectra of a small temperate planet is beyond JWST except in the most favourable cases. The Habitable Worlds Observatory, a NASA flagship concept following the 2020 Decadal Survey, is targeted at this gap. Launch is not before the late 2030s.
Planet-by-planet 3D structure. We measure mass, radius, and integrated atmospheric chemistry. We do not yet measure cloud structure, three-dimensional thermal profiles, or surface features for any exoplanet. The closest we have come is rotational maps of a few hot Jupiters via thermal phase curves.
Why this matters for the lab
LokLab does not do original exoplanet science. We do not have an instrument and we do not propose for observing time. But the exoplanet census informs anything we publish about the cosmos at galactic scale, and it tightens estimates relevant to the Drake-equation parameters several of our research posts touch on. We use the NASA Exoplanet Archive (publicly maintained at exoplanetarchive.ipac.caltech.edu) as our reference and cite its date of access when figures are pulled.
A future calibration study will check our pipeline against a known transit (likely HD 209458b, the first transiting exoplanet, characterised since 1999). That post is planned.
References
- Wolszczan & Frail, Nature, 1992 (pulsar planets).
- Mayor & Queloz, Nature, 1995 (51 Peg b).
- Borucki et al., ApJ, 2010 (Kepler mission overview).
- Howell et al., PASP, 2014 (K2 extended mission).
- Ricker et al., JATIS, 2015 (TESS mission).
- Fulton et al., AJ, 2017 (radius gap).
- JWST early-release exoplanet papers, Nature, 2022-2024.
- NASA Exoplanet Archive, exoplanetarchive.ipac.caltech.edu (continuously updated).
The full bibliography lives at /bibliography.
All counts and rates above are drawn from the public NASA Exoplanet Archive and the peer-reviewed papers referenced. The census evolves weekly; the figures here reflect the state as of mid-2026.