Posted by What is an exoplanet? The short answer — a planet orbiting a star other than the Sun — is correct and almost useless, because the interesting part is everything that answer papers over. Thirty years ago the count stood at zero and serious astronomers still argued about whether other planetary systems existed. Now it runs into the thousands, the diversity has broken most of the models people had in 1990, and the catalogue has quietly become the target list for SETI.
Start with the definition, since it is messier than anyone expects.
The Definition Is Fuzzier Than You’d Expect
The International Astronomical Union’s working definition sets an upper mass limit at roughly thirteen Jupiter masses — the point where an object starts fusing deuterium in its core and arguably becomes a failed star rather than a planet. Below that, planet. Above it, brown dwarf.
The line is not physically clean. Objects have been found on either side of it that formed in the same way, and formation history is arguably the thing that matters. Then there are the rogues: free-floating planetary-mass bodies drifting between the stars, ejected from systems or formed alone, which fit the mass criterion and orbit nothing at all. Microlensing surveys keep finding them. Whether they count as exoplanets is still argued about.
So when someone asks what is an exoplanet, the honest response is that it is a category with a hard edge at the top, a soft edge at the bottom, and an awkward population of orphans nobody has fully agreed how to file.
The First Ones Were Found Around a Dead Star
Popular accounts usually start with 51 Pegasi b in 1995, and they are three years late. Aleksander Wolszczan and Dale Frail announced two planets around the millisecond pulsar PSR B1257+12 in 1992, detected through minute variations in the pulse timing. A pulsar is the collapsed remnant of a supernova. Nobody had expected planets to be there at all, and the result was so strange it took a while for the field to absorb it.
Michel Mayor and Didier Queloz then found 51 Pegasi b around an ordinary Sun-like star in October 1995 — half a Jupiter mass, orbiting every 4.2 days, so close to its star that the surface temperature runs above a thousand degrees. Nothing in the standard formation theory allowed for that. Giant planets were supposed to form far out where ice condenses and stay there. The discovery forced a whole subfield of planetary migration into existence.
Five Ways to Find a World You Cannot See
Almost no exoplanet has ever been looked at directly. The star outshines the planet by a factor of a billion or so in visible light, which is the fundamental obstacle every technique works around.
Radial velocity measures the star’s wobble as the planet tugs it back and forth, read off as a Doppler shift in the stellar spectrum. It found 51 Peg b and it gives you a mass, or at least a lower limit on one.
Transits work the other way — you watch for the star to dim as the planet crosses in front. Kepler, launched in 2009, stared at a fixed field spanning Cygnus and Lyra for four years and monitored around 150,000 stars for dips of a few hundredths of a percent. TESS took over in 2018 with an all-sky survey aimed at brighter, nearer stars. Transits give you a radius, and combined with radial velocity, a density.
Direct imaging works only for young, hot, widely separated giants — the HR 8799 system, with four planets photographed at once, is the poster case. Microlensing catches the momentary brightening when one star passes precisely in front of another and its gravity focuses the light, with any planets adding a spike. And Gaia’s astrometry tracks the star’s position on the sky to microarcsecond precision, picking up the same wobble that radial velocity sees, from a different angle.
What the Census Actually Showed
The headline result is that planets are ordinary. Most stars have them. What surprised people was the shape of the population.
The commonest planets in the galaxy appear to be super-Earths and mini-Neptunes — bodies between one and four Earth radii, a size range our Solar System skipped entirely. There is also a gap in the distribution around 1.5 to 2 Earth radii, identified by Benjamin Fulton and colleagues in 2017, probably carved by atmospheric escape stripping small planets of their gas envelopes.
Hot Jupiters, compact systems like TRAPPIST-1 with seven Earth-sized worlds packed inside what would be Mercury’s orbit, planets on wild eccentric paths, planets orbiting both stars of a binary. Our own system’s tidy architecture is looking less like the norm than the exception.
Kepler alone contributed something over two thousand confirmed planets before a failed reaction wheel ended its primary mission, and TESS has been adding to the total ever since. The confirmed count now runs into the thousands, with thousands more candidates waiting on follow-up.
Worth adding a caution here, because the phrase “potentially habitable” gets attached to press releases far too readily. A transit gives you a radius and an orbital period. That is it. It does not tell you whether the planet has an atmosphere, whether it kept its water, whether its star has been flaring hard enough to strip it bare for three billion years. Several worlds announced as Earth-like on those grounds have since looked considerably less promising once anyone measured them properly.
Why SETI Cares
Frank Drake had to guess. When he pointed a dish at Tau Ceti and Epsilon Eridani in 1960, there was no evidence either star had planets — they were chosen because they were nearby and Sun-like, and that was the best anyone could do.
The catalogue removes the guessing. Breakthrough Listen and other programmes can now prioritise systems with confirmed rocky planets in temperate orbits, which converts an undirected sweep into something closer to a targeted survey. The improvement is real even though the underlying probability that anyone is transmitting stays completely unknown.
Exoplanet work also supplies the other half of the search. Atmospheric spectroscopy, the technique that got JWST a clear carbon dioxide detection on WASP-39b in 2022, is the same technique that would eventually look for industrial pollutants or an oxygen-methane disequilibrium. Biosignatures and technosignatures are read off the same photons.
Who Can See Us
One of the more elegant recent ideas inverts the whole question. If we find planets by watching them transit their stars, then anyone doing the same thing would have to be positioned near the plane of Earth’s orbit to see Earth cross the Sun.
Lisa Kaltenegger and Jackie Faherty worked out that zone using Gaia data and published it in 2021: something like two thousand stars have been in a position to watch Earth transit at some point in the past five thousand years, with several hundred of those inside a range where our radio leakage would already have arrived. It is a finite, ranked list of places from which we are conspicuous. Radio searches have started using it.
Which is a neat closing of the loop. The methods built to answer what is an exoplanet turn out to also define who might have noticed us. SETIworld follows both threads — the new detections and the searches that use them — as the catalogue keeps growing.
