Thirty years ago, the question of whether planets orbited other stars was still technically open. Astronomers suspected they did — the physics suggested they should — but no confirmed detection existed. Then in October 1995 came 51 Pegasi b, and suddenly “suspected” became “confirmed.” The field that didn’t exist had 5,600+ entries in its catalog by 2024.

If you’re new to exoplanets, start here.

The Basic Definition

An exoplanet is a planet that orbits a star other than our Sun. The prefix “exo” means outside — as in outside our solar system. That’s the entire definition. It says nothing about size, composition, temperature, or whether life could exist there. Gas giant orbiting a red dwarf in three days: exoplanet. Rocky world in the habitable zone of a Sun-like star: also an exoplanet.

The word “extrasolar planet” means the same thing. You’ll see both used interchangeably, though “exoplanet” has largely won on brevity.

The Main Types

Hot Jupiters were the first type discovered in large numbers — massive gas planets orbiting extremely close to their stars, completing orbits in days rather than years. 51 Pegasi b is one. They’re easy to detect because their large mass causes a pronounced wobble in their host star, and their short orbital periods mean you don’t have to wait long to confirm repeated transits. They’re also almost certainly not habitable. Surface temperatures often exceed 1,000°C, and they have no solid surface.

Super-Earths are rocky planets larger than Earth but smaller than Neptune, typically 1.5 to 2 times Earth’s radius. This category didn’t exist in our solar system — there’s nothing between Earth and Neptune — which made early discoveries surprising. Many orbit within habitable zones. Whether they’re genuinely habitable depends heavily on their atmospheres, which we’re only beginning to characterize.

Mini-Neptunes are slightly larger than super-Earths and likely have thick hydrogen-helium atmospheres. The boundary between super-Earth and mini-Neptune is actively debated. The “radius gap” — a statistical shortage of planets between 1.5 and 2 Earth radii — suggests these categories are physically distinct, with different interior compositions and atmospheric histories.

Earth-sized rocky planets are what everyone is looking for. They exist — TRAPPIST-1 has four of them, three in the habitable zone — but they’re harder to detect because they’re smaller, and characterizing their atmospheres pushes current telescopes to their limits.

How They’re Found

The transit method is dominant. When a planet passes in front of its star as seen from Earth, it blocks a small fraction of the starlight. A Jupiter-sized planet around a Sun-like star causes a 1% dip. An Earth-sized planet causes a 0.008% dip — roughly 80 parts per million. The Kepler Space Telescope was built to detect dips that small, staring at 150,000 stars for four years without blinking. It found 2,662 confirmed planets.

Radial velocity detects the gravitational pull a planet exerts on its star. As the planet orbits, the star wobbles toward and away from Earth at a speed proportional to the planet’s mass. Modern spectrographs can measure velocity changes of about 1 meter per second — walking pace. This is what Mayor and Queloz used in 1995.

Direct imaging is rare but spectacular: actually photographing the planet. Works only for young, large planets far from their stars, where the brightness contrast with the host star is manageable. HR 8799, a system 133 light-years away, has four directly imaged giant planets. It’s one of the best-studied directly imaged systems we have.

Gravitational microlensing uses the way massive objects bend light. When a star with a planet passes in front of a more distant star, the gravity acts like a lens. The planet adds a brief, distinctive brightening signal. This method finds planets that other techniques miss, including free-floating planets — worlds that were apparently ejected from their home systems and drift through space without a star.

The Habitable Zone Question

The habitable zone is not a guarantee of habitability. It’s the region around a star where a planet with an Earth-like atmosphere could maintain liquid water on its surface. For the Sun, that’s roughly 0.95 to 1.67 AU. Earth is at 1.0 AU. Mars is at 1.52 — inside the zone, yet not habitable.

The zone shifts based on stellar type. Red dwarf stars — M-dwarfs — are cooler and dimmer, so their habitable zones are much closer in. A planet in the habitable zone of an M-dwarf might orbit at 0.05 AU, completing a circuit in weeks. At that distance, tidal forces may lock the planet so one hemisphere always faces the star. Whether that kills habitability or just makes it different is a genuinely open question.

M-dwarfs make up about 70% of all stars. If their planets can be habitable, the universe has a lot more potentially inhabited worlds than the conservative estimate. If tidal locking or stellar flares make them uninhabitable, the estimate drops considerably. This isn’t settled.

Why This Connects to SETI

The Search for Extraterrestrial Intelligence had a targeting problem for decades: no confirmed planetary catalog meant no rational way to prioritize which stars to observe. SETI surveys pointed at G-type stars because they’re like the Sun, and our Sun has a habitable planet, but that reasoning is circular. Now there’s an actual list of rocky planets in habitable zones — over a thousand candidates — and SETI researchers can prioritize accordingly.

The exoplanet catalog also provides statistical grounding. Based on Kepler data, roughly 20–50% of Sun-like stars have a rocky planet in the habitable zone. The Milky Way has about 100 billion stars. The arithmetic, even conservatively, suggests billions of potentially habitable planets in our galaxy alone. None of this means they’re inhabited. But it changes the framing of the search from “maybe there are a few places to look” to “there are a very large number of candidates and we’re working through them.”

What the James Webb Telescope Changed

Webb launched December 2021. By summer 2022 it had already detected CO₂ in an exoplanet atmosphere — WASP-39b, a hot Saturn 700 light-years away. By 2023 it had published observations of TRAPPIST-1b and 1c, finding that 1b likely lacks a thick atmosphere and 1c shows no signs of Venus-like CO₂ abundance.

This isn’t proof of anything yet. It’s calibration and elimination. Understanding which TRAPPIST planets don’t have Venus-like atmospheres narrows the question. The outer planets — 1e, 1f, 1g — are in the habitable zone and haven’t been fully characterized yet. Those observations are coming.

The scientific community isn’t expecting Webb to announce the discovery of life. What it might find is an anomalous spectrum — something that requires explanation — that prompts years of follow-up. That’s how science actually works.

A Field Worth Watching

New exoplanet confirmations arrive weekly. The catalog will cross 10,000 confirmed planets within this decade. Roman and PLATO will add tens of thousands more candidates. Atmospheric characterization will improve as methods are refined and more observing time is allocated to high-priority targets.

Thirty years ago the field didn’t exist. The question “do planets orbit other stars” had no confirmed answer. Now it does — emphatically yes — and the next question, “does life exist on any of them,” is something we have actual instruments pointed at.

How Far Away Are These Worlds?

This is the part that surprises most people. The nearest confirmed exoplanet, Proxima Centauri b, sits 4.24 light-years away. That sounds close until you convert it: about 40 trillion kilometers. The Voyager 1 probe, the fastest object humanity has ever launched into deep space, would take roughly 73,000 years to cover that distance — and Proxima is the closest one.

Most of the planets in the catalog are far worse. Kepler stared at a patch of sky toward the constellation Cygnus where the typical target star sits 1,000 to 3,000 light-years out. We are not visiting these places. We’re not sending probes in any human timeframe. Everything we know about them is carried on light that left the system before the questions we’re asking even existed.

That distance shapes the whole field. It’s why detection is indirect, why a single good atmosphere measurement can take dozens of hours of telescope time, and why the nearby handful of systems — TRAPPIST-1 at 40 light-years, Proxima at 4 — matter far out of proportion to their number. Proximity is the one thing you can’t buy with a bigger budget.

Three Things People Usually Get Wrong

First: an exoplanet in the “habitable zone” is not a habitable planet. The zone only means liquid water is geometrically possible given the right atmosphere. Mars is technically on the edge of the Sun’s zone and it’s a frozen desert.

Second: we have never photographed an Earth-like exoplanet as anything more than, at best, a single pixel. The dramatic illustrations you see — blue oceans, swirling clouds — are artists’ renderings based on a measured radius and orbit, not photographs.

Third: discovering thousands of planets does not mean we’ve found life. We’ve found places. Whether any of them host biology is a separate question that current instruments are only just beginning to probe, and the honest answer today is that we don’t know.

New to space science? SETIworld helps you follow exoplanet discoveries, SETI projects, and the search for life in plain language — join to get started.

References

• NASA Exoplanet Exploration — exoplanets.nasa.gov science.nasa.gov/exoplanets
• ESA Exoplanet Science — esa.int/Science_Exploration
• Winn & Fabrycky, The Occurrence and Architecture of Exoplanetary Systems, Annual Review of Astronomy and Astrophysics 2015
• Kepler Mission Final Catalog — Thompson et al., The Astronomical Journal 2018
• TESS Input Catalog — Stassun et al. 2019
• Schwieterman et al., Exoplanet Biosignatures, Astrobiology 2018 doi.org/10.1089/ast.2017.1729