Exoplanets — Worlds Beyond the Sun
From the first wobble around 51 Pegasi to a planet circling our nearest stellar neighbor — a guide to the worlds we have found, how we found them, and where to look in your own sky.
Contents
What Is an Exoplanet?
An exoplanet — short for “extrasolar planet” — is a planet that orbits a star other than the Sun. For most of human history we knew of only eight planets — the worlds of our own solar system. Today the count is over 5,800 confirmed, and rising every month.
That number is itself a wild understatement. NASA's Kepler space telescope — a planet-hunting mission that stared at one patch of the Milky Way from 2009 to 2018, watching ~150,000 stars for the periodic dimming caused by transiting planets — produced enough statistics to suggest that essentially every star in the galaxy hosts at least one planet, and that small rocky worlds are more common than gas giants. The Milky Way contains a few hundred billion stars. The implication is that planets are not rare cosmic accidents — they are the rule.
None of these worlds can be seen as a disk in any amateur telescope (or even most professional ones). Almost every confirmed exoplanet has been detected indirectly — through tiny effects it has on the light of its host star. The host stars themselves, however, are very much within reach. Some are bright enough to spot with the naked eye on a dark night.
A Brief History
For centuries the existence of other planets was a philosophical question. The first confirmed detections came only in the 1990s, and they came in an order nobody expected.
The very first confirmed exoplanets were found orbiting PSR B1257+12, a millisecond pulsar in Virgo. Aleksander Wolszczan and Dale Frail measured timing irregularities in the pulsar's beacon and showed they had to be caused by orbiting bodies. Two small planets were announced; a third followed in 1994. Pulsars are stellar corpses, so these were not worlds anyone had been hoping to find — but they proved planets existed elsewhere.
Michel Mayor and Didier Queloz announced a planet around 51 Pegasi (now officially named Helvetios), a perfectly ordinary G-type star 50 light-years away. The planet, 51 Pegasi b, is a Jupiter-mass world that whips around its star in just 4.2 days. Nothing in the textbooks predicted gas giants could exist that close to their stars; the discovery rewrote planet-formation theory overnight. Mayor and Queloz shared the 2019 Nobel Prize in Physics for the work.
Pointed at a single patch of sky in Cygnus and Lyra, NASA's Kepler mission single-handedly multiplied the known exoplanet population by an order of magnitude and proved that small, Earth-sized worlds are common. TESS, Kepler's all-sky successor launched in 2018, has been adding nearby targets ever since.
The European Southern Observatory announced Proxima Centauri b — a roughly Earth-mass planet orbiting the red dwarf Proxima Centauri in its habitable zone. Proxima is the closest star to the Sun, just 4.24 light-years away. Suddenly the nearest known exoplanet was also a potentially temperate, rocky world. It is the most-studied exoplanet system we have, and the most plausible target for any future interstellar probe.
How We Find Them
A planet is millions or billions of times fainter than its star and sits inside its glare. Five techniques have produced almost every confirmed detection.
A planet and its star both orbit their shared center of mass. As the star moves toward us its light is slightly blue-shifted; as it moves away, slightly red-shifted. High-resolution spectrographs can measure these shifts down to ~1 m/s — a walking pace. This is how 51 Pegasi b was found and how Proxima b was confirmed. Best for massive, close-in planets.
If a planet's orbit is edge-on from our viewpoint it crosses in front of its star once per orbit, blocking a tiny fraction of the light — typically 0.01% to 1%. Repeated, periodic dips betray a planet. This is the workhorse method for Kepler and TESS, and it accounts for the majority of all known exoplanets. The geometry has to cooperate, so transits only catch a small fraction of existing planets, but the ones it catches are extremely well-characterized: we get the planet's radius, and follow-up radial-velocity measurements yield the mass and therefore the density.
The hardest method — literally photographing the planet. It requires a young, bright, self-luminous planet far from its star, plus heroic optics (coronagraphs, adaptive optics, starshades) to suppress the host's glare. The classic success is the four-planet system around HR 8799, where all four giants have been resolved and tracked along their orbits. Beta Pictoris b is another benchmark. The method also produces cautionary tales: the famous Hubble “image” of Fomalhaut b (2008) was later shown by Gáspár & Rieke (2020) to be an expanding dust cloud from a planetesimal collision, not a planet at all. Direct imaging gives us spectra of the planet's own atmosphere, which is otherwise extremely hard to obtain.
When one star passes precisely in front of another, its gravity bends and magnifies the background star's light. A planet around the foreground star adds a brief secondary spike to that brightening. Microlensing is sensitive to planets at large orbital distances and even to free-floating rogue planets with no host star at all — a population estimated to rival the count of stars.
Astrometry measures the tiny side-to-side wobble of a star against the sky — ESA's Gaia mission is expected to deliver thousands of detections this way. Pulsar timing watches the cadence of a pulsar's radio beam for orbit-induced delays; it is exquisitely precise but only works for the rare handful of stars that are pulsars.
Famous Host Stars You Can Observe
You cannot see the planets themselves, but you can absolutely see their suns. Pointing a telescope at a star you know hosts other worlds is a small, quiet pleasure that nothing else in observing quite matches.
Bright hosts — naked eye or binoculars
An orange giant easily visible to the naked eye. Pollux b is a Jupiter-class planet on a 1.6-year orbit, discovered by radial velocity in 2006.
The closest single sun-like star, just 11.9 light-years away. Multiple super-Earth candidates have been reported, two of them potentially in the habitable zone. A long-time staple of science fiction for good reason.
A young K-type dwarf 10.5 light-years away with a known Jupiter-mass planet and at least two debris belts — a system that probably resembles our own solar system in its youth.
In 1999 this became the first sun-like star confirmed to host multiple planets. At least four are now known.
The headline name in exoplanet history — the first ordinary star ever confirmed to host a planet (1995). Visible to the naked eye under a dark sky and a comfortable binocular target from anywhere.
Dim hosts — the red-dwarf telescope targets
Most of the galaxy's planets orbit M-dwarfs — small, cool, abundant red stars. Individually they are faint, but the closest ones are within reach of any decent telescope.
The closest known exoplanet host, full stop. A red dwarf 4.24 light-years away, gravitationally bound to the brilliant Alpha Centauri AB pair. Proxima b (2016) is roughly Earth-mass and orbits in the habitable zone; a second planet, Proxima c, was added in 2019. Alpha Cen A and B themselves have a long history of planet claims, including Alpha Cen Bb (announced 2012, retracted 2015) and an unconfirmed JWST mid-infrared candidate around Alpha Cen A (2021) — but no confirmed worlds yet. A challenging target from northern Europe (declination −62°) but a must-see from southern latitudes.
A nearby M-dwarf system that became famous in the 2000s as the first place habitable-zone super-Earths were claimed. Several of its planets are confirmed; others remain debated. A historically important system.
A four-planet M-dwarf system — the first multi-planet system found around a red dwarf, and the first to show clear evidence of orbital resonance between its giant planets.
What Kinds of Worlds Are Out There?
The single biggest surprise of the exoplanet era is how unfamiliar most of these worlds are. The solar system, it turns out, is not a representative sample.
Gas giants the size of Jupiter or bigger, orbiting their star closer than Mercury orbits the Sun. Surface temperatures of 1,000 to 3,000 K. They are easy to detect and were the first kind of planet found around a sun-like star — 51 Pegasi b is the prototype.
The most common planet types in the galaxy. Super-Earths are rocky worlds 1–2× Earth's radius; mini-Neptunes are slightly larger (2–4×) with thick gas envelopes. Nothing like them exists in our own solar system — the gap between Earth and Neptune is a peculiarity of the Sun's planetary system, not a universal pattern.
Worlds whose entire year lasts a few hours. Some are so close to their stars that the surface is molten rock and the atmosphere, if any, is rock vapor.
Free-floating worlds drifting through interstellar space without a host star. Microlensing surveys suggest they could be as numerous as stars themselves — an enormous, dark, scattered population.
The grail. Roughly Earth-sized, roughly Earth-mass, orbiting in the habitable zone of a long-lived star. We have a handful of plausible candidates — Proxima b, the inner TRAPPIST-1 worlds, several Tau Ceti candidates — but no firm twin yet.
The Habitable Zone
The habitable zone is the band of orbital distances around a star where a rocky planet with the right kind of atmosphere could plausibly hold liquid water on its surface. Too close, and oceans boil away; too far, and they freeze.
For the Sun the zone runs roughly from inside Earth's orbit to past Mars. For a cool red dwarf like Proxima Centauri it sits much closer in — a few percent of Earth's distance from the Sun. That is why Proxima b, with an 11-day orbit, is still a habitable-zone candidate: its star is far cooler than ours.
“Habitable zone” only means liquid water is geometrically possible. It does not guarantee an atmosphere, a magnetosphere, or anything like a temperate climate. Mars is technically inside the Sun's habitable zone, and yet here we are. Still, it is the right place to start looking, and the count of confirmed habitable-zone planets — visible at the top of the exoplanet catalog — is one of the most-watched numbers in the field.
Explore the Catalog
Nightbase mirrors the full NASA Exoplanet Archive: every confirmed exoplanet, its host star, its discovery method, its orbit, and its measured properties.
Filter by discovery method (radial velocity, transit, imaging…), by planet radius, or by whether the planet sits in its star's habitable zone. Sort by year, distance, period, or mass. The discovery timeline at the top of the page is a good place to start — it tells the story of three decades of detections at a glance.
