Exoplanets — A Guide for Observers

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.

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.

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.

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

• Pollux (Beta Geminorum) — mag 1.1, Gemini. 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.
• Tau Ceti — mag 3.5, Cetus. 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.
• Epsilon Eridani (Ran) — mag 3.7, Eridanus. 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.
• Upsilon Andromedae (Titawin) — mag 4.1, Andromeda. In 1999 this became the first sun-like star confirmed to host multiple planets. At least four are now known.
• 51 Pegasi (Helvetios) — mag 5.5, Pegasus. 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 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.

• Hot Jupiters — 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.
• Super-Earths and Mini-Neptunes — 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.
• Ultra-short-period planets — 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.
• Rogue planets — 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.
• Earth analogues — 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 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.

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.

Nightbase mirrors the full NASA Exoplanet Archive: every confirmed exoplanet, its host star, its discovery method, its orbit, and its measured properties.