Exoplanets

Planets orbiting stars other than the Sun. Before 1992, none were confirmed; as of 2026, thousands are known and hundreds of billions are estimated to exist in the Milky Way alone. The field has transformed from exotic speculation to systematic science with a direct line toward detecting signs of life.

Referenced in james-webb-space-telescope (atmosphere characterisation) and dark-energy (Roman Space Telescope will discover thousands more).

Scale

Exoplanets are extraordinarily faint relative to their host stars — a Sun-like star is roughly one billion times brighter than the reflected light from an Earth-like planet. This makes direct imaging of small planets essentially impossible with current technology; almost all known exoplanets were detected through indirect methods.

Detection Methods

Transit (most prolific) — planet passes in front of its star, causing a tiny, periodic dimming. The *Kepler* telescope used this; so does TESS. Works best for large planets with short orbital periods.

Radial velocity/Doppler — planet pulls its star in a tiny orbit; the star's light shifts bluward and redward periodically. Detects unseen planets from stellar wobble as small as 1 m/s.

Gravitational microlensing — background star's light is briefly amplified by a foreground star's gravity; orbiting planets cause additional anomalies. Sensitive to planets at 1–10 AU.

Astrometry — measuring a star's precise position on the sky as a planet tugs it. Became productive in the 2020s with Gaia data.

Transit timing variation (TTV) — multi-planet systems perturb each other's orbital timing, revealing planets that don't transit.

Pulsar timing — the first confirmed exoplanets (PSR B1257+12, 1992) were discovered around a pulsar this way — an ironic birthplace for the field.

Key Milestones

• 1992: First confirmed exoplanet detection — around a pulsar (PSR B1257+12)
• 1995: First planet around a Sun-like star — 51 Pegasi b (hot Jupiter, 4-day orbit), detected by radial velocity
• 2014: Kepler announces 715 new planets around 305 stars in a single announcement
• 2015: Kepler-452b — "Earth's cousin", near-Earth-size in habitable zone of G-type star

The Habitable Zone

The region around a star where liquid water could exist on a planetary surface with suitable atmospheric pressure — not too hot, not too cold. The exact boundaries depend on the star's luminosity and the planet's atmosphere.

Complications: tidal locking (one hemisphere always facing the star), eccentric orbits, stellar flares on M-dwarf stars, greenhouse effects. An "eyeball planet" (tidally locked, with a hot spot pupil and frozen dark side) could still harbour life in temperate zones.

Biosignatures

Signs of life detectable from a distance:

• Oxygen (O₂) — produced by photosynthesis; unlikely to accumulate abiotically to Earth levels
• Methane + oxygen together — they react with each other; simultaneous presence suggests active production by biology
• Water vapour — necessary for life as we know it

The James Webb Space Telescope is actively characterising exoplanet atmospheres via transmission spectroscopy — measuring which wavelengths of starlight are absorbed as the planet's atmosphere filters it during transit.

Roman Space Telescope (2026)

The Nancy Grace Roman Space Telescope (launching fall 2026) will survey the sky via gravitational microlensing and transit photometry, discovering thousands of exoplanets across a range of orbital distances — including cold planets in wide orbits that Kepler couldn't detect. Its wide-field infrared coverage makes it complementary to Kepler and JWST.

The Big Question

With hundreds of billions of planets in our galaxy alone, many in habitable zones — why haven't we detected signs of intelligent life? The Fermi paradox, the Drake equation, and the search for technosignatures (radio signals, megastructures, industrial pollution) remain open questions that exoplanet science increasingly makes concrete rather than philosophical.