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2. Radial Velocity — Listening to the Star Wobble

A planet and its host star both orbit their common centre of mass. A massive planet causes the star to "wobble" measurably. As the star wobbles toward us, its light is slightly blueshifted; as it wobbles away, it's redshifted. This Doppler shift is the radial velocity signal.

• What we learn: The orbital period and a minimum mass (m·sin(i), where i is the orbital inclination).
• Precision: 51 Pegasi b caused its star to wobble at ~56 m/s. ESPRESSO on the VLT can now detect wobbles as small as 30 cm/s — sufficient in principle to detect Earth-mass planets around nearby stars.
• Complementary to transits: When a planet is detected by both methods, we know both radius and mass, giving us the planet's density — a key clue to composition.

3. Direct Imaging — Taking an Actual Picture

Direct imaging means obtaining photons from the planet itself. It is the most intuitive approach and the most technically demanding.

• Why it's hard: A Sun-like star is roughly a billion times brighter than an Earth-like planet at visible wavelengths. Separating the planet's light requires coronagraphy or sophisticated image processing.
• What works: Young, massive planets on wide orbits — they're hot from formation and easier to resolve from their stars. Notable successes include the HR 8799 system (four giant planets imaged simultaneously) and Beta Pictoris b.
• Future: The Nancy Grace Roman Space Telescope will carry a coronagraph designed to push toward Earth-like parameters.

4. Gravitational Microlensing — The Universe as a Magnifying Glass

When a massive object passes in front of a more distant star, its gravity bends and focuses the background star's light. If the foreground object has a planet, the planet produces a brief additional brightening spike.

• What we learn: The planet-to-star mass ratio and projected separation — but not the orbital period.
• What it finds: Planets at 1–10 AU from their stars, and free-floating "rogue" planets not orbiting any star, which appear to be surprisingly common.
• Key surveys: MOA (Microlensing Observations in Astrophysics) and KMTNet (Korea Microlensing Telescope Network) have detected dozens. The Roman Space Telescope's microlensing survey is expected to detect thousands more.

5. Astrometry — Measuring a Star's Precise Position

Astrometry detects the stellar wobble caused by a planet — but measures the side-to-side motion of the star's position on the sky, rather than the line-of-sight velocity shift.

• Why it's hard from the ground: Earth's atmosphere blurs stellar positions, making sub-milliarcsecond precision impractical for most targets without space-based observations.
• Gaia's role: ESA's Gaia satellite (launched 2013) is measuring over a billion stars to microarcsecond precision. Its final data releases are expected to contain thousands of astrometric planet detections — planets with orbital periods of years, filling a gap other methods can't easily reach.
• Advantage over radial velocity: Astrometry can determine the true mass (not just minimum mass), because it detects the wobble in both sky-plane dimensions.

Why Multiple Methods Matter

Each method finds different kinds of planets in different orbital configurations. The combined picture is one of extraordinary variety: hot Jupiters at 2,000 K; sub-Neptunes with no solar system analogue; super-Earths potentially covered in global oceans; free-floating rogue planets drifting in interstellar space.

Every detection method adds a new class of world to a census that, a generation ago, contained exactly zero confirmed entries beyond our own solar system. That census is still being written.

What Kepler Taught Us: A Universe Full of Planets

The Kepler mission, which monitored roughly 150,000 stars from 2009 to 2018, transformed our picture of planetary demographics. The statistics are striking:

• Planets are common: On average, every star in our galaxy hosts at least one planet. The Milky Way contains something in the range of 100–400 billion planets.
• The most common planet type doesn't exist in our solar system: "Super-Earths" and "mini-Neptunes" — planets with radii between 1.5 and 4 times Earth's — are the most abundant type in Kepler's sample. We have nothing like this between Earth and Neptune. Why our solar system skipped this radius range is an open question.
• The radius gap: There is a pronounced deficit of planets with radii between about 1.5 and 2 Earth radii — the "Fulton gap." The leading explanation is photoevaporation: stellar radiation strips the hydrogen envelopes of close-in planets below a certain mass, leaving behind bare rocky cores (super-Earths) or planets that retained enough gas to become mini-Neptunes.
• Hot Jupiters are rare: Giant planets on short-period orbits occur around only about 1% of sun-like stars. They were the first type discovered because radial velocity and transit methods strongly favour them, creating a selection bias that made them seem more common than they are.
• Habitable zone occurrence: Estimates of η⊕ (eta-Earth) — the fraction of sun-like stars with a roughly Earth-sized planet in the habitable zone — range from about 10% to 50%, depending on assumptions about what "habitable zone" and "Earth-sized" mean. Even the lower estimate implies billions of candidate worlds in the Milky Way.

The Search for Biosignatures

Detecting a planet is one thing. Determining whether it hosts life is another challenge entirely — and it is what drives much of the next generation of exoplanet science.

What would life look like from a distance?

Astronomers look for biosignatures: chemical or physical features of a planet's atmosphere or surface that require a biological explanation, or at least are strongly suggestive of one. The most discussed candidates:

• Oxygen (O₂) and ozone (O₃): On Earth, atmospheric oxygen is almost entirely of biological origin — photosynthesis. Without continuous replenishment, it would react with surface rocks and disappear within a few million years. An oxygen-rich atmosphere therefore strongly implies a continuous source, and life is the best known one. However, abiotic oxygen production from water photodissociation is possible around certain stellar types.
• Methane (CH₄) alongside oxygen: These two gases react quickly and destroy each other. Detecting both simultaneously would imply active production of both — a strong disequilibrium signal that is difficult to explain without biology.
• Nitrous oxide (N₂O): Produced primarily by microbial denitrification on Earth. Some models suggest it may be a more robust biosignature than oxygen in certain stellar environments.
• The red edge: Land plants on Earth produce a sharp reflectance increase at ~700 nm — the chlorophyll edge. A surface biosignature of this kind would be visible in reflected light from a nearby planet observed by a future direct imaging mission.

The detection challenge

JWST can characterise atmospheres of planets around small, nearby M-dwarf stars — this is what the TRAPPIST-1 programme aims to do. But detecting biosignatures is extremely difficult: the signals are weak, the star's intrinsic variability can mimic or mask atmospheric features, and accumulating enough photons for a robust detection requires hundreds of transits for nearby systems, and more for Earth-like planets around sun-like stars.

The next generation of ground-based extremely large telescopes — the European Extremely Large Telescope (ELT, expected 2028), the Thirty Meter Telescope (TMT), and the Giant Magellan Telescope (GMT) — will use high-dispersion coronagraphy to try direct atmospheric characterisation of nearby rocky planets. The Nancy Grace Roman Space Telescope will demonstrate the coronagraph technology needed for future space-based direct imaging missions. If Earth-like biosignatures exist within 10–20 light-years, they may be within reach within the next 20–30 years.