Finding Other Worlds: The Physics of Exoplanet Detection

Planets Beyond the Solar System

For most of human history, the existence of planets around other stars was pure speculation. We could see stars as points of light, but any orbiting planets were lost in the glare — too dim and too close to their host stars to be seen directly.

Then physics provided a way. Not by seeing the planets themselves, but by detecting their gravitational and optical signatures — the subtle ways they influence the light and motion of their parent stars.

The Wobble: Radial Velocity

A planet does not simply orbit a star. Both the planet and the star orbit their common centre of mass. For a massive planet like Jupiter, this makes the star wobble — tracing a small circle or ellipse in space.

We cannot see this tiny spatial wobble directly for distant stars. But we can detect its radial component — the back-and-forth motion along our line of sight — using the Doppler effect. When the star moves toward us, its light is shifted slightly to shorter (bluer) wavelengths. When it moves away, the shift is to longer (redder) wavelengths.

For Jupiter orbiting the Sun at 5 AU, the Sun’s radial velocity variation is about 12 m/s. For Earth, it is just 9 cm/s. Modern spectrographs like ESPRESSO at the Very Large Telescope achieve precisions of about 10 cm/s — enough, in principle, to detect an Earth-mass planet in the habitable zone of a Sun-like star.

In 1995, Michel Mayor and Didier Queloz used this method to discover 51 Pegasi b — a Jupiter-mass planet orbiting its star in just 4.2 days. This “hot Jupiter” was nothing like any planet in our solar system and opened an entirely new field of astronomy. Mayor and Queloz received the 2019 Nobel Prize.

The Shadow: Transit Photometry

If a planet’s orbit is aligned so that it passes between us and its star, we see a transit — a small dip in the star’s brightness as the planet blocks a fraction of the starlight.

The physics is simple geometry. A Jupiter-sized planet transiting a Sun-like star blocks about 1% of the starlight. An Earth-sized planet blocks about 0.01% — a dip of 84 parts per million. Detecting such tiny signals requires photometric precision that was impossible from the ground for most stars but became routine from space.

NASA’s Kepler space telescope (2009–2018) stared at 150,000 stars simultaneously for four years, measuring their brightness every 30 minutes. It discovered over 2,600 confirmed exoplanets and revealed that planets are common — most stars in the Milky Way have at least one planet.

The transit depth gives the planet’s radius (relative to the star). Combined with radial velocity data (which gives mass), we can calculate the planet’s density — and thus infer whether it is rocky, gaseous, icy, or something else entirely.

Transit Spectroscopy: Reading Alien Atmospheres

During a transit, a thin ring of starlight passes through the planet’s atmosphere before reaching us. Different molecules absorb light at characteristic wavelengths — water vapour, carbon dioxide, methane, sodium, and potassium each leave distinct fingerprints in the transmitted spectrum.

By comparing the star’s spectrum during transit with its spectrum outside transit, we can identify molecules in the exoplanet’s atmosphere. The James Webb Space Telescope has detected water vapour, carbon dioxide, and sulfur dioxide in the atmospheres of several exoplanets — the first detailed chemical inventories of alien worlds.

This technique could eventually detect biosignatures — combinations of gases (like oxygen and methane together) that are thermodynamically unstable without a biological source to replenish them. No confirmed biosignature has been found yet, but the instruments to search are now operational.

Gravitational Microlensing

Einstein’s general relativity predicts that massive objects bend light. When a foreground star with a planet passes in front of a distant background star, the foreground star’s gravity acts as a lens, briefly magnifying the background star’s light.

If the foreground star has a planet, the planet adds a brief, sharp spike or dip to the lensing light curve. The duration and shape of this anomaly reveal the planet’s mass and orbital distance.

Microlensing is unique among detection methods: it is sensitive to low-mass planets at large orbital distances, including free-floating planets that orbit no star at all. Surveys suggest there may be billions of rogue planets wandering the galaxy.

The drawback is that microlensing events are one-time occurrences — the alignment is fleeting and unrepeatable, so follow-up observations are difficult.

Direct Imaging

Directly photographing an exoplanet is extraordinarily challenging. A Sun-like star is roughly a billion times brighter than an Earth-like planet in visible light, and the angular separation is tiny.

Coronagraphs block the star’s light with a physical mask, while adaptive optics correct the blurring caused by Earth’s atmosphere. Using these techniques, astronomers have directly imaged several dozen exoplanets — mostly young, massive, hot Jupiters that glow brightly in infrared light at wide separations from their stars.

The Nancy Grace Roman Space Telescope (launching late 2020s) will carry a coronagraph designed to image mature giant planets at closer separations. Future concepts like the Habitable Worlds Observatory aim to directly image and characterise Earth-like planets in the habitable zones of nearby Sun-like stars.

Astrometry: Measuring the Wobble in 2D

While radial velocity measures the star’s wobble along our line of sight, astrometry measures the wobble on the sky — the star’s tiny side-to-side motion. This is technically demanding because the displacements are extraordinarily small — microarcseconds, comparable to the width of a human hair seen from 1,000 km away.

ESA’s Gaia spacecraft is measuring the positions and motions of nearly two billion stars with unprecedented precision. Its data is expected to reveal thousands of new exoplanets through astrometric wobbles, particularly massive planets at large orbital distances — a population underrepresented by other methods.

What We Have Learned

Over 5,700 confirmed exoplanets have revealed a stunning diversity of worlds: hot Jupiters completing orbits in hours, super-Earths with no solar system analogue, mini-Neptunes with thick hydrogen atmospheres, planets orbiting two stars, and worlds in the habitable zone where liquid water could exist.

The physics behind each detection method — the Doppler effect, photometric precision, gravitational lensing, coronagraphy — determines what types of planets we can find. No single method sees everything. Together, they reveal the true population of planets in the galaxy.

The next frontier is characterisation: not just finding planets, but understanding them. What are their atmospheres made of? Do they have magnetic fields? Weather? Oceans? Life? The physics is ready. The telescopes are coming.