Exoplanets, Habitable Zones and the Search for Life
Worlds Beyond Our Solar System
For most of human history, it was impossible to know whether other stars had planets. The first confirmed detections only came in the 1990s, and since then thousands of exoplanets have been found. Detecting them directly is almost impossible — a planet is tiny and dim compared with its host star. Instead, astronomers rely on indirect techniques that reveal a planet’s presence through its effect on the star or on the light reaching us.
The Transit Method
When a planet passes in front of its star as seen from Earth, it blocks a small fraction of the star’s light. This produces a slight, temporary dip in the star’s brightness. The resulting transit light curve dips by a tiny amount — typically less than 1% for an Earth-sized planet — and repeats with the planet’s orbital period.
From the transit light curve, astronomers can determine the planet’s orbital period (time between successive dips), its size relative to the star (depth of the dip), and its orbital distance using Kepler’s third law: \(T^2 = kr^3\). The NASA Kepler and TESS missions used this technique to find thousands of exoplanet candidates.
The depth of a transit dip gives the planet’s size relative to the star; the time between dips gives its orbital period. The planet must orbit in almost exactly the line of sight for a transit to be observable from Earth.
Radial Velocity — The Doppler Wobble
A planet and its star both orbit their common centre of mass. As the star is tugged first towards us then away from us, its spectrum shifts slightly: the spectral lines shift to shorter wavelengths (blue-shift) when the star approaches and to longer wavelengths (red-shift) when it recedes. This Doppler wobble is detectable with highly sensitive spectrographs.
The radial velocity method is most sensitive to massive planets in close orbits (which produce the largest stellar wobble) and was used to find the first confirmed exoplanet around a Sun-like star, 51 Pegasi b, in 1995. It measures the planet’s minimum mass and orbital period but cannot directly determine the planet’s radius.
Astrometry measures the tiny back-and-forth wobble in the star’s position against background stars. This method requires extremely precise positional measurements and is most effective for massive planets orbiting nearby stars. The ESA Gaia mission has demonstrated the precision needed, and several astrometric detections were confirmed after 2020. Astrometry gives the planet’s true mass (not just a minimum mass) and the full three-dimensional orbit, making it complementary to radial velocity measurements.
Requirements for Life
Life as we know it requires a stable supply of liquid water and a source of chemical energy. Liquid water exists only over a specific temperature range, which depends on atmospheric pressure and composition. Astronomers also consider whether a world has a stable atmosphere, a source of complex chemistry (carbon, nitrogen, oxygen compounds), and protection from harmful radiation.
Life Candidates in the Solar System
Several Solar System bodies are considered potentially habitable environments for microbial life:
- Europa (moon of Jupiter) — a subsurface ocean of liquid water is kept warm by tidal heating caused by Jupiter’s gravitational pull. The ocean is covered by an icy shell, but water-rock interactions at the ocean floor could support chemistry similar to deep-sea hydrothermal vents on Earth.
- Enceladus (moon of Saturn) — geysers of water vapour and ice erupting from the south polar region indicate an active subsurface liquid water reservoir. The Cassini spacecraft detected complex organic molecules and molecular hydrogen in these plumes, suggesting ongoing hydrothermal chemistry.
- Titan (moon of Saturn) — has lakes of liquid methane and ethane on its surface, a thick nitrogen atmosphere, and complex organic chemistry. Any life there would need to use methane as a solvent rather than water, representing a very different biochemistry from Earth life.
Europa and Enceladus have subsurface liquid water oceans sustained by tidal heating. Titan has liquid methane lakes and complex organic chemistry. None of these worlds are in the Sun’s habitable zone — internal heating, not sunlight, drives their liquid environments.
The Habitable Zone
The habitable zone (sometimes called the Goldilocks Zone) is the orbital region around a star where a planet with an Earth-like atmosphere could maintain liquid water on its surface. Too close and water evaporates; too far and it freezes. The habitable zone’s location depends on the star’s luminosity: hotter, more luminous stars have habitable zones farther out; cooler, less luminous red dwarf stars have them very close in.
Being in the habitable zone is a necessary but not sufficient condition for life. A planet also needs a suitable atmosphere, geological activity, and protection from stellar flares. Red dwarf stars are of particular interest because they are the most common type of star, but their close-in habitable zones mean planets there face strong tidal locking and intense radiation.
Several confirmed exoplanets lie within the habitable zones of their host stars. Proxima Centauri b, orbiting the nearest star to the Sun at 4.24 light-years, is within the habitable zone of its red dwarf host. The TRAPPIST-1 system, 39 light-years away, contains three planets in or near the habitable zone of an ultracool red dwarf. Determining whether these planets have atmospheres suitable for life is a major goal of the James Webb Space Telescope (JWST), which can analyse starlight filtered through a planet’s atmosphere during transits to identify molecular signatures.
The Drake Equation and SETI
In 1961, Frank Drake proposed an equation to estimate the number of communicating civilisations in the Milky Way. The Drake equation multiplies together factors including: the rate of star formation, the fraction of stars with planets, the fraction of those planets in the habitable zone, the fraction on which life arises, the fraction on which intelligence develops, the fraction that develop communicating technology, and the average lifetime of such civilisations.
Most factors are highly uncertain, but the equation frames the problem scientifically. The SETI programme uses large radio telescopes to search for artificial signals. Radio waves are chosen because they travel at the speed of light across interstellar distances, can carry encoded information, and pass through interstellar dust. A contact would have potential benefits (shared knowledge, new perspectives on cosmic civilisation) but also potential dangers (risks from an unknown civilisation’s intentions or from revealing Earth’s location). The Fermi paradox asks: if intelligent life should be common, why have we found no evidence of it?