Exoplanets and the search for life

An exoplanet is a planet orbiting a star other than the sun. For centuries astronomers suspected such planets existed but had no way to detect them, since planets are tiny and faint next to the stars they orbit.

The first confirmed exoplanets were found in 1992 orbiting a pulsar, the remnant of a dead star. In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first around a sun-like star, work that earned them a share of the 2019 Nobel Prize in Physics. Today more than 5,000 exoplanets have been confirmed.

• Transit method: a planet passing in front of its star causes a tiny, regular dip in brightness; NASA's Kepler and TESS missions found thousands this way.
• Radial velocity: a planet's gravity makes its star wobble, shifting the star's light slightly back and forth in colour.
• Direct imaging: blocking a star's glare to photograph large planets far from their stars.
• Gravitational microlensing: a star and planet passing in front of a more distant star briefly magnify its light.

Exoplanets turned out to be far more varied than the solar system suggested. Hot Jupiters are gas giants orbiting so close to their stars that a year lasts only days. Super-Earths and mini-Neptunes, planets between Earth and Neptune in size, are among the most common types found, yet none exist in our own solar system.

Some planets orbit two stars, like the fictional Tatooine, and some systems pack several planets into orbits smaller than Mercury's. The TRAPPIST-1 system, about 40 light-years away, has seven roughly Earth-sized planets orbiting a small, cool star.

Life as we know it needs liquid water. The habitable zone, sometimes called the Goldilocks zone, is the range of distances from a star where a planet's surface could be neither too hot nor too cold for liquid water, assuming a suitable atmosphere.

Being in the habitable zone does not guarantee a planet is habitable. Its atmosphere, magnetic field, geology and the activity of its star all matter. Many small stars emit powerful flares that could strip away the atmospheres of nearby planets.

When a planet transits its star, some starlight passes through the planet's atmosphere, and molecules there absorb particular colours. By splitting that light into a spectrum, astronomers can identify gases such as water vapour, carbon dioxide and methane.

The James Webb Space Telescope has measured the atmospheres of several exoplanets in this way. Scientists hope to eventually detect biosignatures, combinations of gases, such as oxygen with methane, that would be hard to explain without life. Any such claim will require extraordinary care, since non-living processes can mimic some signals.

The search for life also focuses on our solar system. Mars once had rivers and lakes, and rovers are looking for chemical traces of ancient microbes. Jupiter's moon Europa and Saturn's moon Enceladus have oceans beneath their icy crusts, and Enceladus sprays water plumes containing organic molecules into space.

Missions such as NASA's Europa Clipper, launched in 2024, aim to study these ocean worlds. Meanwhile, projects in the search for extraterrestrial intelligence listen for artificial radio signals. So far no evidence of life beyond Earth has been confirmed.