Are we alone?

Finding exoplanets

A planet orbiting another star is an exoplanet. Exoplanets are very hard to see directly, because they are tiny, faint and lost in the glare of their star, like spotting a firefly next to a lighthouse from thousands of kilometres away. So most are found indirectly, by their effect on the star.

The first exoplanets were found in 1992, orbiting a pulsar. In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first planet around a Sun-like star, and shared the 2019 Nobel Prize in Physics. It was a surprise: a planet half the mass of Jupiter orbiting so close to its star that its year lasts just 4.2 days. Planets like this, called "hot Jupiters", have no equivalent in our Solar System.

The transit method

If a planet's orbit is lined up edge-on to us, the planet passes in front of its star once each orbit and blocks a little of its light. A graph of the star's brightness over time (a light curve) shows regular dips.

• The depth of the dip tells you the planet's size: the fraction of light blocked = (planet radius ÷ star radius)².
• The time between dips is the planet's orbital period, its year. Using Kepler's third law, the period and the star's mass give the orbit's size.

NASA's Kepler space telescope (2009–2018) watched about 150 000 stars continuously and found more than 2600 planets this way. The TESS mission (since 2018) surveys almost the whole sky, looking for planets around nearby bright stars.

The wobble method

A planet and its star both orbit their shared centre of mass, so the star wobbles slightly. When it moves towards us its light is blueshifted, and when it moves away, redshifted (the same Doppler effect used in 6F). Measuring these tiny shifts is called the radial velocity method, and it gives the planet's minimum mass. Jupiter makes the Sun wobble at about 12.5 m/s; the Earth, only about 9 cm/s, slower than a walking pace.

Combining both methods gives the planet's size and its mass, and so its density, which tells us whether it is rocky like Earth or gassy like Neptune.

Gravitational microlensing

When a star passes almost exactly in front of a more distant star, its gravity acts like a lens and briefly magnifies the background star. A planet around the nearer star causes an extra short spike. Australian astronomer Penny Sackett, later Australia's Chief Scientist, led an international team that used telescopes around the world, including in Australia, to watch microlensing events around the clock. The method can find planets thousands of light years away, including ones far from their stars and even "rogue" planets with no star at all.

What makes a planet habitable?

Life as we know it needs liquid water, a source of energy and the right chemical elements. About 96% of the mass of living things is made of just four elements, carbon, hydrogen, oxygen and nitrogen, with phosphorus and sulfur also essential (often remembered as CHNOPS).

The habitable zone (sometimes called the Goldilocks zone) is the range of distances from a star where a planet with an atmosphere like Earth's could have liquid water on its surface. Too close, and water boils away; too far, and it freezes. For a dim red dwarf the zone is tiny and very close to the star; for a hot, bright star it is wide and far out.

Being in the habitable zone isn't enough. Venus is near its inner edge, but a runaway greenhouse effect has made its surface 465 °C. Mars is inside it, but it lost its magnetic field and most of its atmosphere, so liquid water can't last on its surface today. A planet also needs the right mass to hold an atmosphere, and a star that is stable for billions of years.

Red dwarfs are the most common stars, and many have rocky planets in their habitable zones. The nearest exoplanet, Proxima Centauri b (found in 2016), is just 4.2 light years away and in its star's habitable zone. The TRAPPIST-1 system, 40 light years away, has seven Earth-sized planets, three or four in the habitable zone. But red dwarfs often blast their planets with flares, and JWST measurements in 2023 showed the innermost TRAPPIST-1 planets have little or no thick atmosphere. Whether any red-dwarf planet can keep one is a key open question.

Searching for life

Earth's own record helps: some of the oldest evidence of life on Earth is in 3.5-billion-year-old fossil stromatolites (layered mounds built by microbes) in the Pilbara region of Western Australia. Living stromatolites still grow at Shark Bay. Places like the Pilbara are used to test the instruments and methods used to search for life on Mars.

• Mars. Rovers have shown Mars once had lakes and rivers. In 2024 Perseverance found rock in Jezero Crater with tiny "leopard spot" patterns that, on Earth, are often made by microbes. They aren't proof of life, because chemistry alone might make them, so scientists want the samples brought back to Earth for testing.
• Europa and Enceladus. These icy moons of Jupiter and Saturn have salty liquid-water oceans under their ice. Cassini flew through jets of water from Enceladus and found salts, organic molecules and phosphorus. NASA's Europa Clipper launched in October 2024 and will reach Jupiter in 2030 to study whether Europa could support life.
• Exoplanet atmospheres. When a planet transits, a little starlight passes through its atmosphere, and the gases absorb certain wavelengths. JWST can read these absorption lines. A biosignature is a gas, such as oxygen with methane, that is hard to explain without life. In 2023 JWST found methane and carbon dioxide on the planet K2-18b, and in 2025 one team claimed a gas that on Earth is only made by life. Other scientists reanalysed the data and found the signal was weak and could be explained in other ways. This is a good example of how science treats an extraordinary claim: it needs extraordinary evidence, checked by independent teams.
• SETI (the Search for Extraterrestrial Intelligence) looks for signals that only technology could make. The Breakthrough Listen project uses Murriyang at Parkes among others. In 2020 it found a candidate signal, BLC1, that seemed to come from the direction of Proxima Centauri. Careful checks showed it was interference from equipment on Earth.

So far there is no confirmed evidence of life beyond Earth. But as the saying goes, absence of evidence is not evidence of absence: we have barely begun to look.

Where we fit in: the cosmic calendar

Imagine squeezing the whole 13.8-billion-year history of the universe into one calendar year, so that each day is about 38 million years and each second is about 440 years.

Modern humans have existed for only about 0.002% of the age of the universe. Yet in that short time, and mostly in the last century, we have worked out how old the universe is, what stars are made of, and where the atoms in our bodies came from.