Exoplanets: Worlds Around Other Stars

For most of human history, the only planets anyone knew were the handful that wander across our own sky. Astronomers assumed other stars had planets too, but they had no way to see them. A planet is tiny, dark and sits right next to a star billions of times brighter, like a firefly beside a searchlight. Then in 1995 a team in Switzerland found a planet half the mass of Jupiter orbiting the star 51 Pegasi. Within three decades the count passed five thousand. We now think planets outnumber stars.

Finding what you cannot see

Almost no exoplanet has been photographed directly. Instead, astronomers watch the star. The first method looks for a wobble. A planet and its star orbit a common centre of mass, so the star moves in a tiny circle, toward us and away from us. That motion shifts the colour of its light very slightly, blue then red then blue, and sensitive instruments can detect the rhythm. The size of the wobble tells you the planet's mass.

The second method, which has found most known planets, looks for a dip. If a planet's orbit happens to carry it between us and its star, the star dims by a minuscule amount, typically less than one percent, for a few hours. The dip repeats every orbit like clockwork. NASA's Kepler telescope stared at 150,000 stars for four years watching for exactly this, and found more than 2,600 planets. The depth of the dip tells you the planet's size. Combine size with mass and you know its density, which tells you whether it is rock, ice or gas.

A zoo of worlds

Our solar system turned out to be a poor guide to what is out there. The very first discoveries were "hot Jupiters", giant planets orbiting closer to their stars than Mercury does to the Sun, completing a year in a few days, with daysides hot enough to vaporise iron. Nobody expected such things; giant planets are supposed to form far out where it is cold. Many must have migrated inward after forming.

Then came "super-Earths" and "mini-Neptunes", planets between the size of Earth and Neptune. There is nothing like them in our system, yet they appear to be the most common planets in the galaxy. Some may be rocky worlds with thick atmospheres; others could be water worlds with oceans hundreds of kilometres deep. There are planets orbiting two stars at once, planets around dead stars, and planets drifting through space alone, flung out of their systems entirely.

The habitable zone

The question everyone asks is whether any of them could host life. Astronomers start with a simple test: is the planet at the right distance from its star for liquid water to exist on the surface, neither boiling nor frozen? That band is called the habitable zone. Earth sits in the Sun's. Dozens of roughly Earth-sized planets have been found in the habitable zones of other stars.

The most famous system is TRAPPIST-1, a small, cool red star 40 light-years away with seven planets, all roughly Earth-sized, three in the habitable zone. The whole system would fit inside Mercury's orbit. Being in the zone, though, is not enough. Venus is nearly in ours and is a furnace; Mars is at its edge and is a frozen desert. Atmosphere matters as much as distance.

Sniffing alien air

When a planet transits its star, a little starlight passes through the planet's atmosphere on its way to us, and the gases in that atmosphere leave fingerprints in the light. The James Webb Space Telescope, launched in 2021, is sensitive enough to read them. It has already found water vapour, carbon dioxide, methane and sulphur dioxide in the atmospheres of distant worlds. The dream is to detect a combination of gases, oxygen alongside methane for example, that on Earth is produced only by living things.

What we still don't know

We do not yet know whether any exoplanet has life, or even a surface ocean. We do not know why super-Earths are so common when our solar system has none, or how hot Jupiters end up so close to their stars. And despite thousands of discoveries, we have not found a true twin of Earth: a rocky planet of the same size, in the same orbit, around a star like the Sun. That is partly because such planets are the hardest to detect. Within a generation, new telescopes should tell us whether they are rare, or everywhere.