How planets are found

On 11 November 2026 NASA’s Exoplanet Archive listed 6,464 planets round other stars. Hardly any of them has ever been seen. Nearly all were found by watching a star very closely and catching it doing something that only a planet could cause. Four methods account for all but a handful.

The wobble came first for stars like the Sun. A planet does not simply circle its star. The two circle each other, so the star is swung round in a small circle of its own. As it comes towards us its light is squeezed a little bluer, and as it goes away its light is stretched a little redder. In 1995 this showed a giant planet going round the star 51 Pegasi once every four days.

The second has found the most by far. If a planet’s path happens to cross the face of its star as seen from here, the star dims a tiny bit each time it passes. One space telescope, Kepler, stared at 150,000 stars for four years, waiting for dips that came back on time. Dips like that have given 4,797 of the planets on the list.

The third is the obvious one, and the hardest: take a picture. This is a real one. The planet in it was first photographed in 2004. The red dot is a planet about five times the mass of Jupiter. It can be seen only because it is young and still glowing with heat, and because the thing it circles is a dim brown dwarf, not a bright star. Only 98 planets have been found this way.

The fourth uses gravity as a lens. When one star drifts exactly in front of another, far more distant one, its gravity bends the far star’s light and makes it brighten for a while. If the nearer star has a planet, the planet adds a short flicker of its own. This has found 292 planets, and it is best at the ones far out from their stars.

No single method tells the whole story. A dip gives how wide a planet is. A wobble gives how heavy it is. Put the two together and you have its density, which says whether it is a ball of rock or a ball of gas. That is why astronomers try to catch the same planet two ways.