Planets round other stars

The Sun is a star, and it has planets. For centuries people wondered whether other stars have planets too. Until the 1990s nobody knew.

Now we know that most of them do. More than 6,000 planets have been found round other stars, and there are probably more planets in our galaxy than there are stars.

They are very hard to see. A planet gives out no light of its own, and it sits right beside a star that is millions or billions of times brighter. Looking for one is like looking for a moth beside a lighthouse from hundreds of kilometres away.

So astronomers watch the star instead. If a planet passes in front of its star, the star dims by a tiny amount for a few hours. If the dimming comes back again and again, always the same, there is a planet going round.

Many of the planets found this way are nothing like ours. Some are giants so close to their star that a year lasts three days.

A planet round another star is called an exoplanet. The first round a star like the Sun was found in 1995 by Michel Mayor and Didier Queloz, who shared a Nobel Prize for it in 2019. It orbits the star 51 Pegasi, about 50 light years away. By late 2025 the count of confirmed planets had passed 6,000, and it grows every month.

Hardly any of these have been photographed. Nearly all were found through what the planet does to its star.

Method	What is measured	Share of discoveries	Tells you
Transit	The star dims slightly as the planet crosses in front	About three quarters	The planet’s size and the length of its year
Wobble	The star’s light shifts bluer, then redder, as the planet tugs it to and fro	About a fifth	The planet’s mass (strictly, the least it can be) and its year
Direct picture	The planet’s own faint glow, with the star’s light blocked out	About 1 in 100	What its light is made of
Lensing	A distant star brightens briefly as a planet’s gravity bends its light	A few in 100	Mass, including of planets far from their star

Transits. The dip is small. Jupiter crossing the Sun would dim it by 1 part in 100. The Earth would dim it by less than 1 part in 10,000, for about 13 hours, once a year. Catching that needs a telescope above the atmosphere watching without a break. The Kepler spacecraft stared at about 150,000 stars in one patch of sky from 2009 to 2013 and found more than 2,600 planets. Its successor, TESS, has been surveying nearly the whole sky since 2018.

A transit only happens if the orbit is edge-on to us. For a planet at the Earth’s distance from a star like the Sun, the chance is about 1 in 200. So for every planet found this way, there are many more that never cross.

The wobble. A star and its planet both circle their shared centre of mass. The star’s circle is small, but its light gives it away. Dark lines in its spectrum shift a little towards blue as it comes towards us and towards red as it goes away. Jupiter moves the Sun at about 12 metres a second, the speed of a sprinter. The Earth moves it at 9 cm a second. The planet of 51 Pegasi moves its star at more than 50 metres a second, which is why it was found first.

Both together. A transit gives a planet’s size and the wobble gives its mass. Mass divided by volume is density, and density says whether a planet is rock, water or gas.

What has been found. Surveys find most easily what is large and close to its star, so the list is not a fair sample. Even so, some results are clear.

• Hot Jupiters. Giant planets with years of a few days. The planet of 51 Pegasi has about half Jupiter’s mass and goes round in 4.2 days. They are rare, found round about 1 Sun-like star in 100, but easy to detect.
• Super-Earths and sub-Neptunes. Planets between the Earth and Neptune in size are among the commonest kind known. The Solar System has none.
• Small rocky planets are common, particularly round red dwarf stars. The nearest star of all, Proxima Centauri, 4.25 light years away, has a planet of at least the Earth’s mass with a year of 11 days.

Our own arrangement, with small planets inside and giants outside on nearly circular orbits, is one pattern among many. How common it is cannot yet be said, because a copy of the Solar System would be very hard to find with today’s instruments.

Reading the air. During a transit, a little starlight passes through the planet’s atmosphere on its way to us, and the gases there leave their marks in the spectrum. The Webb telescope has measured water vapour, carbon dioxide and other gases this way in the air of giant planets. Doing the same for a small rocky planet is at the limit of what is possible today.