51 Peg b: A World Beyond Expectation

On the anniversary of one of the most transformative discoveries in astronomy, author Niall Deacon takes us on a journey around our solar system and beyond.


Twenty five years ago today the discovery of the first planet found orbiting another star similar to the Sun was announced. This result began a rush of scientific creativity that has created one of the most exciting and dynamic fields in all of science. But it wasn’t the planet anyone had been expecting to find.

Our solar system has eight planets. They span a wide range of temperatures, sizes, compositions and distances from the Sun. As humans we love putting things into categories and luckily the planets in the solar system all seem to fall into one of three categories. Firstly you have the terrestrial planets: Mercury, Venus, Earth and Mars. These are rocky and have relatively thin atmospheres. Then you have the gas giants: Jupiter and Saturn. These are much more massive than the terrestrial planets. They still probably have rocky cores but these are surrounded by enormous atmospheres of hydrogen and helium. The final category of worlds is the ice giants: Uranus and Neptune. These worlds have rocky cores and atmospheres of hydrogen and helium but with an intermediate layer consisting of strange high pressure forms of water. methane and ammonia that these worlds accreted from chunks of ice in the frigid outer regions of the early solar system.

Looking at the order of the worlds in our solar system you can see a pattern emerge. The terrestrial planets are closest to the Sun with orbital periods of a few months to a few years, then come the gas giants circling the Sun every ten to thirty years, then the ice giants which take eighty to a hundred and sixty five years to orbit our central star. So of course when imagining other solar systems astronomers thought they would follow a similar pattern. How wrong they were.

It’s very hard to directly take a picture of a planet orbiting another star. Firstly exoplanets (planets orbiting other stars) are much fainter than stars. Secondly, because exoplanets orbit another star they appear very close to that star when viewed from the Earth. Being faint and appearing close to a bright star makes it hard to directly take a picture of an exoplanet.

As a result astronomers started looking for clever tricks to find planets orbiting other stars. One such trick was to make use of a subtle motion a planet induces on the star it is orbiting. It’s easy to picture a planet orbiting a star. The star stays in the middle and the planet goes round it. This picture isn’t strictly true. The planet and the star both orbit a point in-between them called the centre of mass. This means that when a planet goes around in its orbit, the star also makes a tiny orbit around the centre of mass. This motion produces a subtle change in the star’s spectrum (the star’s light split into different colours) that can be spotted from a telescope on Earth.

Like many astronomers around the world Michel Mayor and Didier Queloz had been studying stars similar to the Sun, looking for subtle changes in each star’s spectrum that might be caused by an orbiting planet. They noticed a strange pattern in the spectrum of a star called 51 Peg. The star seemed to be moving around, back and forward in a tiny orbit. But the period of the pattern was very short, repeating once every four days. So this must be a planet orbiting very close to its parent star. But the size of the pattern was huge, suggesting a planet at least half as large as Jupiter.

On the 6th of October 1995 Mayor and Queloz announced their discovery of 51 Peg b, a planet at least half the mass of Jupiter, orbiting its star seven times closer than Mercury orbits the Sun. This was a world beyond expectation, truly alien compared to our own solar system worlds.

In the intervening 25 years astronomers have discovered over 4000 exoplanets. Some have been similar to our own familiar Earth, some utterly alien. There are rocky worlds with the right temperature for liquid water, worlds torn to shreds by dead stars, worlds floating through the cold wilderness of space alone, worlds found by some of the largest telescopes on Earth, and worlds found with some of the smallest telescopes used by professional astronomers.

51 Peg b is one of the exoplanets discussed in Twenty Worlds: The Extraordinary Story of Planets Around Other Stars. Available now from Reaktion Books.

– Niall Deacon

To read more about exoplanets and their discoveries, order Twenty Worlds from our online shop now.

Niall Deacon is an astronomy researcher and writer, and lives in Heidelberg, Germany. His research focuses on failed stars called brown dwarfs and giant planets orbiting other stars.