First look
The idea, in plain words
Every dot here is a real planet going round another star. There are more than 6,000 of them, as NASA’s list stood on 11 November 2026. Each is placed by two things that have been measured: how wide it is, and how long it takes to go once round its star. Small planets are at the bottom and giants at the top. Short years are on the left and long ones on the right.
Now put our own eight planets on the same chart. They sit off to one side, where there are hardly any dots. Most of the planets found so far are far closer to their stars than ours are. More than four in every five have a year shorter than Mercury’s 88 days.
Part of that is the way we look. A big planet close to its star tugs harder on the star and blocks more of its light, and it comes round again every few days, so it is the easiest kind to catch. Planets like ours, small or far out, are much harder to find. Many more must be there than we have seen.
The first kind to turn up was a shock. In 1995 a giant planet was found going round a star like the Sun once every four days. Nobody had expected a giant so close in. Hundreds of these hot Jupiters are known now. They are heated to thousands of degrees, and they are thought to have formed further out and moved inward.
The commonest kind found so far lies between the Earth and Neptune in size. More than half of the planets on the chart are in that band, and the Solar System has nothing in it at all. The smaller ones are called super-Earths and may be rock. The larger are called mini-Neptunes and are probably wrapped in deep gas. Nobody yet knows what most of them are really like.
Some are stranger still. A planet called 55 Cancri e, 41 light years away, goes round its star in 18 hours. It always keeps the same face to the star. That side is at about 2,400 degrees, hot enough for the rock to be molten.
Hardly any of these worlds has been seen directly. This is one that has. The bright spot is a planet a few times as heavy as Jupiter, still forming inside the disc of gas and dust round a very young star. The star itself has been blocked out so that its glare does not drown the planet.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Exoplanet
- A planet that orbits a star other than the Sun.
- Hot Jupiter
- A giant planet that orbits very close to its star, with a year lasting only a few days.
- Super-Earth
- A planet bigger than the Earth but smaller than Neptune.
- Gas giant
- A very large planet made mostly of hydrogen and helium, with no solid surface: Jupiter and Saturn.
- Ice giant
- A large planet made mostly of water, ammonia and methane under a hydrogen and helium atmosphere: Uranus and Neptune.
- Terrestrial planet
- A planet made mostly of rock and metal with a solid surface: Mercury, Venus, the Earth and Mars.
- Tidal locking
- When a moon turns exactly once for each orbit, so the same face always points at its planet.
- Protoplanetary disc
- The flat, spinning disc of gas and dust round a newborn star, out of which its planets grow.
Where this comes from
- NASA Exoplanet Archive, Planetary Systems Composite Data (fetched 11 November 2026)
- NASA, Exoplanet types
- NASA, Gas giants and hot Jupiters
- NASA, Super-Earths
- ESO, First confirmed image of a newborn planet caught with the VLT (2018)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.