There are around four thousand known planets, all but eight of them orbiting suns other than our own. They're called exoplanets or extrasolar planets, and some of them are mind-boggling. Here are half a dozen exotic specimens.

The first hot Jupiter
Two Swiss astronomers made history in 1995 when they discovered the first exoplanet orbiting a Sun-like star: 51 Pegasi b.

Exoplanets are named using a lower case letter after the star's name. 51 Pegasi is the star and “b” is for the first planet discovered. If someone finds another one, it will be 51 Pegasi c. Unofficially, 51 Pegasi b was known as Bellerophon, after the legendary Greek hero who rode Pegasus the winged horse. Since then it has officially been named Dimidium for reasons which escape me.

In the Solar System, planets near the Sun are small and rocky; the large gas planets are farther out. Astronomers were amazed to find a giant planet circling its star in just over four Earth days. At first, everyone assumed that the new planet was some sort of monster rocky planet, only later realizing that it was a gas giant. 51 Pegasi b was the first of the planets now known as hot Jupiters.

Hot Jupiters are tidally locked to their stars, so the same side always faces the star, just as the same side of the Moon always faces Earth. For hot Jupiters this means that one side is vastly hotter than the other. Our own Jupiter has a fairly even temperature.

Yet hot Jupiters aren’t all alike. Here are two quite different ones.

Space ember
One of the hottest and blackest planets known is HD 149026 b. It orbits the star numbered 140926 in the Henry Draper (HD) catalog.

Gas giants are mostly hydrogen and helium. However HD 149026 b not only contains heavier elements, but also has a large dense core. The temperature is over 2000 °C (3700 °F) on the sunny side, hot enough to melt iron and boil silicon. The atmosphere probably consists of metal oxides like vanadium and titanium.

The opaque upper layers absorb almost all of its star's light, reflecting nothing back. This would make the planet blacker than coal, but it radiates heat, so HD 149062 b would glow like a dying ember.

It's a corker
Although HD 149026 b is dense, HAT P-1 b would bob like a cork if there were a big enough cosmic pool. Its density is about a quarter that of water. HAT P-1 b was discovered by HAT, a robotic telescope network monitoring the sky for transits (the dimming of a star when a planet passes in front of it). Its diameter is over 170,000 km (107,000 miles).

Tatooine?
Interestingly, in addition to its corklike density, HAT P-1 b orbits a star in a double star system.

Scientists had argued about whether binary stars could have planets, because of the complex gravitational forces involved. The argument was settled by finding planets such as HAT P-1 b in binary systems orbiting one of the stars.

The first exoplanet found orbiting both of its suns is Kepler-16 (AB) b. Kepler-16 (AB) b isn't a hot Jupiter snuggled up next to a star. It completes an orbit every 229 Earth days, similar to the time it takes Venus to orbit the Sun. Both stars are cooler than our Sun, and the planet is far enough from its star to be a chilly -100 °C (-150 °F).

But you couldn't stand on Kepler-16 (AB) b to watch the suns set, because it's a gas planet similar to Saturn. Nonetheless George Lucas was pleased by its discovery in 2011. In the Star Wars films, Luke Skywalker's home planet Tatooine has a double sun.

It rains rock
CoRoT-7 b, discovered by the French space telescope CoRoT, is a rocky planet, but it's not like home. It's much larger than Earth, so is a superearth. Like the hot Jupiters, it's much closer to its star than Mercury is to the Sun, and is tidally locked.

CoRoT-7 b's shady side is -187 °C (-369 °F), but the sunny side is 2300 °C (4200 °F), hot enough to melt rock. Its surface would have lava oceans. And its atmosphere would be vaporized rock, so when it rains, the atmosphere would condense into pebbles, not raindrops.

Not all exoplanets are big and hot, but these are the easiest to find. (See “Searching for Exoplanets” to learn more.) However, NASA's Kepler mission found many smaller planets. Kepler is a space telescope which has been hunting for habitable planets. Some of its smaller finds have also been in the habitable zone of their stars. The habitable zone is the area in which water could exist as a liquid.

Planet-sized diamond
To finish, here's something completely different.

The very first extrasolar planets were discovered in 1992 by Aleksander Wolszczan and Dale Frail. They were orbiting a pulsar, which was a bigger shock than the hot Jupiters that followed. A pulsar is a spinning neutron star that gives out strong radio signals. It's the end product of a supernova explosion, so no one expected planets. Yet a number of pulsars are now known to have planets.

Pulsar PSR J1719-1438 and its planet PSR J1719-1438 b were once a double star system. When the massive star exploded as a supernova, the two stars stayed separate instead of merging. However the pulsar's gravity stripped its companion of everything except its carbon core. On Earth, carbon in the Earth's interior, under high pressure, forms diamonds. Astronomers think this has happened to the former star. A planet-sized diamond! The ultimate bling.