Our Solar System was the only one we knew. Astronomers used it to explain how star systems were made from the leftovers of star formation, and why small rocky planets were closer to the Sun and gas giants farther away. But beware the theory based on only one example! We now know of thousands of planets around other suns, and the theories need revising. Here are a few of the astonishing planets we've found.

KELT-9 b, the hottest planet
Jupiter is five times farther from the Sun than the Earth is. However, a hot Jupiter like KELT-9 b is a giant planet that orbits close to its star – much closer than Mercury is to the Sun. About 650 light years away in the constellation Cygnus, this hot Jupiter is zipping around its star KELT-9 every 36 hours. It's not only thirty times closer to the star than Earth is to the Sun, but its star is twice as hot as the Sun.

KELT-9 b is tidally locked, so its dayside always faces the star. The cool side of the planet is 2000°C (3600°F), and the sunny side is more than twice as hot. The planet is hotter than most stars in the Galaxy. In fact, its atmosphere is quite starlike, containing vaporized heavy elements such as iron and titanium.

You may wonder why, if KELT-9 b is so starlike, that it isn't a star. But the tremendous heating is caused by KELT-9 – it's external to the planet. The heat of a star is generated internally by nuclear fusion.

This ferocious heat is evaporating the planet's atmosphere. If we could check out KELT-9 in a few million years, there would be no sign of the planet. All of its gas would have boiled away.

TrES-2 b, the darkest planet
As a star name, GSC 03549-02811 lacks the appeal of, say, Antares or Bellatrix, but the star is of great interest. It's similar in type and age to the Sun, and also has at least one planet. The star is 750 light years away in the constellation Draco (the Dragon), and unlike the Sun, it's the primary star of a binary system. The companion is an orange dwarf separated from it by 230 AU. (One AU – astronomical unit – is the distance from the Earth to the Sun.)

The usual system for naming an exoplanet would make the planet GSC 03549-02811 b. Fortunately, the planet is called TrES-2 b which is, at least, shorter. (Its name is derived from the Trans-Atlantic Exoplanet Survey.)

The planet TrES-2 b is nothing like any of the Solar System planets. It's another hot Jupiter, and it's also the darkest known planet. About as reflective as coal, it returns less than 1% of the light falling on it.

But why is TrES-2 b so dark? There seem to be three reasons. (1) Although Jupiter is brightened by its reflective ammonia clouds, at 1000°C (1800°F) it's too hot for clouds to form on TrES-2 b. (2) Probably, sodium and potassium and other light-absorbing chemicals, are vaporized into the atmosphere. (3) The first two reasons don't entirely account for the darkness, so there's another factor that astronomers don't yet understand.

Nonetheless despite its lack of reflectivity, TrES-2 b isn't completely black. According to researcher David Spiegel, “It's so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove."

PSR B1620-26 b, the oldest planet
Messier 4 (M4) is a globular cluster 12,400 light years away in the constellation Scorpius. A globular cluster is a densely-populated star cluster where the stars are pulled together by their mutual gravity into a spherical shape. M4 is somewhat more open than most globular clusters, but it has a dense core. Having stars close together might interfere with planet formation.

So astronomers wondered: could there be planets in a globular cluster? The answer turns out to be yes. In 1993 a surprising planet was discovered in M4. Its official designation is PSR B1620-26 b, but as it's 12.7 billion years old – only a billion years younger than the Universe – popular names for it are Methuselah and the Genesis planet.

Besides its age and location, Methuselah is an extraordinary planet in other ways. It was the first planet discovered in a circumbinary orbit. This means that it's in a binary star system, and it orbits both of the stars.

Methuselah is a giant with 2.5 times the mass of Jupiter, but what are the two stars like? They have at least one feature in common: they're both dead. The primary star PST B1620-26 is a pulsar. That's a rapidly rotating neutron star, which is the compressed core of a massive star destroyed in a supernova explosion. The secondary star WD B1620-26 is a white dwarf, which is the remnant of a sunlike star that collapsed, without an explosion, after its fuel was used up.

Piecing together the story of this complicated little system, astronomers think that the planet formed around the sunlike star that's now a white dwarf. While passing through the crowded core of M4, the pair was captured by a neutron star. Such a close stellar encounter would generally be rare in the Galaxy because the stars are so far apart, but it's possible in this higher density environment.

When the sunlike star used up its hydrogen fuel and swelled into a red giant, some of its matter spilled onto the neutron star, speeding up its rotation to form a pulsar. Eventually the red giant exhausted its fuel and became a white dwarf. The pulsar and the dwarf are now orbiting each other, and Methuselah orbits the pair at a distance of 23 AU. That's somewhat farther away than Uranus is from the Sun.

There could be a sad ending for Methuselah in the very distant future. Eventually, the trio will probably be disturbed by an encounter with another star. If so, the planet is likely to be ejected from M4 into the great empty space between the stars.