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Our Solar System was the only one we knew. Astronomers used it to explain how star systems were made. 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.

Exoplanets - Hottest, Darkest, Oldest

We can't actually see KELT-9 b, it's too close to its extremely bright star. It was discovered using the Kilodegree Extremely Little Telescope (KELT), which was able to detect a tiny drop in the starlight as the planet passes between us and the star. So for a picture we have to rely on an artist's impression of KELT-9 b made using the available data.

From the data, the discovery team tells us that the planet's dayside (about 4300°C) glows a bright orange and the nightside (a mere 2000°C) would be a deep, dark red. The star's extreme heat and UV radiation are evaporating the planet's atmosphere at a rate of several million tonnes a second.

[Image credit: NASA/ESA/G. Bacon]

TReS-2 b (also known as Kepler-1 b) is as dark as coal or black acrylic paint. It reflects almost no light. It's shown in the artist's impression as black against its star. However, researchers say that although it doesn't reflect any starlight, it would have a soft reddish glow simply because it is so hot - rather like an ember.

The planet PSR B1620-26 b (aka Methuselah) is located in the globular cluster M4. There are many images of M4, but here's an image that locates it in the sky. This amazing wide field (about 50 degrees across) view of the Galactic plane took over 100 hours of sky imaging plus a lot of digital processing.

On the far left, the band of the Milky Way runs diagonally. Along the far right is the colorful Rho Ophiuchus region - the bright orange star is Antares (the heart of the Scorpion. There are a number of famous nebulae in front of the Milky Way band. Note that ,mostly, red means emission nebulae glowing from excited hydrogen gas. Blue reflection nebulae, where dust is scattering the red light, but reflecting the blue. You'll find M4 and several other Messier objects on the right hand side.

Image Credit & License: Rogelio Bernal Andreo (Deep Sky Colors)

A number of planets orbiting stars outside the Solar System have been directly imaged, but most of the 4000 known exoplanets haven't been. Very few have been discovered by direct imaging. How do astronomers find what they can't see?

Searching for Exoplanets