WASP_17b is a huge planet, twice the size of Jupiter that orbits a yellow-white dwarf star similar to the Sun, around 1,000 light years from Earth. It’s considered a ‘hot Jupiter’ due to the extreme proximity of its orbit with its parent star. It has a density around half that of Jupiter and has one of the lowest known densities of all the planets. It’s a combination of the baking heat it endures as well as the tidal forces of its nearby host star’s gravity, which is suspected to have caused WASP_17b to inflate to its enormous size and low density.

WASP_17b has an estimated equatorial radius of just over 136,000 kilometres (84,500 miles), which makes it bigger than some small main-sequence stars. This includes OGLE_TR_122B, a tiny star with a mass that borders on the lower limit for hydrogen fusion in stars. This star is just over half the size of the giant exoplanet, but 50 times denser.

WASP_17b was discovered in 2009 and though its size made it a compelling subject, its orbit was of even more interest. Other objects in the same system were orbiting in the right direction but WASP_17b was travelling contrary to the spin of its host star. This retrograde orbit may have been caused by a close encounter with another object that caused the planet to slingshot in the opposite direction.

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