Twinkle, twinkle little star: The diamond planet - the shrunken core of a once-huge star - is a chunk of crystallised carbon measuring 40,000 miles across, five times the diameter of Earth

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Fun science fact:
FYI Your girlfriend's new engagement ring is bigger than yours. No matter how big her rock is, it's no match for BPM 37093, a.k.a. Lucy. What's that, you say? It's a star that's made of diamond. Located in the constellation Centaurus, Lucy has 10 billion trillion trillion carats.
(Brings a whole new meaning to the song "Lucy in the Sky With Diamonds."

 

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http://www.dailymail.co.uk/sciencetech/article-2030138/Is-biggest-rock-Astronomers-believe-entire-planet-diamonds.html