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51 Pegasi b: Your First Exoplanet While there is much debate over which exoplanet discovery is the ‘first’, 51 Pegasi b stands out. Discovered in 1995, it is changing the way we see the universe and our place in it. The exoplanet is about half the mass of Jupiter, with a seemingly impossible, star-hugging orbit of only 4.2 Earth days. Not only was it the first planet confirmed to orbit a sun-like star, it also ushered in a new class of planets called hot Jupiters: hot massive planets orbiting closer to their stars than Mercury