<\/p>\n “This isn’t what we expected at all,” Max Kroft, lead author of the study, said in the statement<\/a>. “Dense planets like this aren’t uncommon, but they’re usually small rocky planets similar to Earth or Mercury. This planet is two and a half times bigger than the Earth.”<\/p>\n GJ 523b was initially identified as a candidate by NASA’s Transiting Exoplanet Survey Satellite<\/a> (TESS), which searches for periodic dips in starlight caused when planets cross, or transit, their host stars. Researchers followed up with observations from the ground-based WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona, using a spectrograph to measure the planet’s gravitational tug on its star.<\/p>\n Combining those observations, the team calculated that GJ 523b has a mass about 23.5 times that of Earth, a radius 2.55 times larger and a density of about 126.82 grams per cubic inch. It circles its star<\/a> every 17.75 days. The planetary system is also relatively young, at an estimated nearly 170 million years old, according to the study.<\/p>\n You may like<\/p>\n That combination of size, mass and youth poses a puzzle for planet formation models. Planets begin by building cores of rock and metal, which can then pull in hydrogen and helium from the disk of gas and dust surrounding a young star. In our own solar system<\/a>, giant planets such as Jupiter and Saturn are thought to have begun rapidly accumulating their massive gaseous envelopes once their growing cores reached roughly 20 times Earth’s mass.<\/p>\n GJ 523b is already about 23 times Earth’s mass<\/a>, putting it beyond that threshold. Yet instead of developing into a gas-rich world, its unusually high density suggests it has relatively little gas and remains predominantly rocky \u2014 raising the question of why it followed such a different path.<\/p>\n “The question is, why didn’t this planet do that, if it’s 20 times the size of Earth?” Kroft said in the statement.<\/p>\n One possibility is that GJ 523b initially formed with a thick atmosphere that was later stripped away. Another is that it formed through a collision<\/a> between two planets, creating a larger rocky world while blasting much of their gaseous envelopes into space, according to the statement.<\/p>\n![\"Space\"](\"https:\/\/www.space.com\/media\/img\/spc-logo-w.svg\"){kind=logo}
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