Astronomers were looking for alien gas giants around bright stars when they suddenly came across an unusual find: a hyper-inflated planet. Orbiting around the brightest star in the Souther Hemisphere known to host a transiting planet is one of the ‘puffiest’ planets in the universe. Though nearly 40% bigger than Jupiter, it’s only a fifth as massive as our backyard gas giant. That puts it at a density nearing styrofoam — incidentally, the stuff school children often use to build model solar systems.  Beyond the sheer ‘wow’ factor, this research might lead to opportunities that test the atmospheres of faraway planets and look for signs of life.

Artist impression of KELT-11b, a ‘styrofoam’-density exoplanet orbiting a bright star in the southern hemisphere. Credit; Walter Robinson/Lehigh University.

The ‘styrofoam planet’ orbits KELT-11, the sixth brightest transit star discovered so far. KELT-11 — located some 320 light-years away– lives fast, though. In only a hundred million years, the star will run out of fuel and turn into a red giant, engulfing our adorable styrofoam planet called KELT-11b. Already, KELT-11b is dangerously close to its parent star traveling in an orbit that lasts less than five days.

‘Extraordinarily inflated’

The system was named in honor of the astronomical survey responsible for the discovery, the Kilodegree Extremely Little Telescope or KELT for short. It’s made up of two telescopes, one in Arizona and the other in South Africa, that collectively scan the night’s sky for about five million stars. The astronomers who operate KELT, among them more than three dozen collaborators from Vanderbilt University, Ohio State, and the South African Astronomical Observatory to name a few, look for stars that seem to dim slightly at regular intervals. This behaviour indicates a planet must be orbiting a parent a star, momentarily eclipsing the star as it passes between it and Earth. The dimming or ‘transit’ is verified by measuring the gravitational ‘wobble’ (a slight tug a planet exerts on the parent star).

Most of the planets outside the solar system, something which scientists call ‘exoplanets’, orbit around faint stars which makes measuring their properties difficult. Unlike other exoplanet-hunting instruments like Kepler, the KELT survey is specifically designed and tasked with finding valuable planets orbiting around very bright stars.

Though the team led by Joshua Pepper, astronomer and assistant professor of physics at Lehigh University, was setting out to find gas giants in bright star systems, no one expected to come across such a low mass, large planet like KELT-11b.

It’s still not clear what might have caused KELT-11b to be so inflated — about twice as big as scientists can explain. Pepper and colleagues hope to learn more once powerful telescopes like Hubble, Spitzer, and hopefully the James Webb Space Telescope, will try to peer through KELT-11b’s atmosphere. The fact that the puffy exoplanet’s atmosphere is so big, however, will provide a good opportunity to develop novel techniques meant to identify chemicals in the atmosphere. Ultimately, such measurement techniques might assess the habitability of certain planets, the authors reported in The Astronomical Journal.

“We don’t know of any real Earthlike planets or stars for which we can measure their atmospheres, though we expect to discover more in future years,” Pepper said in a statement. “These (giant gas) planets are the gold standards or testbeds for learning how to measure the atmospheres of planets.”

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