Exoplanet As Dense As Styrofoam Might Teach Us A Lot About Planetary Atmospheres

Artist's impression of KELT-11b. Walter Robinson/Lehigh University

KELT-11b is quite the unusual object. It’s the brightest transiting exoplanet visible in the southern hemisphere, it’s one-fifth as massive as Jupiter, and it orbits its star very closely. It’s also so puffy that its density is similar to Styrofoam. KELT-11b would easily float in water.

As reported in the Astrophysical Journal, KELT-11b is a phenomenal target for astronomers. Its inflated atmosphere, which is estimated to be over 2,760 kilometers (1,700 miles), can be used as a training ground for our instruments, which can then be used on more complex objects.

The object was discovered as part of the KELT (Kilodegree Extremely Little Telescope) survey. KELT uses two small robotic telescopes, one in South Africa and one in Arizona, to hunt exoplanets. They are looking for transiting exoplanets, ones that pass in front of stars, with the telescopes able to spot them by measuring dips in the light they get from the stars.

"The KELT project is specifically designed to discover a few scientifically valuable planets orbiting very bright stars, and KELT-11b is a prime example of that," lead author Joshua Pepper, from Lehigh University, said in a statement. "We were very surprised by the amazingly low density of this planet. It's extremely big for its mass. It's got a fifth of the mass of Jupiter but is puffed up into this really underdense planet."

KELT-11b is located 320 light-years from Earth, with astronomers catching it in its twilight years. It orbits a star that's larger and more massive than the Sun, and will expand into a red giant and engulf the planet. KELT-11b is already so close that it orbits the star in just five days.

While location and composition play a role, astronomers are still trying to figure out the exact mechanism behind over-inflated planets like KELT-11b. This one is the third-lowest density planet whose radius and mass have been accurately measured.

The team hope to gather follow-up observations of KELT-11b with better instruments in order to understand the complexity of this planet's atmosphere, hopefully in preparation for what we might be doing down the line.

"We don't know of any real Earth-like planets or stars for which we can measure their atmospheres, though we expect to discover more in future years," Pepper added. "These (giant gas) planets are the gold standards or testbeds for learning how to measure the atmospheres of planets."

When the James Webb Space Telescope comes online in early 2019, it will push our limits on what we can resolve, with many more Earth-like objects studied just as easy as KELT-11b.

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