Since the largest exoplanets are the easiest to spot and study, it makes sense that distant worlds with large “puffy” atmospheres would provide the best and most abundant information about the envelope of gases surrounding these faraway planets. But the challenge comes in finding planets with such large atmospheres.
According to researchers at Lehigh University, KELT-11b has by far the largest atmospheric scale height of any ever studied, including our own solar system. Scale height describes the height at which an atmosphere “hugs” a planet, and while the scale height of Saturn’s atmosphere is about 60 km (37 miles) and Earth’s is about 8 km (5 miles), KELT-11b’s is a whopping 2,763 km (1,716 miles).
And the oddities of KELT-11b don’t stop there. The planet itself is one of the most puffy and inflated planets in the known universe, with the density of a Styrofoam ball — the kind that students use to simulate planetary bodies in classroom presentations.
“It is highly inflated, so that while it’s only a fifth as massive as Jupiter, it is nearly 40 percent larger, making it about as dense as Styrofoam, with an extraordinarily large atmosphere,” said Joshua Pepper, astronomer and assistant professor of physics at Lehigh University, who led the study.
Pepper called KELT-11b an “extreme” version of a gas planet. It is orbiting very close to its host star, circling it in less than five days. The star, KELT-11, has started using up its nuclear fuel and is evolving into a red giant, so the planet will eventually be engulfed by its star and not survive the next hundred million years.
But this star is extremely bright — the brightest known transiting exoplanet host that can be seen from the southern hemisphere — and is the sixth brightest transit host to date. Such a bright star allows for precise measurements of the planet’s atmosphere.