Astronomers are facing a persistent mystery: a trio of ultra-low density exoplanets, orbiting a distant star, remain obscured by a haze so thick that even the powerful James Webb Space Telescope (JWST) cannot penetrate it. These worlds, dubbed “cotton candy planets” due to their exceptionally low densities, challenge current understanding of planetary formation.
The Enigmatic Kepler-51 System
The planets – Kepler-51b, c, and d – reside 2,615 light-years away in a four-planet system. They were initially discovered using NASA’s Kepler Space Telescope, which detected them passing in front of their star. Measurements of the planets’ sizes and masses reveal an anomaly: they are comparable in size to Saturn, yet have only a fraction of its mass.
Specifically, planets 51b, c, and d have radii 7.1, 9, and 9.7 times that of Earth, while their masses are only 3.7, 5.6, and 5.6 times Earth’s mass, respectively. Saturn, in comparison, has a mass 95 times greater than Earth. This makes them unusually fluffy, with densities more akin to spun sugar than rocky or gaseous planets.
The Problem with the Haze
Attempts to analyze the atmospheres of these planets using both the Hubble Space Telescope and JWST have failed. The haze surrounding Kepler-51d is so dense that spectroscopic analysis, which relies on identifying the chemical fingerprint of atmospheric molecules, has yielded no results.
“A star’s light is filtered through the atmosphere of the planet before it reaches our telescopes,” explained Jessica Libby-Roberts of the University of Tampa. “If we look across a range of wavelengths… we get a sort of fingerprint of the planet’s atmosphere that reveals its composition.” However, no such fingerprint has emerged, suggesting the haze is overwhelming any underlying atmospheric signals.
Why This Matters
The existence of these ultra-low density planets throws a wrench into current models of gas giant formation. Gas giants like Jupiter and Saturn form around dense cores that gravitationally attract gas from the surrounding protoplanetary disk. Kepler-51’s planets, however, appear to have small cores and bloated atmospheres – a combination that defies explanation.
The system is also relatively young (approximately 500 million years old), raising the possibility that the planets are in a transitional state. The star’s activity may be stripping away the outer gases, meaning these worlds might not remain so fluffy for long.
The Future of Research
Another team is currently analyzing the atmosphere of Kepler-51b using JWST’s NIRSpec instrument. If successful, these observations could provide clues about the origins of all three oddball planets in the system. For now, the mystery of the “cotton candy planets” remains unsolved, highlighting the vast unknown still lurking within the cosmos.
The Kepler-51 system presents a unique challenge to planetary scientists: how can gas giants form with such low densities and such thick atmospheres? Further observations will be crucial in unraveling this cosmic riddle.
