An exoplanet so gentle that it could float on water, have been there an ocean massive sufficient, is continuous to frustrate astronomers by concealing its closest secrets and techniques with a layer of haze thicker than any ever seen on a planet earlier than.
The haze is so thick that not even the imaginative and prescient of the James Webb House Telescope (JWST) can penetrate it, leaving the thriller of how this ultra-low density world and its sibling planets all shaped unsolved for now.
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Kepler-51d is a member of a four-planet system orbiting a younger Solar-like star 2,615 gentle years away. They have been found by NASA’s Kepler House Telescope, which noticed the planets transiting their star. From the quantity of the star’s gentle blocked throughout the transits, astronomers deduced the dimensions of the worlds, and from transit timing variations — the best way every planet’s gravity pulls and pushes on the opposite planets, various precisely when they’re seen to transit — their plenty have been measured. Planets 51b, c and d have 7.1, 9 and 9.7 instances the radius of Earth, respectively, making them about the identical dimension as Saturn.
Nonetheless, planets b, c and d have plenty solely 3.7, 5.6 and 5.6 instances higher than Earth’s, respectively. Saturn, however, has a mass 95 instances greater than Earth. So, these worlds are the same dimension to Saturn, however a lot (a lot) much less large. (The fourth planet within the system, e, was solely found in 2024 and its mass and radius are but to be measured to any diploma of accuracy.)
It’s outstanding that the densities of planets 51b, c and d have extra in frequent with cotton sweet (or sweet floss as we name it within the U.Ok.!) than with the planets we’re extra aware of.
As such, Kepler-51d and its fellow ultra-low density worlds are fully alien to the planets in our personal photo voltaic system. Take the gasoline giants Jupiter or Saturn, for instance, which have massive, dense and well-defined cores that on their very own are ten instances extra large than Earth. These cores shaped first after which their gravitational pull attracted plenty of gasoline from the planet-forming disk that encircled the Solar 4.5 billion years in the past.
In distinction, the ultra-low density worlds of Kepler-51 “have tiny cores and large atmospheres giving them a density akin to cotton sweet,” mentioned Libby-Roberts. It’s not clear how these small cores may have accreted comparatively massive quantities of gasoline.
So in quest of solutions, when Libby-Roberts was at Penn State College she led a staff in 2020 to look at the Kepler-51 system spectroscopically utilizing the Hubble House Telescope‘s Large-Subject Digicam 3. The aim was to search for indicators of the chemical composition of the ambiance across the planets, which may present clues as to how removed from their star these worlds shaped, and the way they subsequently got here to be so tenuous. Given their low density, they’re undoubtedly wealthy in hydrogen and helium, the 2 lightest and most typical components within the universe, however the varied hint gases current of their ambiance may inform us extra about their origin.
But Hubble discovered no signal of any chemistry, main Libby-Roberts and her colleagues to suspect that there could possibly be a featureless haze swamping the ambiance of the planets.
Now, Libby-Roberts has returned to the Kepler-51 system, utilizing the JWST’s Close to Infrared Spectrometer (NIRSpec) to attempt to probe more durable into the ambiance of Kepler-51d within the hope of detecting its chemical composition.
They aimed to perform this by way of transit spectroscopy. When Kepler-51d transits its star, a few of its star’s gentle filters by the planet’s ambiance. Any molecules current can take in sure wavelengths of the star’s gentle, which ought to present up within the star’s spectrum as absorption strains.
“A star’s gentle is filtered by the ambiance of the planet earlier than it reaches our telescopes,” mentioned Libby-Roberts. “If we glance throughout a variety of wavelengths, throughout a spectrum, we get a kind of fingerprint of the planet’s ambiance that reveals its composition.”
But the spectrum nonetheless confirmed no indicators of the chemistry of 51d’s ambiance, which means that the haze that’s current should be the thickest ever encountered on an exoplanet if even NIRSpec, working at longer wavelengths than Hubble, can’t see by it.
“It appears similar to the haze we see on Saturn’s largest moon Titan, which has hydrocarbons like methane, however at a a lot bigger scale,” mentioned co-researcher Suvrath Mahadevan at Penn State. “Kepler-51d appears to have an enormous quantity of haze, virtually the radius of Earth.”
There are at present no planet-formation fashions that may clarify how such low density worlds can kind, notably so near their star — if 51b, c and d have been transported to our photo voltaic system they’d all be packed right into a area properly contained in the orbit of Venus.
“It is doable that [51d] shaped additional away and moved inward, however we’re nonetheless left with a ton of questions on how this planet — and the opposite planets on this system — shaped,” mentioned Libby-Roberts. “What’s it about this method that created these three actually oddball planets, a mixture of extremes that we have not seen wherever else?”
It’s doable that we’re seeing these planets in a transitory section. The system is half a billion years outdated, so younger in comparison with our 4.5-billion-year-old photo voltaic system. Being younger, the Kepler-51 star continues to be fairly lively and its stellar wind can be stripping away the outer gases of the ultra-low density planets. Maybe if we got here again in a billion years’ time, a lot of every planets’ gasoline may have been whittled away forsaking a small core.
Some solutions may nonetheless be forthcoming. A separate staff is performing NIRSpec observations of Kepler-51b to attempt to discover proof of the composition of its ambiance. They may as a substitute discover that additionally it is lined in haze, but when they’re profitable, then the clues these observations present may also apply to 51c and d.
Then measurements of Kepler-51d are reported within the 16 March difficulty of The Astronomical Journal.
