An exoplanet so light that it would float on water, were there an ocean large enough, is continuing to frustrate astronomers by concealing its closest secrets with a layer of haze thicker than any ever seen on a planet before.
The haze is so thick that not even the vision of the James Webb Space Telescope (JWST) can penetrate it, leaving the mystery of how this ultra-low density world and its sibling planets all formed unsolved for now.
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Kepler-51d is a member of a four-planet system orbiting a young Sun-like star 2,615 light years away. They were discovered by NASA’s Kepler Space Telescope, which observed the planets transiting their star. From the amount of the star’s light blocked during the transits, astronomers deduced the size of the worlds, and from transit timing variations — the way each planet’s gravity pulls and pushes on the other planets, varying exactly when they are seen to transit — their masses were measured. Planets 51b, c and d have 7.1, 9 and 9.7 times the radius of Earth, respectively, making them about the same size as Saturn.
However, planets b, c and d have masses only 3.7, 5.6 and 5.6 times greater than Earth’s, respectively. Saturn, on the other hand, has a mass 95 times more than Earth. So, these worlds are a similar size to Saturn, but much (much) less massive. (The fourth planet in the system, e, was only discovered in 2024 and its mass and radius are yet to be measured to any degree of accuracy.)
It is remarkable that the densities of planets 51b, c and d have more in common with cotton candy (or candy floss as we call it in the U.K.!) than with the planets we are more familiar with.
As such, Kepler-51d and its fellow ultra-low density worlds are completely alien to the planets in our own solar system. Take the gas giants Jupiter or Saturn, for example, which have large, dense and well-defined cores that on their own are ten times more massive than Earth. These cores formed first and then their gravitational pull attracted masses of gas from the planet-forming disk that encircled the Sun 4.5 billion years ago.
In contrast, the ultra-low density worlds of Kepler-51 “have tiny cores and huge atmospheres giving them a density akin to cotton candy,” said Libby-Roberts. It is not clear how these small cores could have accreted relatively large amounts of gas.
So in search of answers, when Libby-Roberts was at Penn State University she led a team in 2020 to observe the Kepler-51 system spectroscopically using the Hubble Space Telescope‘s Wide-Field Camera 3. The purpose was to look for signs of the chemical composition of the atmosphere around the planets, which could provide clues as to how far from their star these worlds formed, and how they subsequently came to be so tenuous. Given their low density, they are undoubtedly rich in hydrogen and helium, the two lightest and most common elements in the universe, but the various trace gases present in their atmosphere could tell us more about their origin.
Yet Hubble found no sign of any chemistry, leading Libby-Roberts and her colleagues to suspect that there could be a featureless haze swamping the atmosphere of the planets.
Now, Libby-Roberts has returned to the Kepler-51 system, using the JWST’s Near Infrared Spectrometer (NIRSpec) to try and probe harder into the atmosphere of Kepler-51d in the hope of detecting its chemical composition.
They aimed to accomplish this via transit spectroscopy. When Kepler-51d transits its star, some of its star’s light filters through the planet’s atmosphere. Any molecules present can absorb certain wavelengths of the star’s light, which should show up in the star’s spectrum as absorption lines.
“A star’s light is filtered through the atmosphere of the planet before it reaches our telescopes,” said Libby-Roberts. “If we look across a range of wavelengths, across a spectrum, we get a sort of fingerprint of the planet’s atmosphere that reveals its composition.”
Yet the spectrum still showed no signs of the chemistry of 51d’s atmosphere, meaning that the haze that is present must be the thickest ever encountered on an exoplanet if even NIRSpec, operating at longer wavelengths than Hubble, cannot see through it.
“It seems very similar to the haze we see on Saturn’s largest moon Titan, which has hydrocarbons like methane, but at a much larger scale,” said co-researcher Suvrath Mahadevan at Penn State. “Kepler-51d seems to have a huge amount of haze, almost the radius of Earth.”
There are currently no planet-formation models that can explain how such low density worlds can form, particularly so close to their star — if 51b, c and d were transported to our solar system they would all be packed into a region well inside the orbit of Venus.
“It’s possible that [51d] formed further away and moved inward, but we are still left with a ton of questions about how this planet — and the other planets in this system — formed,” said Libby-Roberts. “What is it about this system that created these three really oddball planets, a combination of extremes that we haven’t seen anywhere else?”
It is possible that we are seeing these planets in a transitory phase. The system is half a billion years old, so young compared to our 4.5-billion-year-old solar system. Being young, the Kepler-51 star is still quite active and its stellar wind will be stripping away the outer gases of the ultra-low density planets. Perhaps if we came back in a billion years’ time, much of each planets’ gas will have been whittled away leaving behind a small core.
Some answers could still be forthcoming. A separate team is performing NIRSpec observations of Kepler-51b to try and find evidence of the composition of its atmosphere. They might instead find that it is also covered in haze, but if they are successful, then the clues those observations provide might also apply to 51c and d.
Then measurements of Kepler-51d are reported in the 16 March issue of The Astronomical Journal.
