This illustration depicts the Sun-like star Kepler 51 and three giant planets that NASA’s Kepler space telescope discovered in 2012–2014. These planets are all roughly the size of Jupiter but a tiny fraction of its mass. This means the planets have an extraordinarily low density, more like that of Styrofoam rather than rock or water, based on new Hubble Space Telescope observations. The planets may have formed much farther from their star and migrated inward. Now their puffed-up hydrogen/helium atmospheres are bleeding off into space. Eventually, much smaller planets might be left behind. The background starfield is correctly plotted as it would look if we gazed back toward our Sun from Kepler 51’s distance of approximately 2,600 light-years, along our galaxy’s Orion spiral arm. However, the Sun is too faint to be seen in this simulated naked-eye view. Credit: NASA, ESA, and L Hustak, J Olmsted, D Player and F Summers (STScI). Public Domain via Wikimedia Commons.

A thick layer of haze around the ultra-low-density planet Kepler-51d likely obscures not only the strange planet’s composition, but also its origin.

A team led by Penn State researchers used NASA’s James Webb Space Telescope (JWST) to take a deeper look at the ‘super-puff’ planet that defies planetary formation models.

However, the thickest layer of haze found on a planet yet makes discerning the chemical elements in the planet’s atmosphere – and any clues to the planet’s formation – challenging.

The Kepler-51 system

Kepler-51 is a star located about 2,615 light-years away from Earth in the constellation Cygnus.

The system has four known planets, at least three of which are unusual low-density super-puff planets that are about the size of Saturn but only a few times the mass of Earth.

Kepler-51d is the coolest and least dense of the planets in this system.

Jessica Libby-Roberts, Center for Exoplanets and Habitable Worlds Postdoctoral Fellow at Penn State at the time of the research and first author of the paper, said: “We think the three inner planets orbiting Kepler-51 have tiny cores and huge atmospheres giving them a density akin to cotton candy.

“These ultra-low-density super-puff planets are rare, and they defy conventional understanding of how gas giants form.

“And if explaining how one formed wasn’t difficult enough, this system has three!”

According to Libby-Roberts, gas giants typically have a dense core and therefore a strong gravitational pull that attracts and retains gases.

These planets typically form further away from their star, whose gravitational pull also attracts gases, much like the gas giants of our solar system are located outside the asteroid belt.

But Kepler-51d doesn’t have a dense core, and its distance to its star is approximately the same distance as Venus is from the Sun.

Libby-Roberts, who is now an assistant professor of physics and astronomy at the University of Tampa, said: “Kepler-51 is a relatively active star, and its stellar winds should easily blow away the gases from this planet, though the extent of this mass-loss over Kepler-51d’s lifetime remains unknown.

“It’s possible that the planet 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.

“What is it about this system that created these three really oddball planets, a combination of extremes that we haven’t seen anywhere else?”

Infer information

Because of the ultra-low densities, the researchers said they believe these super-puff planets have a substantial amount of the lightest elements, hydrogen and helium, but they also expect to find other elements.

By clarifying the elements that make up the atmosphere of Kepler-51d, the researchers can also infer information about the environment and location in which the planet formed.

Although planets at these distances cannot be directly imaged from Earth, researchers can observe the star’s light, which dims when a planet passes in front of, or transits, its host star from the perspective of Earth.

Libby-Roberts said: “A star’s light is filtered through the atmosphere of the planet before it reaches our telescopes.

“If a certain molecule is present in the atmosphere that absorbs a certain wavelength of light, like how different coloured objects on Earth absorb different wavelengths of light, it can block the light at that wavelength.

“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.”

Libby-Roberts and her colleagues previously observed Kepler-51d with NASA’s Hubble Space Telescope, which observed near-infrared wavelengths of about 1.1 to 1.7 microns.

The improved technology of JWST’s Near-Infrared Spectrograph allowed the team to extend observations to 5 microns, with the potential to provide a more nuanced atmosphere ‘fingerprint’.

However, the researchers did not see any clear dips in the star’s intensity at specific wavelengths.

Suvrath Mahadevan, Verne M Willaman Professor of Astronomy and Astrophysics in the Penn State Eberly College of Science and an author of the paper, advised: “We think that the planet has such a thick haze layer that is absorbing the wavelengths of light we looked at, so we can’t actually see the features underneath.

“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.

“Kepler-51d seems to have a huge amount of haze, almost the radius of Earth, which would be one of the largest we’ve seen on a planet yet.”

Alternatives

The researchers did consider alternatives to a large haze layer but largely ruled them out.

For example, if the planet had rings and was tilted, it’s possible the rings would block the star’s light, making the planet appear larger and in this case less dense than it really is.

However, the researchers said that rings would block light at different wavelengths in a consistent manner and would have to be angled in a very specific way to create these results.

Libby-Roberts said: “Instead, we see a linear trend, with more light being blocked at longer wavelengths.

“This is unusual, and the simplest explanation is thick haze. Rings would have to be short-lived, composed of very particular materials, and situated in just the right angle, which seems unlikely, but we can’t completely rule it out.

“If we could observe the planet at even longer wavelengths, such as with JWST’s Mid Infrared Instrument, we might be able to detect the materials that would be in a ring or see the full extent of the haze layer.”

Unique?

The researchers said that observations of other super-puff planets could also shed light on Kepler-51d. For example, another research team is currently analysing JWST observations of Kepler-51b, which could help clarify if all super-puff planets are hazy, if they all have similar structure and environments they were formed in, or if 51d is unique.

Libby-Roberts said: “Before astronomers found planets outside our Solar System, we thought we had a pretty good grasp on how planets formed.

“But we started to find exoplanets that didn’t match our Solar System at all, and we have these alien worlds that really challenge our understanding of planet formation.

“We haven’t found a solar system like ours yet, and being able to explain how all these different planets formed helps us understand how we fit into the big picture and our place in the Universe.”

A paper describing the planet is published the Astronomical Journal.