New findings suggest why

The study of “exoplanets,” a science fiction-sounding name for all the planets in the cosmos beyond our own solar system, is a relatively new field. Exoplanet researchers, like those at the University of Kansas ExoLab, primarily use data from space telescopes such as the Hubble Space Telescope and the Webb Space Telescope. Whenever news headlines talk about “Earth-like” planets or planets with the potential to support humanity, they are talking about exoplanets within our own Milky Way.

Jonathan Brande, doctoral student at the ExoLab at the University of Kansas, has just published his results in Letters from the astrophysical journal showing new atmospheric details in a set of 15 exoplanets similar to Neptune. Although none can sustain humanity, a better understanding of their behavior could help us understand why we don’t have a small Neptune, when most solar systems seem to feature a planet of this class.

“Over the past several years at KU, I have focused on studying the atmospheres of exoplanets through a technique known as transmission spectroscopy,” Brande said. “When a planet transits, that is, it moves between our line of sight and the star it orbits, light from the star passes through the planet’s atmosphere and is absorbed by the different gases present. By capturing a spectrum of the star, passing the light through an instrument called a spectrograph, which is equivalent to passing it through a prism: we observe a rainbow and measure the luminosity of the different colors that compose it. Varying areas of brightness or darkness in the spectrum reveal the light-absorbing gases in the planet’s atmosphere.

Using this methodology, Brande published a paper several years ago regarding the “hot Neptune” exoplanet TOI-674 b, in which he presented observations indicating the presence of water vapor in its atmosphere. These observations were part of a larger program led by Brande’s advisor, Ian Crossfield, associate professor of physics and astronomy at KU, aimed at observing the atmospheres of Neptune-sized exoplanets.

“We want to understand the behavior of these planets, given that those slightly larger than Earth and smaller than Neptune are the most common in the galaxy,” Brande said.

This recent paper summarizes observations from this program, integrating data from additional observations to explain why some planets appear cloudy while others are clear.

“The goal is to explore the physical explanations behind the distinct appearances of these planets,” Brande said.

Brande and his co-authors particularly noted regions where exoplanets tend to form clouds or hazes high in their atmospheres. When such atmospheric aerosols are present, the KU researcher said the hazes can block light filtering through the atmosphere.

“If a planet has a cloud just above the surface with hundreds of kilometers of clear air above it, starlight can easily pass through the clear air and be absorbed only by the specific gases present in that part atmosphere,” Brande said. “However, if the cloud is positioned very high, the clouds are generally opaque across the entire electromagnetic spectrum. Although hazes have spectral characteristics, for our work, where we focus on a relatively narrow range with Hubble, they also produce predominantly flat spectra.”

According to Brande, when these aerosols are present in the upper atmosphere, light cannot filter clearly.

“With Hubble, the gas we are most sensitive to is water vapor,” he said. “If we observe water vapor in a planet’s atmosphere, that’s a good indication that there are no clouds high enough to block its absorption. Conversely, if water vapor is not observed and only a flat spectrum is visible, even though we know the planet should have an extended atmosphere, this suggests the likely presence of clouds or mists at higher altitudes. »

Brande led the work of an international team of astronomers on the paper, including KU’s Crossfield and collaborators at the Max Planck Institute in Heidelberg, Germany, a cohort led by Laura Kreidberg, and researchers at the University of Texas at Austin, led by Caroline. Morley.

Brande and his co-authors approached their analysis differently from previous efforts by focusing on determining the physical parameters of Neptune’s small atmospheres. In contrast, previous analyzes often involved fitting a single model spectrum to observations.

“Typically, researchers take an atmospheric model with a pre-calculated water content, scale it, and move it to match the planets observed in their sample,” Brande said. “This approach indicates whether the spectrum is clear or cloudy but provides no information about the amount of water vapor or the location of clouds in the atmosphere.”

Instead, Brande employed a technique known as “atmospheric recovery.”

“This involved modeling the atmosphere based on various planetary parameters such as the amount of water vapor and the location of clouds, going through hundreds and thousands of simulations to find the best-fitting configuration,” he said. he declared.

“Our retrievals gave us a spectrum of best-fit models for each planet, from which we calculated how cloudy or clear the planet appeared. Then we compared these measured clearnesses to a separate suite of models from Caroline Morley , which allowed us to see that our results are consistent with expectations for similar planets. By examining the behavior of clouds and haze, our models indicated that clouds were better suited than mists.

“The sedimentation efficiency parameter, reflecting the compactness of the clouds, suggests that the observed planets had relatively low sedimentation efficiencies, resulting in fluffy clouds. These clouds, made up of particles such as droplets of “water, remained high in the atmosphere due to their low tendency to sediment.”

Brande’s findings provide insight into the behavior of these planetary atmospheres and sparked “considerable interest” when he presented them at a recent meeting of the American Astronomical Society.

Other results

Additionally, Brande is part of an international observing program, led by Crossfield, which has just announced the discovery of water vapor on GJ 9827d, a planet as hot as Venus, located 97 light years away. Earth in the constellation Pisces.

The observations, made with the Hubble Space Telescope, show that the planet may be just one example of the water-rich planets in the Milky Way. They were announced by a team led by Pierre-Alexis Roy of the Trottier Institute for Research on Exoplanets at the University of Montreal.

“We were looking for water vapor in the atmospheres of sub-Neptune-like planets,” Brande said. “Pierre-Alexis’ paper is the last of this main effort because it took about 10 or 11 orbits or transits of the planet to make the water vapor detection. Pierre-Alexis’ spectrum was integrated into our article as one of our trending data points., and we included all the planets from their proposal and others studied in the literature, thus strengthening our results. We were in close communication with them during the process of redaction of both articles to ensure we used appropriate updated results and accurately reflected their findings.”