By Julia Levy,
Medill News Service, March 9
Planetary physicist Raymond Pierrehumbert of Oxford University treats sub-Neptune exoplanets as boulders scattered throughout the universe. Akin to how rock formations and glaciers give scientists a better understanding of the Earth’s geologic history, exoplanets create a clearer picture of how various chemicals came together to make planets, stars and ultimately the Milky Way galaxy.
Pierrehumbert is focused on researching “sub-Neptunes,” a category of exoplanet. In particular, he looks for low-density planets that are larger than Earth but with less than three times the radius of the Earth. Neptune has a radius of nearly four times the Earth’s.
Sub-Neptunes are not present in our Solar System, but are the most common type of planet in the universe. Many sub-Neptunes are dominated with materials of rock and iron, similar to our Solar System’s rocky planets. Studying sub-Neptunes takes a mentality no different from that of studying rocks on Earth.
“I’ve seen various people out in the field doing exposure each day getting excited saying, ‘I’ve got a new boulder. I haven’t seen this boulder before. I can ask this boulder, where did you come from? How long have you been exposed?’ Well, to make this transition, just think, these planets are my boulders,” Pierrehumbert said, a comment welcomed with an audience chuckle at the annual fall Comer Climate Conference in Wisconsin.
Introducing Pierrehumbert, Columbia University environmental scientists Sidney Hemming turned the conference over to the speaker “who’s going to tell us about something extraterrestrial.” Pierrehumbert researches exoplanets, planets that orbit stars other than the sun.
Studying exoplanets also allows us to analyze the differing extent of the greenhouse gas effect in atmospheres other than on Earth, where various amounts of heat from an exoplanet’s star are trapped by its atmospheric gas, warming the exoplanet relative to how much of the star’s radiation is trapped, he said.
Pierrehumbert’s research includes the theoretical underpinnings of how these exoplanets work. To experimentally find planets outside of the Solar System, there are two main methods. The first is the transit method, where an exoplanet’s shadow is seen moving across the star it orbits. The second is the radial velocity method, where a star appears to slightly “wobble” due to the gravitational pull of an orbiting planet on the star. Both methods have their pros and cons and the use of each method depends on the relative mass, size and position between the planet and its star.
“For most of the planets that we know about out there, we know their size, but it’s only for a subset that we also know the mass. So the sweet spot is to know both the size and mass of the planet,” Pierrehumbert said.
Pierrehumbert believes that habitable sub-Neptunes exist, but that they are farther from their stars than any of the planets scientists are currently capable of observing.
Habitable Worlds Observatory (HWO), a large space telescope, optimized for observation in a variety of wavelengths, designed to search and identify habitable exoplanets. HWO will have the technology to observe light reflected from an exoplanet even when it is not passing in front of its host star.
Pierrehumbert predicts HWO will open up a completely new window for astronomers. HWO is expected to launch into Earth orbit in May 2027.
“We’re interested in detecting liquid water on the surface of these planets, which we might be able to do by some combination of modeling,” Pierrehumbert said. “There’s some possibility you might even be able to see sunlight, reflection of water or reflection of the starlight off of the water. The most basic thing is to first find the atmospheres and then see if they have carbon dioxide and water vapor in them. Because that’s the start for building a habitable planet.”
In contrast, some sub-Neptunes actually contain lava. Pierrehumbert is proposing to observe a very hot sub-Neptune that is so hot that rock vapors evaporate from an ocean entirely composed of lava. Pierrehumbert hypothesizes the rock vapor on such a planet could form clouds of either a black, gritty, silicon monoxide or an oxidized quartz. These clouds would be prominent enough that they could possibly be visible to astronomers with the James Webb Space Telescope, JWST.
As for which sub-Neptune Pierrehumbert believes is the best candidate for scientists to see such clouds for the first time, the public will have to wait. Pierrehumbert is keeping the name of the exoplanet a secret for now.
“If it works, it will be the first sub-Neptune for which we have direct evidence of the magma ocean,” Pierrehumbert said.
For what Pierrehumbert and his team will actually see during observation, there are many predicted possibilities. Pierrehumbert is hoping for a clear enough atmosphere to detect either silicon monoxide or silane, because the amount of either would allude to understanding the composition of its surface. There could be a more disappointing result where some thick, unknown clouds block his team’s signal, but even then, scientists could postulate what kind of cloud would induce this effect.
The second worst case scenario is a flat line in the observed spectrum, meaning the team would have no spectral features to work with.
The worst-case scenario?
“We get nothing, really nothing, it’s worse than a flat line,” Pierrehumbert said. “Sometimes that happens, but it hasn’t happened to us yet.”
The annual fall Comer Climate Conference in Wisconsin is an opportunity for scientists from across the planet to showcase their work about climate change. Topics this year included reconstructing the fluctuations of mountain glaciers and ice sheets, updates on Holocene glacial chronology and determining the age of some of the oldest ice cores in the world located in Allan Hills, Antarctica through argon dating.
Photo at top: NASA rendering of a sub-Neptune exoplanet. “This artist’s concept shows what the hot sub-Neptune exoplanet TOI-421 b could look like. It is based on spectroscopic data gathered by Webb, as well as previous observations from other telescopes on the ground and in space,” according to NASA. Credits: NASA, ESA, CSA, Dani Player (STScI)