Just a few short decades ago, scientists were discovering the first planets beyond our solar system. Now, thousands of such exoplanets are known, and researchers are using state-of-the-art observations to probe the chemical compositions of their atmospheres.
A team recently detected hydrogen sulfide—the compound that gives rotten eggs their characteristic stench—in the atmosphere of HD 189733 b, an exoplanet roughly 60 light-years from Earth. This discovery suggests it likely formed via the accretion of smaller bodies known as planetesimals, rather than by collapsing from a cloud of gas and dust, the team suggested.
HD 189733 is a Sun-like star in the constellation Vulpecula (“little fox”), which sweeps high across the summertime night sky at northern latitudes. The star is too faint to be seen with the naked eye, but sensitive instruments attached to large telescopes have no trouble spotting its light. In 2005, astronomers published observations of HD 189733 that revealed the star to be wobbling ever so slightly. That motion indicated the presence of an unseen orbiting planet, which scientists called HD 189733 b.
Like Jupiter, but Not
Researchers have inferred that HD 189733 b is roughly 10% more massive than Jupiter. But unlike Jupiter, which is roughly 780 million kilometers (484 million miles) from the Sun, HD 189733 b’s orbit takes it to within 5 million kilometers of HD 189733.
“It orbits very close to its star,” said Guangwei Fu, an astronomer at Johns Hopkins University in Baltimore and lead author of the new study.
“The planet blocks a different amount of starlight depending on what gas is present in the planet’s atmosphere.”
That orbital arrangement contributes to a lucky coincidence: As seen from Earth, HD 189733 b regularly appears to pass directly across the face of its host star. Fu and his colleagues recently exploited those so-called transits to probe the exoplanet’s atmosphere.
By comparing observations of HD 189733 with and without HD 189733 b in front of it, the team measured how starlight dimmed as it traveled through the planet’s atmosphere. Those data revealed the chemical compounds that make up HD 189733 b’s atmosphere. “The planet blocks a different amount of starlight depending on what gas is present in the planet’s atmosphere,” said Fu.
Going to Space
Fu and his collaborators analyzed data from the James Webb Space Telescope. Making observations from space was key, Fu said, because he and his team were measuring near-infrared light (with wavelengths of roughly 2–5 micrometers). Our planet’s atmosphere absorbs strongly in that part of the electromagnetic spectrum, muddling the signal for ground-based telescopes.
The Hubble Space Telescope, also situated beyond much of Earth’s atmosphere, is limited to detecting wavelengths shorter than about 2.5 micrometers. It’s therefore unable to observe the signatures of several important compounds, Fu said. “Carbon dioxide, carbon monoxide, and hydrogen sulfide are all beyond 2–3 microns.”
“You can get a holistic view of what the atmosphere is made of.”
The researchers started by reconfirming earlier findings that the atmosphere of HD 189733 b contains water. Those observations allowed the team to estimate the abundance of oxygen. However, it’s critical to detect other compounds as well, Fu said. Astronomers refer to any elements other than hydrogen or helium as metals, and measuring an array of metals is important, he said. “You can get a holistic view of what the atmosphere is made of.”
Fu and his colleagues also detected carbon dioxide, carbon monoxide, and hydrogen sulfide. The discovery of hydrogen sulfide on this world is a first for an exoplanet, said Aurora Kesseli, an astronomer at the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena who was not involved in the research.
That’s important because measuring hydrogen sulfide sheds light on the abundance of sulfur in HD 189733 b’s atmosphere. And that measurement, paired with information about oxygen and carbon from observations of carbon dioxide and carbon monoxide, allowed the researchers to estimate HD 189733 b’s so-called metallicity—essentially, its abundance of metals compared with its abundance of hydrogen.
HD 189733 b’s metallicity was a few times higher than the Sun’s and comparable to that of Jupiter. That’s expected for an exoplanet of this mass and radius, Kesseli said. Planets that are more massive tend to have lower metallicities, she said, so this result also jibes with other observations of exoplanets made by the James Webb Space Telescope.
Enter the Planetesimals
Determining HD 189733 b’s metallicity furthermore helps constrain how this planet likely formed. There are two generally accepted models of planetary formation. In one model, known as core accretion, planets are built up over time as smaller bodies—known as planetesimals—collide with one another. In the other, known as gravitational instability, planets form from the collapse of a cloud of gas and dust.
HD 189733 b’s metallicity suggests that it formed from the buildup of planetesimals, Fu and his collaborators concluded in their study, which was published in Nature. Planets that formed from the gravitational collapse of a cloud of gas and dust, on the other hand, tend to have lower metallicities because more hydrogen gets incorporated into those worlds, Fu said.
Fu and his team aren’t finished yet with HD 189733 and its orbiting planet. They’re currently studying other observations from the James Webb Space Telescope obtained at 5–25 micrometers that they hope will reveal the presence of clouds on HD 189733 b. Those clouds, if they exist, will likely be made of silicon, Fu said. “This planet is way too hot to have water clouds.” The James Webb Space Telescope has really revolutionized our study of exoplanets, he said. “It’s been a game changer for exoplanet science.”
—Katherine Kornei (@KatherineKornei), Science Writer
Citation: Kornei, K. (2024), Smells like an exoplanet, Eos, 105, https://doi.org/10.1029/2024EO240341. Published on 1 August 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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