The James Webb Space Telescope is beginning to reveal previously unknowable facts about the composition of exoplanets — about the presence or absence of atmospheres around the exoplanets and the makeup of any atmospheres that are detected.
The results have been coming in for some months and they are a delight to scientists. And as with most things about exoplanets, the results are not always what were expected.
For instance, gas giant planets orbiting our Sun show a clear pattern; the more massive the planet, the lower the percentage of “heavy” elements (anything other than hydrogen and helium) in the planet’s atmosphere.
But out in the galaxy, the atmospheric compositions of giant planets do not fit the solar system trend, an international team of astronomers has found.
Researchers discovered that the atmosphere of exoplanet HD149026b, a “hot jupiter” given the name “Smertrios” that orbits a Sun-like star, is super-abundant in the heavier elements carbon and oxygen – far above what scientists would expect for a planet of its mass.
In its “early release” program for exoplanet results, JWST also observed WASP-39 b, a “hot Saturn” (a planet about as massive as Saturn but in an orbit tighter than Mercury) orbiting a star some 700 light-years away.
The atmosphere around the planet provided the first detection in an exoplanet atmosphere of sulfur dioxide (SO2), a molecule produced from chemical reactions triggered by high-energy light from the planet’s parent star.
The Trappist-1 system –seven Earth-sized planets orbiting a red dwarf star only 40 light-years away — is another subject of great interest and JWST has provided some exciting results there too.
While the first Trappist-1 planet studied — the one nearest to the star — apparently has no atmosphere, JWST was able to in effect take the planet’s temperature. The telescope captured thermal signatures from the planet, which is another first.
To make these measurements (and more) JWST was used to track the planets as they passed in front of their stars, allowing some of the star’s light to filter through the planet’s atmosphere.
Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the colors that are missing tell astronomers which molecules are present. By viewing the universe in infrared light, JWST can pick up chemical fingerprints that can’t be detected in visible light.
“We are going to be able to see the big picture of exoplanet atmospheres,” said Laura Flagg, a researcher at Cornell University and a member of the international team working on the WASP-39b data, published in Nature. “It is incredibly exciting to know that everything is going to be rewritten.”
For instance, Shang-Min Tsai, a researcher at the University of Oxford in the United Kingdom and lead author of the paper explaining the origin of sulfur dioxide in WASP-39b’s atmosphere, described some of the new insights JWST is providing.
“This is the first time we see concrete evidence of photochemistry – chemical reactions initiated by energetic stellar light – on exoplanets,” he said in a release. “I see this as a really promising outlook for advancing our understanding of exoplanet atmospheres.”
The evidence was that first detection of sulfur dioxide surrounding an exoplanet.
Sulphur dioxide is released by volcanism on terrestrial worlds – such as Earth, Venus and Jupiter’s moon Io – but in gas giants like Jupiter or WASP-39b it must come from a different source.
In our local gas giants, for instance, sulphur deep in the atmosphere exists as hydrogen sulphide gas. But as this is churned up to higher altitudes the energetic photons of ultraviolet rays from the Sun break apart the hydrogen sulphide and drive further chemical reactions that produce sulphur dioxide.
Such UV-driven reactions (photochemistry) also take place in Earth’s atmosphere and produce the ozone layer by driving reactions of oxygen molecules. This data on WASP-39b is the first concrete indication of photochemistry being crucial in the atmospheric composition of exoplanets too.
Here is another exoplanet world explored with JWST:
A giant exoplanet known as VHS 1256b which obits around two Suns (like Tatooine from “Star Wars”) , was find turbulent clouds made of silicates, similar to sand here on Earth
“VHS 1256b is about four times farther from its stars than Pluto is from our sun, which makes it a great target for Webb,” said Brittany Miles, an astrophysicist at the University of Arizona and lead author on the new study, in a press release . “That means the planet’s light is not mixed with light from its stars.”
The spectra showed signs of clouds made of silicates, which periodically rain down into the depths of the planet, moving about in an atmosphere as hot as a flame, around 1,500 degrees Fahrenheit (815 degrees Celsius). Silicate clouds have no equivalent here on Earth, other than maybe being in a cloud of hot sand.
“The finer silicate grains in its atmosphere may be more like tiny particles in smoke,” said University of Edinburgh astrophysicist Beth Biller, part of the research team, in the press release. “The larger grains might be more like very hot, very small sand particles.”
This exoplanet was described in a recently published article in Astrophysical Journal Letters.
Returning to Smertrios, the fact that its atmosphere has a variety of heavy elements — unlike gas giants in our solar system — is some of what makes investigation exoplanets so exciting, and so surprising.
“It appears that every giant planet is different, and we’re starting to see those differences thanks to JWST,” said Jonathan Lunine, the David C. Duncan Professor in the Physical Sciences in the College of Arts and Sciences and a co-author of the study, published in Nature.
“In this paper, we have determined how many molecules there are relative to the primary component of the gas, which is hydrogen, the most common element in the universe. That tells us quite a lot about how this planet formed.”
The giant planets of our solar system exhibit a nearly perfect correlation between both overall composition and atmospheric composition and mass, said Jacob Bean, professor of astronomy and astrophysics at the University of Chicago and lead author of the paper. Extrasolar planets show a much greater diversity of overall compositions, but scientists didn’t know how varied their atmospheric compositions were until this analysis of HD149026b.
“We have shown definitively that the atmospheric compositions of giant extrasolar planets do not follow the same trend that is so clear in the solar system planets,” Bean said in a release. “Giant extrasolar planets show a wide diversity in atmospheric compositions in addition to their wide diversity of overall compositions.”
Most of the success researchers have had in identifying particular chemicals in exoplanet atmospheres has been with large gas giant planets. Observatories before JWST — including the Hubble — do not have the observing power to investigate atmospheres on the scale of smaller, terrestrial planets.
Determining the composition of gas giant atmospheres is important for sure. But ultimately terrestrial planets like our own are the prize for astronomers and astrobiologists, because only they can under certain circumstances be habitable and perhaps even inhabited — if only by some microbes.
This is why the Trappist-1 system is so compelling to researchers. The seven planets are all small and rocky, and the system is even relatively close at 40 light-years away.
In the first year of JWST observing all the Trappist-1 planets will be studied, and some by more than one team.
Trappist-1b was the first to show some results, which led to the conclusion that the planet had no atmosphere.
Last November and December, JWST searched for an atmosphere around the planet by looking for heat radiating from it, according to Thomas Greene, an astronomer at NASA’s Ames Research Center. He and his colleagues report their results in Nature.
Although the negativbe finding might sound disappointing, scientists say that the work illustrates the power of JWST and bodes well for future results from the TRAPPIST-1 system.
Previous studies with the Hubble and Spitzer space telescopes, using a different technique, showed that TRAPPIST-1b — the innermost planet in the system — probably doesn’t have a large puffy atmosphere made mostly of hydrogen. But researchers couldn’t rule out whether it has a dense atmosphere, as Earth might have had billions of years ago.
JWST looked at TRAPPIST-1 in mid-infrared wavelengths of light — 20 times redder than the human eye can see — to see how that radiation changed as TRAPPIST-1b moved behind the star. By measuring the brightness of the star and planet together compared with that of the star alone, astronomers could calculate how much came from the planet.
It’s not surprising that TRAPPIST-1b has no atmosphere,because it is pummeled with four times as much radiation as Earth receives from the Sun. TRAPPIST-1 is also scorched by stellar flares and other activity that sends radiation across its planets, potentially scouring away atmospheres.
Still, understanding these conditions is crucial because M dwarf stars — cool, dim stars such as TRAPPIST-1 — often have Earth-sized planets orbiting them. And M dwarf star, or red dwarfs, are the most common stars in the night sky.
Many more exoplanet atmosphere reports will be coming in the weeks, months and years ahead.
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