Using JWST, scientists observe silicate clouds and measure the temperature of exoplanets

by Haygen Warren

With help from the joint NASA/ESA/CSA James Webb Space Telescope, two teams of scientists have made groundbreaking, first-of-their-kind discoveries on two exoplanets. One team utilized the immense power of Webb’s Mid-Infrared Instrument (MIRI) and Near-Infrared Spectrograph (NIRSpec) instruments to observe and characterize clouds and molecules in the atmosphere of an exoplanet located 40 light-years away.

The second team of scientists also utilized MIRI to measure the temperature of a rocky exoplanet in the TRAPPIST-1 star system, located around 40 light-years away from Earth. The teams determined the temperature by measuring the infrared light emitted by the planet — the first time any light emitted by a rocky exoplanet has been detected and measured by a telescope.

Webb Characterizes Clouds and Molecules in Exoplanet’s Atmosphere

Throughout the last few decades, scientists have continued to learn more and more about the atmospheres of exoplanets scattered throughout the Milky Way. Using telescopes like Hubble and NASA’s Transiting Exoplanet Survey Satellite (TESS), scientists could characterize the atmospheres of exoplanets for several years. However, scientists typically have to identify one atmospheric feature at a time — until now.

Webb’s latest exoplanet observations continue to display the true power of the infrared telescope, as teams were able to extract the data for multiple atmospheric features from a single observation of the exoplanet by Webb. No other telescope identified as many exoplanetary features in a single observation as Webb did in this one observation of the exoplanet.

Artist’s illustration of VHS 1256b, with swirling silicate clouds seen in the planet’s upper atmosphere. (Credit: NASA/ESA/CSA/Joseph Olmsted (STScI))

The exoplanet, named “VHS 1256b,” is located around 40 light-years away from Earth around a tightly-locked binary star system. With an orbital period of approximately 10,000 years, the planet orbits its host stars at a distance that is around four times the distance between Pluto and our Sun.  This made VHS 1256b a fantastic target for Webb, as the light emitted by the exoplanet wouldn’t be mixed in with the light emitted by its host stars — meaning a coronagraph or the commonly-used transit method wouldn’t be needed when collecting data on the exoplanet.

Although its orbital distance and period are nothing short of extreme, its rotational period is fairly normal, completing a single rotation in 22 hours. VHS 1256b’s strange characteristics continue to impress, though, as each time the planet completes a single rotation, its thick atmosphere rises, mixes, and moves dramatically – so much so that VHS 1256b’s intense brightness changes, caused by its ever-changing atmosphere, make it the most variable planetary-mass object ever discovered. Swirling in the upper atmosphere of the exoplanet are silicate clouds, which churn in scorching atmospheric temperatures as high as 830 degrees Celsius.

What’s more, Webb’s data also indicated the presence of water, methane, carbon monoxide, and carbon dioxide. Led by Brittany Miles of the University of Arizona, the team’s observations represent the most molecules ever detected in an exoplanetary atmosphere from a single observation.

The silicate clouds in VHS 1256b’s atmosphere can remain at high altitudes due to the planet’s low gravity — unlike many other brown dwarf exoplanets, which have stronger gravitational forces. Webb was able to collect data on the clouds because of their high atmospheric altitude, and the data showed both large and small silicate particles in these silicate clouds, which can be seen on a spectrum created by Webb’s NIRSpec and MIRI instruments.

The emission spectrum of VHS 1256b, measured by the James Webb Space Telescope. (Credit: NASA/ESA/CSA/J. Olmsted (STScI)/Brittany Miles (University of Arizona)/Sasha Hinkley (University of Exeter)/Beth Biller (University of Edinburgh)/Andrew Skemer (University of California, Santa Cruz))

“We’re seeing a lot of molecules in a single spectrum from Webb that detail the planet’s dynamic cloud and weather systems,” said Miles.

“The finer silicate grains in its atmosphere may be more like tiny particles in smoke. The larger grains might be more like very hot, very small sand particles,” said co-author Beth Biller of the University of Edinburgh in Scotland.

In addition to the high-altitude silicate clouds, VHS 1256b’s tumultuous atmosphere may be caused by the planet’s age. At only 150 million years old, VHS 1256b is quite young in astronomical terms, meaning it is likely still in the later stages of its formation and has not yet begun to cool to normal planetary temperatures.

However, though these findings are groundbreaking, they’re just the initial findings from Miles et al. As the team continues to comb through the data, they’ll likely find more elements or structures in the atmosphere of the exoplanet. Furthermore, the data has been pulled from only a single observation of the exoplanet. Future observations of VHS 1256b could reveal even more about its volatile atmosphere and extreme characteristics.

“We’ve identified silicates, but better understanding which grain sizes and shapes match specific types of clouds is going to take a lot of additional work. This is not the final word on this planet – it is the beginning of a large-scale modeling effort to fit Webb’s complex data,” Miles said.

“There’s a huge return on a very modest amount of telescope time. With only a few hours of observations, we have what feels like unending potential for additional discoveries,” Biller added.

Miles et al.’s data and results, titled “The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b,” was published on March 22 in The Astrophysical Journal Letters. 

Webb Measures the Temperature of an Exoplanet

In addition to Webb’s groundbreaking observations of VHS 1256b, a second team of international scientists used Webb’s MIRI to measure the temperature of a rocky exoplanet located approximately 40 light-years away from Earth. Named TRAPPIST-1b, the planet is part of the well-known TRAPPIST-1 system — a collection of seven rocky exoplanets orbiting a red dwarf star.

While star systems of rocky exoplanets are not uncommon, the exoplanets of the TRAPPIST-1 system are of particular interest to scientists because of their similarities in size and mass to the inner rocky planets of our solar system. Interestingly, the orbits of all seven exoplanets could fit within the orbit of Mercury, but given the small size of their host star, the energy some of the planets receive is comparable to the energy Earth receives from the Sun.

TRAPPIST-1b, the innermost planet of the TRAPPIST-1 system, orbits at an orbital distance of about one-hundredth that of Earth’s, receiving nearly four times the amount of energy Earth receives from our Sun. Given TRAPPIST-1b’s close proximity to its host star, it does not lay within TRAPPIST-1’s habitable zone. Nonetheless, any information collected on the planet will prove to be useful for scientists trying to characterize its sibling exoplanets and the evolution of the TRAPPIST-1 and other red dwarf systems.

Artist’s illustration of TRAPPIST-1b, with red dwarf TRAPPIST-1 seen in the background. (Credit: NASA/ESA/CSA/J. Olmsted (STScI))

“It’s easier to characterize terrestrial planets around smaller, cooler stars. If we want to understand habitability around M stars, the TRAPPIST-1 system is a great laboratory. These are the best targets we have for looking at the atmospheres of rocky planets,” said co-author Elsa Ducrot of the French Alternative Energies and Atomic Energy Commission (CEA) in France.

Since their discovery in 2017, scientists have been trying to learn more about each of the seven exoplanets. However, until last year, scientists didn’t have access to telescopes powerful enough to measure specific details on each exoplanet.

With the release of Webb’s first images in July 2022, several teams of scientists were already planning to use the telescope to observe the TRAPPIST-1 exoplanets. Thomas Greene, lead author of the study and scientist at NASA’s Ames Research Center, and his international team of astrophysicists were one of those teams and were ready to utilize the mid-infrared capabilities of Webb and MIRI — a capability which, to that point, had not been available.

“These observations really take advantage of Webb’s mid-infrared capability. No previous telescopes have had the sensitivity to measure such dim mid-infrared light,” Greene said.

Greene et al. made the temperature measurements by investigating the thermal emission, or the heat energy radiated by a planet, from TRAPPIST-1b. Planets radiate heat in the form of infrared light, and with Webb’s incredible sensitivity to infrared light, measuring the thermal characteristics of TRAPPIST-1b was sure to yield impressive results.

The team found that TRAPPIST-1b’s dayside temperature sits at about 500 Kelvin, or 227 degrees Celsius — suggesting that the planet may not have an atmosphere. Former observations of TRAPPIST-1b using NASA’s Hubble Space Telescope and infrared Spitzer Space Telescope found no evidence of an atmosphere around the exoplanet. However, the observations were not enough to completely rule out the possibility of a dense atmosphere existing around the planet.

Graphic comparing the measured temperature of TRAPPIST-1b to computer models, Earth, and Mercury. (Credit: NASA/ESA/CSA/J. Olmsted (STScI)/Thomas Greene (NASA Ames)/Taylor Bell (BAERI)/Elsa Ducrot (CEA)/Pierre-Olivier Lagage (CEA))

“There are ten times as many of these [red dwarf] stars in the Milky Way as there are stars like the Sun, and they are twice as likely to have rocky planets as stars like the Sun. But they are also very active ­– they are very bright when they’re young, and they give off flares and X-rays that can wipe out an atmosphere,” Greene said.

For Greene et al. to determine the existence of an atmosphere around TRAPPIST-1b with confidence, they had to use the team’s groundbreaking temperature measurement.

“This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness. If it has an atmosphere to circulate and redistribute the heat, the dayside will be cooler than if there is no atmosphere,” said co-author Pierre-Olivier Lagage of CEA.

Webb collected the data used for the temperature measurement by performing a technique called secondary eclipse photometry. When using the technique, MIRI measured the change in brightness of the entire TRAPPIST-1 system as the planet moved behind the star. Given that TRAPPIST-1b emits its own infrared light, Greene et al. simply subtracted the light from only TRAPPIST-1 from the light of both the planet and star combined to calculate the exact amount of infrared light being emitted by TRAPPIST-1b. To ensure their data was correct, the team collected and analyzed data for five secondary eclipses.

The total change in brightness between the star and the planet is less than 0.1%, once again highlighting the immense power and precision of Webb and its world-class suite of instruments.

“There was also some fear that we’d miss the eclipse. The planets all tug on each other, so the orbits are not perfect. But it was just amazing: The time of the eclipse that we saw in the data matched the predicted time within a couple of minutes,” said researcher Taylor Bell of the Bay Area Environmental Research Institute, who performed data analysis for Greene et al.

Graphic showing the light curve created by the secondary eclipse of TRAPPIST-1b behind TRAPPIST-1. (Credit: NASA/ESA/CSA/J. Olmsted (STScI)/Thomas Greene (NASA Ames)/Taylor Bell (BAERI)/Elsa Ducrot (CEA)/Pierre-Olivier Lagage (CEA))

“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements,” Ducrot explained.

As mentioned, the temperature measurement marks the first time any light emitted by a rocky exoplanet has been directly detected and measured by a telescope. What’s more, the temperature data is giving scientists valuable insights into red dwarf star systems and the characteristics of the exoplanets that orbit them.

“There was one target that I dreamed of having, and it was this one. This is the first time we can detect the emission from a rocky, temperate planet. It’s a really important step in the story of discovering exoplanets,” said Lagage, who assisted with the development of MIRI.

The data used in Greene et al.’s study was collected as part of Webb’s Guaranteed Time Observation (GTO) program 1177. The program is one of eight planned GTO programs that are expected to further characterize and explore the TRAPPIST-1 system and its exoplanets. Webb is currently performing additional secondary eclipse observations of TRAPPIST-1b, which scientists are hoping will allow them to capture a full phase curve for the planet, which would show how the brightness of the planet changes across its entire orbit of TRAPPIST-1. Having the data from a full phase curve would give scientists insight into how the planet’s temperature changes from TRAPPIST-1b’s dayside and nightside — allowing them to determine the existence of an atmosphere with high confidence.

Greene et al.’s study, titled “Thermal Emission from the Earth-sized Exoplanet TRAPPIST-1 b using JWST,” was published in the journal Nature on March 27. 

(Lead image: (top left) Artist’s illustration of VHS 1256b. (bottom right) Artist’s illustration of TRAPPIST-1b. Credit: NASA/ESA/CSA/Joseph Olmsted (STScI))

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