The JWST has found an exoplanet unlike any other. This unique world has an atmosphere almost entirely composed of water vapour. Astronomers have theorized about these types of planets, but this is the first observational confirmation.
The unique planet is GJ 9827 d. It’s about twice as large as Earth and three times as massive, and it orbits a K-type star about 100 light years away. The Kepler Space Telescope first discovered it during its K2 extension. In 2023, astronomers studied it with the Hubble Space Telescope. They detected hints of water vapour and described it as an ocean world.
“This is the first time we’re ever seeing something like this.”
Eshan Raul, University of Wisconsin – MadisonHowever, the JWST results show that the atmosphere is almost completely comprised of water vapour.
The results are in new research published in The Astrophysical Journal Letters titled “JWST/NIRISS Reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d.” The lead author is Caroline Piaulet-Ghorayeb from the University of Montréal’s Trottier Institute for Research on Exoplanets.
Astronomers have wondered if steam planets can exist. Some thought that life could exist on them in the cooler, higher layers of their atmospheres. Others think it’s extremely unlikely. But there was no evidence to go on until now.
“This is the first time we’re ever seeing something like this,” said Eshan Raul, who analyzed the JWST data of GJ 9827 d as an undergraduate student at the University of Michigan. “To be clear, this planet isn’t hospitable to at least the types of life that we’re familiar with on Earth. The planet appears to be made mostly of hot water vapor, making it something we’re calling a ‘steam world.'”
However, every exoplanet teaches us something. GJ 9827 d and its unique atmosphere will help scientists understand exoplanets better in general.
“If these are real, it really makes you wonder what else could be out there.”
Eshan Raul, University of Wisconsin – MadisonThe researchers used transmission spectroscopy to detect the exoplanet’s atmosphere. As the planet passes in front of its star, the atmosphere absorbs certain wavelengths of light in the starlight’s spectrum. Different chemicals absorb different wavelengths and reveal their presence.
The observations show that GJ 9827 d’s atmosphere is more than 31% water vapour by volume and has very high metal enrichment. The observations also show that no hydrogen or helium is escaping.
The exoplanet’s atmosphere may be strange, but in other ways, the planet itself is common. It’s a sub-Neptune, a planet larger than Earth but smaller than Neptune. Sub-Neptunes are the most common type of exoplanet we’ve found in the Milky Way.
This discovery is about more than sub-Neptunes and steam worlds. It’s about one of the key challenges in exoplanet atmospheres: the clouds-metallicity degeneracy.
When astronomers use transmission spectroscopy to examine and characterize an exoplanet’s atmosphere, high metallicity and clouds can produce the same signal. High metallicity can produce smaller spectral features, and clouds can also mute and flatten spectral features. Clouds can also mask the presence of molecular absorbers below the cloud deck. As a result, when scientists see a relatively flat spectrum or muted features, they struggle to determine if they’re seeing a metal-rich atmosphere with intrinsically small features or a low-metallicity atmosphere that’s partially obscured by clouds.
This research has broken the stalemate between clouds and metallicity.
Piaulet-Ghorayeb and her co-authors combined previous Hubble Space Telescope observations of GJ 9827 d with JWST observations. The JWST used its NIRISS (Near-Infrared Imager and Slitless Spectrograph) and SOSS (Single Object Slitless Spectroscopy) to analyze the exoplanet’s atmosphere during two transits. This provided enough wavelength coverage and precision to break the clouds-metallicity degeneracy. This is the first conclusive observation of a high-metallicity and water-rich atmosphere.
“This is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.”
Ryan MacDonald, Astrophysicist, University of Wisconsin This figure from the research shows GJ 9827 d’s two transits observed by the JWST. The broad wavelength coverage and the precision broke the clouds-metallicity degeneracy. Image Credit: Piaulet-Ghorayeb et al. 2024.Almost all the exoplanet atmospheres that have been characterized are mostly made of the lighter elements hydrogen and helium. These atmospheres are similar to Jupiter and Saturn in our Solar System. They’re nothing like Earth and its life-friendly atmosphere.
“GJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules like the terrestrial planets of the solar system,” Piaulet-Ghorayeb said. “This is a huge step.”
Though GJ 9827 d isn’t habitable as far as our understanding of life goes, other exoplanets with similar metallicity are desirable targets in the search for life. Now that astronomers have broken the clouds-metallicity degeneracy, it changes our understanding of those planets and scientists’ ability to discern them. It’s all thanks to the JWST and its observing prowess.
Ryan MacDonald is a co-author of the new research and is a U-M astrophysicist and NASA Sagan Fellow. “Even with JWST’s early observations in 2022, researchers were discovering new insights into the atmospheres of distant gas giants,” MacDonald said, referring to the JWST’s spectroscopic characterizations of exoplanet atmospheres.
But those atmospheres were primarily composed of light gases, not heavier metals. These observations take us deeper into the atmospheres of sub-Neptunes. And though they’re the most common type of exoplanet in our galaxy, our Solar System is without one.
“Now we’re finally pushing down into what these mysterious worlds with sizes between Earth and Neptune, for which we don’t have an example in our own solar system, are actually made of,” MacDonald said. “This is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.”
The atmospheric steam didn’t jump out of the JWST observations. JWST produces an enormous amount of data, and to make sense of it, astronomers use modelling tools based on sampling algorithms and machine learning techniques. They typically employ several different models and work with all of the results to arrive at the most likely interpretation of the data.
The process of determining an atmosphere from data is called atmospheric retrieval. A 2023 paper presented a catalogue of 50 different atmospheric retrieval codes used by exoplanet scientists. The lead author of that paper is none other than Ryan MacDonald, a co-author of this new research. MacDonald wrote the software that analyzed and retrieved GJ 9827 d’s atmosphere, and co-author Raul used that software.
Raul generated millions of model atmospheres that matched the JWST observations before settling on the steam world model. In a sense, Raul was the first person to see proof that steam worlds exist.
“It was a very surreal moment,” said Raul, who is now working toward his doctorate at the University of Wisconsin-Madison. “We were searching specifically for water worlds because it was hypothesized that they could exist.”
“If these are real, it really makes you wonder what else could be out there.”
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While new rockets and human missions to the Moon are in the press, NASA is quietly thinking through the nuts and bolts of a long-term presence on the Moon. They have already released two white papers about the lunar logistics they’ll require in the future and are now requesting proposals from companies to supply some serious cargo transportation. But this isn’t just for space transport; NASA is also looking for ground transportation on the Moon that can move cargo weighing as much as 2,000 to 6,000 kg (4,400 to 13,000 pounds.)
In a recent press release, NASA asked U.S. industry to submit proposals for logistics ideas and solutions to help the agency land and move cargo on the lunar surface during the upcoming Artemis missions.
“NASA relies on collaborations from diverse partners to develop its exploration architecture,” said Nujoud Merancy, deputy associate administrator, strategy and architecture in the Exploration Systems Development Mission Directorate at NASA Headquarters in Washington. “Studies like this allow the agency to leverage the incredible expertise in the commercial aerospace community.”
In the two white papers, NASA outlined the “gaps” they have lunar logistics and mobility as part of its Moon to Mars architecture. In the first paper, “Lunar Logistics Drivers, Needs,” NASA said that as the Artemis missions and goals are conceptualized and planned, it is imperative to accurately predict logistics and resupply needs, not only for mission goals but for the very important need of keeping the humans alive and healthy. They need to have a good plan and the ability to transport landed cargo and exploration items from where they are delivered to where they are used.
Graph showing approximate logistics item needs for representative lunar surface missions. Credit: NASA.“The total amount of logistics items required to keep the crew alive and healthy, to maintain systems, and to perform productive science and utilization can be relatively large,” the authors wrote. “It can also heavily influence the design of the architecture and exploration missions. The architecture must therefore be based on comprehensive, accurate estimates of logistics item needs and include assessment of a suitable logistics sub-architectures to deliver those needs.”
How to provide various things like food, water, air, spare parts, and other similar products required to sustain life, as well as maintain all the various systems and structures are key to having productive science and utilization activities. NASA also expects they will need to move all these supplies around on the Moon, including to the lunar South Pole where they plan to send crews in the future. The paper outlines the importance of accurately predicting logistics resupply needs, as they can heavily influence the overall architecture and design of exploration missions.
An artist’s conception shows NASA’s generic concept for the Lunar Terrain Vehicle. (NASA Illustration)NASA’ said their current planned lunar mobility elements, such as the Lunar Terrain Vehicle and Pressurized Rover, have a capability limit of about 1,760 pounds (800 kilograms) and will primarily be used to transport astronauts around the lunar surface. However, future missions could include a need to move cargo totaling around 4,400 to 13,000 pounds (2,000 to 6,000 kg). That’s why NASA wants input from companies who have experience in this area.
But to be able to move cargo around to various places on the Moon, NASA first needs to get the supplies to the lunar surface. The second white paper, “Lunar Surface Cargo,” looks at the lunar surface cargo delivery needs, compares those needs with current cargo lander capabilities, and outlines considerations for fulfilling this capability gap. NASA said that access to a diverse fleet of cargo landers would empower a larger lunar exploration footprint, and that a combination of international partnerships and U.S. industry-provided landers could supply the concepts and capabilities to meet this need.
“Given diverse cargo needs of varying size, mass, delivery cadence, and operational needs, a diverse portfolio of cargo lander capabilities will be necessary to achieve NASA’s Moon to Mars Objectives,” the paper says. “Encouraging the development of varied cargo lander concepts and capabilities will be key to establishing a long-term lunar presence for science and exploration.”
Planned and potential cargo to the lunar surface. Credit: NASAWhile the request for proposals doesn’t explicitly seek new concepts for landing vehicles, it does ask for integrated assessments of logistics that can include transportation elements.
“We’re looking for industry to offer creative insights that can inform our logistics and mobility strategy,” said Brooke Thornton, industry engagement lead for NASA’s Strategy and Architecture Office. “Ultimately, we’re hoping to grow our awareness of the unique capabilities that are or could become a part of the commercial lunar marketplace.”
Got ideas? Check out NASA’s Request for Proposals.
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Most of the diverse elements in the Universe come from supernovae. We are, quite literally, made of the dust of those long-dead stars and other astrophysical processes. But the details of how it all comes about are something astronomers strive to understand. How do the various isotopes produced by supernovae drive the evolution of planetary systems? Of the various types of supernovae, which play the largest role in creating the elemental abundances we see today? One way astronomers can study these questions is to look at presolar grains.
These are dust grains formed long before the formation of the Sun. Some of them were cast out of older systems as a star fired up its nuclear furnace and cleared its system of dust. Others formed from the remnants of supernovae and stellar collisions. Regardless of its origin, each presolar grain has a unique isotopic fingerprint that tells us its story. For decades, we could only study presolar grains found in meteorites, but missions such as Stardust have captured particles from comets, giving us a richer source for study. Observations from radio telescopes such as ALMA allow astronomers to look at the isotope ratios of these grains at their point of origin. We can now study presolar grains both in the lab and in space. A new study compares the two, focusing on the role of supernovae.
Pair of presolar grains from the Murchison meteorite. Credit: Argonne National Laboratory, Department of EnergyWhat they found was that the physical gathering of presolar grains will be crucial to understanding their origins. For example, Type II supernovae, also known as [core-collapse supernovae,](https://briankoberlein.com/post/supernovas-tale/) are known to produce Titanium-44, which is an unstable isotope. Through decay processes, this can create an excess of Calcium-44 in presolar grains. But grains cast off from young star systems also have a Calcium-44 excess. In the first case, the grains form with titanium, which then decays to calcium, while in the second case, the grains form with calcium directly. We can’t distinguish between the two just by looking at the isotope ratios. Instead, we have to look at the specific distribution of Calcium-44 within the grain. The team found that using nanoscale secondary ion mass spectrometry (NanoSIMS) they could distinguish the origin of grains found in meteorites. Similar complexities are seen with isotopes of silicon and chromium.
Overall, the study proves that we will need much more study to tease apart the origins of the presolar grains we gather. But as we better understand the grains we gather here on Earth, they should help us unravel a deeper understanding of how elements are forged in the nuclear furnaces of large stars.
Reference: Liu, Nan, et al. “Presolar grains as probes of supernova nucleosynthesis.” arXiv preprint arXiv:2410.19254 (2024).
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