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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. <\/p>\n
<\/p>\n The unique planet is GJ 9827 d. It\u2019s 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. <\/p>\n \u201cThis is the first time we\u2019re ever seeing something like this.\u201d<\/p>\n Eshan Raul, University of Wisconsin \u2013 Madison<\/cite><\/p><\/blockquote>\n<\/figure>\n However, the JWST results show that the atmosphere is almost completely comprised of water vapour.<\/p>\n The results are in new research published in The Astrophysical Journal Letters titled \u201cJWST\/NIRISS Reveals the Water-rich \u201cSteam World\u201d Atmosphere of GJ 9827 d.\u201d The lead author is Caroline Piaulet-Ghorayeb from the University of Montr\u00e9al\u2019s Trottier Institute for Research on Exoplanets. <\/p>\n 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\u2019s extremely unlikely. But there was no evidence to go on until now. <\/p>\n \u201cThis is the first time we\u2019re ever seeing something like this,\u201d said Eshan Raul, who analyzed the JWST data of GJ 9827 d as an undergraduate student at the University of Michigan. \u201cTo be clear, this planet isn\u2019t hospitable to at least the types of life that we\u2019re familiar with on Earth. The planet appears to be made mostly of hot water vapor, making it something we\u2019re calling a \u2018steam world.’\u201d<\/p>\n However, every exoplanet teaches us something. GJ 9827 d and its unique atmosphere will help scientists understand exoplanets better in general. <\/p>\n \u201cIf these are real, it really makes you wonder what else could be out there.\u201d<\/p>\n Eshan Raul, University of Wisconsin \u2013 Madison<\/cite><\/p><\/blockquote>\n<\/figure>\n The researchers used transmission spectroscopy to detect the exoplanet\u2019s atmosphere. As the planet passes in front of its star, the atmosphere absorbs certain wavelengths of light in the starlight\u2019s spectrum. Different chemicals absorb different wavelengths and reveal their presence. <\/p>\n The observations show that GJ 9827 d\u2019s 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. <\/p>\n The exoplanet\u2019s atmosphere may be strange, but in other ways, the planet itself is common. It\u2019s a sub-Neptune, a planet larger than Earth but smaller than Neptune. Sub-Neptunes are the most common type of exoplanet we\u2019ve found in the Milky Way.<\/p>\n This discovery is about more than sub-Neptunes and steam worlds. It\u2019s about one of the key challenges in exoplanet atmospheres: the clouds-metallicity degeneracy. <\/p>\n When astronomers use transmission spectroscopy to examine and characterize an exoplanet\u2019s 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\u2019re seeing a metal-rich atmosphere with intrinsically small features or a low-metallicity atmosphere that\u2019s partially obscured by clouds. <\/p>\n This research has broken the stalemate between clouds and metallicity.<\/p>\n 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\u2019s 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. <\/p>\n \u201cThis is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.\u201d<\/p>\n Ryan MacDonald, Astrophysicist, University of Wisconsin<\/cite><\/p><\/blockquote>\n<\/figure>\n 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\u2019re nothing like Earth and its life-friendly atmosphere. <\/p>\n \u201cGJ 9827 d is the first planet where we detect an atmosphere rich in heavy molecules like the terrestrial planets of the solar system,\u201d Piaulet-Ghorayeb said. \u201cThis is a huge step.\u201d<\/p>\n Though GJ 9827 d isn\u2019t 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\u2019 ability to discern them. It\u2019s all thanks to the JWST and its observing prowess. <\/p>\n Ryan MacDonald is a co-author of the new research and is a U-M astrophysicist and NASA Sagan Fellow. \u201cEven with JWST\u2019s early observations in 2022, researchers were discovering new insights into the atmospheres of distant gas giants,\u201d MacDonald said, referring to the JWST\u2019s spectroscopic characterizations of exoplanet atmospheres. <\/p>\n 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\u2019re the most common type of exoplanet in our galaxy, our Solar System is without one. <\/p>\n \u201cNow we\u2019re finally pushing down into what these mysterious worlds with sizes between Earth and Neptune, for which we don\u2019t have an example in our own solar system, are actually made of,\u201d MacDonald said. \u201cThis is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.\u201d<\/p>\n The atmospheric steam didn\u2019t 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. <\/p>\n 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\u2019s atmosphere, and co-author Raul used that software. <\/p>\n 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. <\/p>\n \u201cIt was a very surreal moment,\u201d said Raul, who is now working toward his doctorate at the University of Wisconsin-Madison. \u201cWe were searching specifically for water worlds because it was hypothesized that they could exist.\u201d<\/p>\n \u201cIf these are real, it really makes you wonder what else could be out there.\u201d<\/p>\n\n
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Like this:<\/h3>\n