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Our solar system has a wide variety of planet types, from tiny rocky Mercury to huge puffy gas giant Jupiter to distant ice giant Uranus. But beyond our own system, there are even more types of exoplanet out there, including water worlds covered in ocean and where life could potentially thrive. Now, researchers using the James Webb Space Telescope have identified a new and exotic type of planet called a steam world, which has an atmosphere almost entirely composed of water vapor.
The planet, called GJ 9827 d, was examined by the Hubble Space Telescope earlier this year and had researchers so intrigued that they wanted to go back for a closer look using Webb. They found that the planet, which is around twice the size of Earth, had a very different atmosphere from the typical hydrogen and helium that is usually seen. Instead, it was full of hot steam.
“This is the first time we’re ever seeing something like this,” said researcher Eshan Raul of the University of Michigan in a statement. “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.’”
To look at the planet’s atmosphere, the researchers used Webb’s Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument that can split light into different wavelengths to see what something is composed of in a technique called transmission spectroscopy. This is easier to do with lighter elements like hydrogen and helium, so being able to use this technique for a heavier element like water means scientists can now start to investigate more diverse planetary atmospheres.
“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,” said fellow researcher Ryan MacDonald. “This is a crucial proving step towards detecting atmospheres on habitable exoplanets in the years to come.”
As this is such a new area of research, the discovery required new software written by the team so Raul, who is an undergraduate student, was the first person to see direct evidence that steam worlds existed.
“It was a very surreal moment,” said Raul, now a doctoral candidate 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.”
The research is published in The Astrophysical Journal Letters.
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