Studying a unique class of ultra-hot exoplanets, astronomers at NASA’s Hubble Space Telescope might be in the mood to dance to the Calypso party song “Hot, Hot, Hot.” It’s because these puffy Jupiter-sized worlds are so precariously close to their parent star that they’re roasting in boiling temperatures above 3,000 degrees Fahrenheit. It’s hot enough to vaporize most metals, including titanium. They have the hottest planetary atmospheres ever seen.
In two new papers, teams of Hubble astronomers report on strange weather patterns on these scorching worlds. One planet is raining vaporized rock, and another is seeing its upper atmosphere become warmer rather than cooler because it is “sunburned” by intense ultraviolet (UV) radiation from its star.
This research goes beyond the simple discovery of strange and offbeat planetary atmospheres. Studying extreme weather gives astronomers greater insight into the diversity, complexity, and exotic chemistry taking place on distant worlds across our galaxy.
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“We still don’t have a good understanding of weather in different planetary environments,” said David Sing of Johns Hopkins University in Baltimore, Maryland, co-author of two reported studies. “When you look at Earth, all of our weather forecasts are still finely tuned to what we can measure. But when you go to a distant exoplanet, you have limited predictive powers because you haven’t built a general theory on the how everything in an atmosphere fits together and responds to extreme conditions Even if you know the basics of chemistry and physics, you don’t know how it will manifest in complex ways.
In an April 7 newspaper article Nature, astronomers describe Hubble observations of WASP-178b, located about 1,300 light-years away. On the daytime side, the atmosphere is cloudless and enriched with silicon monoxide gas. Because one side of the planet is permanently facing its star, the scorching atmosphere moves to the night side at super-hurricane speeds exceeding 2,000 miles per hour. On the dark side, silicon monoxide can cool enough to condense into rock that rains down from the clouds, but even at dawn and dusk the planet is hot enough to vaporize the rock. “We knew we had seen something really cool with this silicon monoxide feature,” said Josh Lothringer of Utah Valley University in Orem, Utah.
In an article published in the January 24 issue of Astrophysical Journal Letters, Guangwei Fu of the University of Maryland, College Park, reported a super hot Jupiter, KELT-20b, located about 400 light-years away. On this planet, a burst of ultraviolet light from its parent star creates a thermal layer in the atmosphere, much like Earth’s stratosphere. “Until now, we never knew how the host star directly affects a planet’s atmosphere. There have been a lot of theories, but now we have the first observational data,” Fu said.
In comparison, on Earth, ozone in the atmosphere absorbs UV light and raises temperatures in a layer between 7 and 31 miles above the Earth’s surface. On KELT-20b, the star’s UV radiation heats metals in the atmosphere, creating a very strong thermal inversion layer.
The evidence came from Hubble’s detection of water in near-infrared observations and the detection of carbon monoxide by NASA’s Spitzer Space Telescope. They radiate through the warm, transparent upper atmosphere that is produced by the inversion layer. This signature is unique compared to what astronomers see in the atmospheres of hot Jupiters orbiting cooler stars, like our Sun. “KELT-20b’s emission spectrum is quite different from other hot Jupiters,” Fu said. “This is irrefutable proof that planets do not live in isolation but are affected by their host star.”
Although super-hot Jupiters are uninhabitable, this type of research helps pave the way for a better understanding of the atmospheres of potentially habitable terrestrial planets. “If we can’t understand what’s happening on super hot Jupiters where we have strong and reliable observational data, we won’t have a chance to understand what’s happening in the fainter spectra observing terrestrial exoplanets. “, said Lothringer. “This is a test of our techniques that allows us to build a general understanding of physical properties such as cloud formation and atmospheric structure.”
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
Illustration of KELT-20b Credit: NASA, ESA, Leah Hustak (STScI)