Studying a unique class of ultrahot exoplanets, astronomers at NASA’s Hubble Space Telescope may be in the mood to dance to Calypso’s party song “Hot, Hot, Hot.” That’s because these bloated, Jupiter-sized worlds are so precariously close to their parent star that they’re roasting in scalding temperatures above 3,000 degrees Fahrenheit. That’s hot enough to vaporize most metals, including titanium. They have the hottest planetary atmospheres ever seen.
In two new articles, Hubble teams of astronomers report strange weather conditions on these hot worlds. It’s raining down vaporized rock on one planet, and another’s upper atmosphere is getting hotter instead of cooler because it’s being “sunburned” by its star’s intense ultraviolet (UV) radiation.
This research goes beyond simply finding strange and wacky planetary atmospheres. Studying extreme weather gives astronomers a better understanding of the diversity, complexity, and exotic chemistry taking place on remote worlds throughout our galaxy.
“We still don’t have a good understanding of climate in different planetary environments,” said David Sing of Johns Hopkins University in Baltimore, Maryland, co-author of two studies that are being reported. “When you look at Earth, all of our weather predictions 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 about how everything in an atmosphere comes together and responds to extreme conditions. Even though you know basic chemistry and physics, you don’t know how it’s going to manifest in complex ways.”
In an article in the April 7 magazine Nature, astronomers describe Hubble observations of WASP-178b, located about 1,300 light-years away. On the day side, the atmosphere is cloudless and enriched with silicon monoxide gas. Because one side of the planet permanently faces its star, the torrid atmosphere spins toward the night side at super-hurricane speeds in excess of 2,000 miles per hour. On the dark side, silicon monoxide can get cool enough to condense into rock that rains down from clouds, but even at dawn and dusk, the planet is hot enough to vaporize rock. “We knew we had seen something really interesting 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 is creating a thermal layer in the atmosphere, much like Earth’s stratosphere. “Until now we never knew how the host star directly affected the atmosphere of a planet. There have been many theories, but now we have the first observational data,” Fu said.
By comparison, on Earth, ozone in the atmosphere absorbs ultraviolet light and raises temperatures in a layer between 7 and 31 miles above the Earth’s surface. In KELT-20b, ultraviolet radiation from the star heats metals in the atmosphere, creating a very strong inversion layer.
The evidence came from Hubble’s detection of water in near-infrared observations and from NASA’s Spitzer Space Telescope detection of carbon monoxide. They radiate through the hot, transparent upper atmosphere that is produced by the inversion layer. This signature is unique 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 compelling evidence 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 figure out what’s going on in superhot Jupiters where we have strong, reliable observational data, we won’t have a chance to figure out what’s going on in fainter spectra when looking at terrestrial exoplanets,” Lothringer said. “This is a test of our techniques that allows us to develop a general understanding of physical properties, such as cloud formation and atmospheric structure.”
The Hubble Space Telescope is an international cooperative project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages 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, DC
Illustration of KELT-20b Credit: NASA, ESA, Leah Hustak (STScI)