LAWRENCE, Kan. — As Earth telescopes scan the stars for planets that may support life, they also find plenty of things that leave astronomers scratching their heads. Researchers say a planet they call a “hot Neptune” falls right into the head-scratching category. Not only does this scolding hot world travel around its sun in less than a day, it has an atmosphere that “shouldn’t exist.”
University of Kansas astronomer Ian Crossfield says new data from NASA’s TESS and Spitzer space telescopes is providing the first look at an exoplanet’s atmosphere the satellites captured. Planet LTT 9779b, which sits about 260 light-years from Earth and is so close to its home star it doesn’t have ground for a probe to land on.
“This planet doesn’t have a solid surface, and it’s much hotter even than Mercury in our solar system — not only would lead melt in the atmosphere of this planet, but so would platinum, chromium and stainless steel,” Crossfield explains in a media release. “A year on this planet is less than 24 hours — that’s how quickly it’s whipping around its star. It’s a pretty extreme system.”
This “hot Neptune’s” wild ride around its sun isn’t the only thing shocking scientists. Unlike most planets which lose their water and atmosphere in a close orbit, the study on LTT 9779b reveals a cloud layer has somehow survived here.
“For the first time, we measured the light coming from this planet that shouldn’t exist,” the assistant professor of physics & astronomy says. “This planet is so intensely irradiated by its star that its temperature is over 3,000 degrees Fahrenheit and its atmosphere could have evaporated entirely. Yet, our Spitzer observations show us its atmosphere via the infrared light the planet emits.”
Shedding light on LTT 9779b
Study authors add NASA’s telescopes measured infrared light coming from the planet as it rotates 360 degrees on its axis. Measuring this light reveals a world’s temperature and where the hotter and cooler spots are. In Earth’s case, the planet is hottest a few hours after noon. On LTT 9779b, scientists find its hottest right at noon when the planet’s sun is right overhead.
“The planet is much cooler than we expected, which suggests that it is reflecting away much of the incident starlight that hits it, presumably due to dayside clouds,” co-author Nicolas Cowan from Montreal’s McGill University adds. “The planet also doesn’t transport much heat to its nightside, but we think we understand that: The starlight that is absorbed is likely absorbed high in the atmosphere, from whence the energy is quickly radiated back to space.”
Why do they call it ‘hot Neptune?’
The main thing Neptune, our solar system’s eighth planet, and the galaxy’s “hot Neptunes” have in common are their size. Our Neptune is about four times as wide as the Earth. NASA calls it an ice giant that is dark, cold, and whipped by supersonic winds sitting about 2.8 billion miles away from the Sun.
Hot Neptunes however, sit in a region around their suns that is typically devoid of planets with atmospheres. Most exoplanets which are that close to their star are either gigantic gas worlds (hot Jupiters) or rocky planets with no atmosphere left. Crossfield says finding LTT 9779b is an interstellar rarity.
“We think this is because hot Neptunes aren’t massive enough to avoid substantial atmospheric evaporation and mass loss.”
‘Training wheels’ for finding habitable worlds
While no one from Earth is going to be visiting this world, researchers say finding an atmosphere on LTT 9779b is good practice for finding Earth-like planets.
“What our measurements so far show us are what we call the spectral absorption features — and its spectrum indicates carbon monoxide and or carbon dioxide in the atmosphere. We’re starting to get a handle on what molecules make up its atmosphere. Because we see this, and because of how this global temperature map looks, it also tells us something about how the winds are circulating energy and material around through the atmosphere of this mini gas planet,” Crossfield explains.
“If anyone is going to believe what astronomers say about finding signs of life or oxygen on other worlds, we’re going to have to show we can actually do it right on the easy stuff first. In that sense these bigger, hotter planets like LTT 9779b act like training wheels and show that we actually know what we’re doing and can get everything right.”
Astronomers say NASA’s next big mission, the James Webb Space Telescope, will also be targeting this hot Neptune for observation.
The study appears in the journal Astrophysical Journal Letters.
This article was first published October 24, 2020.