A team led by a University of Kansas astronomer analyzed data from NASA’s TESS and Spitzer space telescopes to represent for the first time the atmosphere of a very unusual type of exoplanet dubbed “Hot Neptune.”
The findings concerning the recently discovered planet LTT 9779b were published today in Letters from the Astrophysical Journal. The article details the very first spectral atmospheric characterization of any planet discovered by TESS, the first global temperature map of any planet TESS with an atmosphere and a hot Neptune whose emission spectrum is fundamentally different from the many “hot Jupiters” studied previously.
“For the first time, we have measured the light coming from this planet that shouldn’t exist,” said Ian Crossfield, assistant professor of physics and astronomy at KU and lead author of the article. “This planet is so intensely irradiated by its star that its temperature exceeds 3,000 degrees Fahrenheit and its atmosphere could have evaporated entirely. Yet our Spitzer observations show us its atmosphere via infrared light emitted by the planet.
While the LTT 9779b is extraordinary, one thing is for sure: people probably wouldn’t like it much there.
“This planet does not have a solid surface, and it is even much hotter than Mercury in our solar system – not only would lead melt in this planet’s atmosphere, but so would platinum, chromium. and stainless steel, ”Crossfield said. “A year on this planet is less than 24 hours – that’s how fast it revolves around its star. It’s a pretty extreme system. “
The hot Neptune LTT 9779b was discovered last year, becoming one of the first Neptune-sized planets discovered by NASA’s TESS planet-hunting mission. Crossfield and his co-authors used a technique called “phase curve” analysis to analyze the atmospheric composition of the exoplanet.
“We measure the amount of infrared light emitted by the planet as it rotates 360 degrees on its axis,” he said. “Infrared light tells you the temperature of something and where the hottest and coldest parts of this planet are – on Earth, it’s not the hottest at noon; it’s hotter a few hours in the afternoon. But on this planet, it’s actually the hottest around noon. We see most of the infrared light coming from the part of the planet when its star is directly above the head and much less from other parts of the planet.
Reading the temperature of the planet is seen as a way to characterize its atmosphere.
“The planet is much cooler than we expected, which suggests that it reflects much of the incident starlight that hits it, possibly from daytime clouds,” the co-worker said. -author Nicolas Cowan of the Institute for Research on Exoplanets (iREx) and McGill University in Montreal, who assisted in the analysis and interpretation of thermal phase curve measurements. “The planet doesn’t carry much heat to its nocturnal part either, but we understand that: starlight that is absorbed is probably absorbed high in the atmosphere, from where the energy is quickly sent back into the atmosphere. space.”
According to Crossfield, the results are just a first step in a new phase of exoplanetary exploration as the study of exoplanet atmospheres gradually shifts to smaller and smaller planets.
“I wouldn’t say we understand everything on this planet now, but we’ve measured enough to know that this is going to be a really fruitful subject for future study,” he said. “It’s already targeted for observations with the James Webb Space Telescope, which is NASA’s next multi-billion dollar flagship large space telescope to be commissioned in a few years. What our measurements are showing us so far are what we call spectral absorption characteristics – and its spectrum indicates carbon monoxide and / or carbon dioxide in the atmosphere. We are starting to understand which molecules make up its atmosphere. Because we see this, and because of how this world temperature map looks, it also tells us something about how the winds move energy and matter through the atmosphere of this gaseous mini planet.
Crossfield explained the extreme rarity of Neptune-like worlds found near their host stars, a region typically so devoid of planets that astronomers call it the “hot desert of Neptune.”
“We believe this is because the hot Neptunes are not massive enough to avoid substantial atmospheric evaporation and mass loss,” he said. “So the closest hot exoplanets are either massive hot Jupiters or rocky planets that have long lost most of their atmospheres.
A complementary article to this research led by Diana Dragomir, assistant professor of physics and astronomy at the University of New Mexico, studies the atmospheric composition of the expoplanet via secondary eclipse observations with the Spitzer infrared camera (IRAC ) hot Neptune.
Although the LTT 9779b is not suitable for colonization by humans or any other known life form, Crossfield said evaluating its atmosphere would refine techniques that could one day be used to find more welcoming planets for life.
“If anyone is to believe what astronomers are saying about finding signs of life or oxygen on other worlds, we’re going to have to show that we can actually do it on the easy things first,” says -he. “In that sense, these bigger, hotter planets like LTT 9779b act as training wheels and show that we really know what we’re doing and that we can do everything right.
Crossfield said his glimpse into the atmosphere of such a strange and distant planet was also inherently valuable.
“As a person who studies them, there is a lot of interesting planetary science we can do to measure the properties of these planets – just like people study the atmospheres of Jupiter, Saturn, and Venus – although we don’t believe that these will shelter life. , “he said.” They are always interesting and we can learn more about the formation of these planets and the larger context of planetary systems.
Crossfield said there is still a lot of work to be done to better understand LTT 9779b and similar undiscovered Hot Neptunes. (A companion article regarding the atmospheric composition of LTT 9779b via analysis of its secondary eclipse “spectrum” is in publication, which Crossfield co-wrote.)
“We want to continue to observe it with other telescopes so that we can answer more questions,” he said. “How is this planet able to conserve its atmosphere? How did it form in the first place? Was it initially larger but lost some of its original atmosphere? If so, then why isn’t its atmosphere just a scaled-down version of the atmosphere of larger, ultra-hot exoplanets? And what else could be hiding in its atmosphere?
Some of the KU researcher’s co-authors also plan to continue studying the unlikely exoplanet.
“We have detected carbon monoxide in its atmosphere and that the permanent side of the day is very hot, while very little heat is transported to the night side,” said Björn Benneke of iREx and the University of Montreal . “Both results make LTT 9779b say there is a very strong signal to observe, making the planet a very intriguing target for future detailed characterization with JWST. We are also planning much more detailed phase curve observations with NIRISS on JWST. “
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