‘Hot Neptune’ found 260 light-years away that should not exist

Published: Nov. 28, 2020 at 2:42 PM CST
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LAWRENCE, Kan. (WIBW) - A new study at the University of Kansas details the atmosphere on ‘hot Neptune’ which is 260 light-years away and should not exist.

The University of Kansas says a team led by one of its astronomers has crunched data from NASA’s TESS and Spitzer telescopes to portray for the first time the atmosphere of a highly unusual kind of exoplanet that has been named ‘hot Neptune.’

KU said the findings concern the recently found planet LTT 9779b and were published in Astrophysical Journal Letters. It said the paper describes the very first spectral atmospheric characterization of any planet discovered by TESS, the first global temperature map of any TESS Plante with an atmosphere and a hot Neputen whose emission spectrum is fundamentally different from the larger ‘hot Jupiters’ that have previously been studied.

“For the first time, we measured the light coming from this planet that shouldn’t exist,” said Ian Crossfield, assistant professor of physics & astronomy at KU and lead author of the paper. “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.”

According to the university, while LTT 9779b is extraordinary, people probably would not like it there very much.

“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 said. “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.”

KU said Hot Neptune LTT 9779b was discovered in 2019 and became one of the first Neptune sized planets discovered by NASA’s all-sky Tess planet-hunting mission. It said Crossfiled and his coauthors used “phase curve” analysis in order to parse the exoplanet’s atmospheric makeup.

“We measure how much infrared light was being 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 hotter and cooler parts of this planet are — on Earth, it’s not hottest at noon; it’s hottest a couple of hours into the afternoon. But on this planet, it’s actually hottest just about at noon. We see most of the infrared light coming from the part of the planet when its star is straight overhead and a lot less from other parts of the planet.”

According to KU, readings of the planet’s temperature is seen as a way to characterize its atmosphere.

“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,” said co-author Nicolas Cowan of the Institute for Research on Exoplanets (iREx) and McGill University in Montreal, who helped in the analysis and interpretation of the thermal phase curve measurements. “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.”

Crossfield said the results are just a step into a new phase of exoplanetary exploration as the study of these atmospheres moves toward smaller and smaller planets.

“I wouldn’t say we understand everything about this planet now, but we’ve measured enough to know this is going to be a really fruitful object for future study,” he said. “It’s already being targeted for observations with the James Webb Space Telescope, which is NASA’s next big multibillion-dollar flagship space telescope that’s going up in a couple of years. 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.”

According to Crossfield, the extreme rarity of Neptune-like worlds found close to host stars are part of a region that is usually empty of planets. He said astronomers call it the “hot Neptune desert.”

“We think this is because hot Neptunes aren’t massive enough to avoid substantial atmospheric evaporation and mass loss,” he said. “So, most close-in hot exoplanets are either the massive hot Jupiters or rocky planets that have long ago lost most of their atmospheres.”

KU said a companion paper to the research is being led by Diana Dragomir, an assistant professor of physics and astronomy at the University of New Mexico, investigates the explanet’s atmospheric makeup through secondary eclipse observations with the Spitzer Infrared Array Camera of the hot Neptune.

According to KU, while LTT 9779b is not suitable for colonization by humans or any other known life form, Crossfiled said evaluating its atmosphere would improve techniques that someday could be used to find a more welcoming planet for life.

“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,” he said. “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.”

Crossfield said his look into the atmosphere of such a strange and distant planet was also valuable on its own merits.

“As someone who studies these, there’s just a lot of interesting planetary science we can do in measuring the properties of these planets — just like people study the atmospheres of Jupiter, Saturn and Venus — even though we don’t think those will host life,” he said. “They’re still interesting, and we can learn about how these planets formed and the broader context of planetary systems.”

According to Crossfield, much work is still to be done in order to better understand LTT 9779b and similar hot Neptunes that have not yet been discovered.

“We want to continue observing it with other telescopes so that we can answer more questions,” he said. “How is this planet able to retain its atmosphere? How did it form in the first place? Was it initially larger but has lost part of its original atmosphere? If so, then why is its atmosphere not just a scaled-down version of the atmospheres of ultra-hot larger exoplanets? And what else might be lurking in its atmosphere?”

KU said some of the researcher’s coauthors on the paper also plan to continue studying the improbable exoplanet.

“We detected carbon monoxide in its atmosphere and that the permanent dayside is very hot, while very little heat is transported to the night side,” said Björn Benneke of iREx and the Université de Montréal. “Both findings make LTT 9779b say that there is a very strong signal to be observed making the planet a very intriguing target for future detailed characterization with JWST. We’re now also planning much more detailed phase curve observations with NIRISS on JWST.”

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