Very unusual “hot Neptune” exoplanet 260 light years away that “shouldn’t...

This artist’s impression shows LTT 9779b near the star it is orbiting, highlighting the ultra-hot day side (2000 Kelvin) of the planet and its fairly roasted night side (about 1000 K). Photo credit: Ethen Schmidt

A team led by an astronomer from the University of Kansas has compiled data from NASA’s Tess and Spitzer Space Telescopes to show the atmosphere of a most unusual kind for the first time Exoplanet called a “hot Neptune. ”

The results for the recently found planet LTT 9779b were published today in Astrophysical Journal Letters. The paper describes the very first spectral atmospheric characterization of a planet discovered by TESS, the first global temperature map of a TESS planet with an atmosphere and a hot Neptune whose emission spectrum is fundamentally different from the many larger “hot Jupiters” previously studied.

“For the first time we have measured the light that comes from this planet and should not exist,” said Ian Crossfield, assistant professor of physics and 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 completely evaporated. However, our Spitzer observations show us its atmosphere via the infrared light that the planet emits. ”

While LTT 9779b is exceptional, one thing is certain: people probably wouldn’t like it very much there.


“For the first time we have measured the light that comes from this planet and should not exist,” said Ian Crossfield, assistant professor of physics and astronomy at KU and lead author of the paper. “This planet is so intensely radiated from its star that its temperature is above 3,000 degrees Fahrenheit and its atmosphere could have completely evaporated. However, our Spitzer observations show us its atmosphere via the infrared light that the planet emits. Photo credit: Ethen Schmidt, University of Kansas

“This planet has no solid surface and is even hotter than mercury in our solar system – not only would lead melt in this planet’s atmosphere, but platinum, chrome and stainless steel as well,” said Crossfield. “A year on this planet lasts less than 24 hours – that’s how fast it lashes for its star. It’s a pretty extreme system. ”

Hot Neptune LTT 9779b was only discovered last year, making it one of the first Neptune-sized planets to be discovered by NASA’s TESS planet-hunting mission. Crossfield and his co-authors used a technique called “phase curve” analysis to analyze the exoplanet’s atmospheric composition.

This artist’s impression shows the roughly-to-scale LTT 9779 system with the hot Neptune-sized planet on the left and its bright star nearby on the right. The trail of material flowing from the planet is hypothetical, but likely, based on the intense irradiation of this planet. Photo credit: Ethen Schmidt

“We measure how much infrared light is emitted by the planet when 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 – Earth isn’t hottest at noon; It’s hottest a couple of hours in the afternoon. But it is actually hottest on this planet around noon. We see most of the infrared light from part of the planet when its star is directly over it and much less from other parts of the planet. ”

Readings of the planet’s temperature are seen as a way to characterize its atmosphere.

“The planet is much cooler than expected, which suggests that it is reflecting much of the incident starlight it hits, presumably due to daytime clouds,” said co-author Nicolas Cowan of the Institute for Exoplanet Research (iREx) and McGill University in Montreal who helped with the analysis and interpretation of the thermal phase curve measurements. “The planet doesn’t carry much heat to its night side either, but we think we understand: the starlight it absorbs is likely to be absorbed high in the atmosphere, from where the energy is quickly radiated back into space.”

According to Crossfield, the results are only a first step into a new phase of exoplanetary exploration, as the study of the exoplanet’s atmosphere is steadily moving towards ever smaller planets.

This artist’s impression shows LTT 9779b traversing the star it is orbiting. This transit blocks a significant part of the star’s light for a short time and so the planet was discovered for the first time by NASA’s TESS mission. Photo credit: Ethen Schmidt

“I wouldn’t say we understand all about this planet now, but we’ve measured enough to know that this will be a really fruitful object for future study,” he said. “It is already used for observations James Webb WeltraumteleskopThis is NASA’s next large multi-billion dollar space telescope to be commissioned in a few years. Our measurements so far show what we call spectral absorption features – and its spectrum shows carbon monoxide and / or carbon dioxide in the atmosphere. We are beginning to understand what molecules the atmosphere is made up of. Because we see that, and because this global temperature map looks like, it also tells us something about how the winds circulate energy and material through the atmosphere of this mini-gas planet. ”

Crossfield explained the extreme rarity of Neptune-like worlds that are close to their host stars. This region, which is usually so devoid of planetary astronomers, calls it the “hot Neptune desert”.

“We believe this is because hot neptunes are not massive enough to avoid significant atmospheric evaporation and mass losses,” he said. “Most of the hot exoplanets nearby are either the massive hot Jupiter or the rocky planets that long ago lost most of their atmosphere.”

A companion paper to this research, led by Diana Dragomir, Assistant Professor of Physics and Astronomy at the University of New Mexico, examines the atmospheric composition of the exoplanet through secondary eclipse observations with the Spitzer Infrared Array Camera (IRAC) of hot Neptune .

Although LTT 9779b is not suitable for human colonization or any other known form of life, Crossfield said assessing its atmosphere would improve techniques that could one day be used to find more welcoming planets to live on.

“If anyone is to believe what astronomers are saying about finding life signs or oxygen on other worlds, we have to show that we can get it right with the simple things first,” he said. “In that sense, these larger, hotter planets like the LTT 9779b act like training wheels and show that we actually know what we’re doing and can do everything right.”

Crossfield said his look into the atmosphere of such a strange and distant planet was valuable in itself.

“As someone studying these, there is just a lot of interesting planetary science that we can use to measure the properties of these planets – just like humans study the atmospheres of planets Jupiter, Saturn and Venus – although we don’t believe that these will harbor life, ”he said. “They are still interesting, and we can learn how these planets were formed and what context the planetary systems have.”

Much remains to be done to better understand LTT 9779b and similar hot Neptunes that have not yet been discovered, according to Crossfield. (At the same time, an accompanying paper on the atmospheric composition of LTT 9779b through analysis of its secondary eclipse spectrum will be published, which Crossfield co-wrote.)

“We’d like to keep watching it with other telescopes so we can answer more questions,” he said. “How can this planet preserve its atmosphere? How did it even come about? Was it bigger in the beginning but lost part of its original atmosphere? If so, why isn’t its atmosphere just a scaled-down version of the atmosphere of ultra-hot larger exoplanets? And what else could lurk in its atmosphere? ”

Some of the KU researcher’s co-authors also plan to continue studying the unlikely exoplanet.

“We detected carbon monoxide in its atmosphere and determined that the permanent day side is very hot, while very little heat is transported to the night side,” said Björn Benneke of iREx and the Université de Montréal. “Both results suggest that LTT 9779b is seeing a very strong signal, making the planet a very interesting target for future detailed characterization with JWST. We are now also planning much more detailed phase curve observations with NIRISS on JWST. ”

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