In the last few decades, a peculiar distribution of exoplanets has appeared in astronomers' data: planets that are close to their stars are either really big like Jupiter or small like Earth. Neptune-sized worlds in this region are rare.
To learn more, a study recently published in The Astrophysical Journal Letters reports on the atmospheric composition of one of these hot Neptunes, as they are known. Planet LTT9779 b is so close to its star that it has a day-side temperature of over 2,000°C (3,660°F), making it the first entry in the ultra-hot Neptune category.
The research suggests the planet is tidally locked, with one side in perennial light and the other in constant darkness. Astronomers also caught the planet crossing the face of its star, which allowed them to catch a glimpse of the atmosphere and discover the presence of carbon monoxide.
"For the first time, we measured light coming from a planet that shouldn't exist! This ultra-hot Neptune is a 'medium-sized' exoplanet that orbits very close to its star (it takes just 19 hours to complete an orbit), but its low density indicates that it still has an atmosphere weighing at least 10 percent of the planet's mass," lead author Diana Dragomir, an assistant professor from the University of New Mexico, said in a statement.
The peculiar finding of carbon monoxide may provide much-needed insight into how some of these hot Neptunes manage to survive. Previously, astronomers believed that the reason hot planets were either huge and gassy or small and rocky had to do with their ability to keep hold of their atmosphere. As a star tries to erode away its atmosphere, the planet either has the gravity to keep it or not.
However, LTT9779 b is 2 billion years old, so it can’t just be gravity. It's possible the planet's atmospheric composition contributes to its ability to resist the effects of its star, as the team estimates the atmosphere weighs about 10 percent of the whole planet.
The planet was discovered by NASA’s Transiting Exoplanet Survey Satellite and observations were conducted by the now-retired NASA Spitzer Space Telescope. To better understand the atmosphere of this and other hot Neptunes, we'll have to wait for the James Webb Space Telescope to come online in the next few years.
"LTT9779 is one of those super-exciting targets, a very rare gemstone for our understanding of hot Neptunes. We believe 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," added co-author Professor Björn Benneke, from the Université de Montréal. "Both findings make LTT9779b say that there is a very strong signal to be observed making the planet a very intriguing target for future detailed characterization with JWST."