The discovery of the unusual exoplanet LTT 9779b was first announced a month ago. Only 260 light-years away, the planet was immediately identified as an excellent candidate for further study of its curious atmosphere. But it turns out we didn’t even have to wait too long to learn more.
LTT 9779b is slightly larger than Neptune orbiting a Sun-like star – quite normal for now. But two things are really special. It is so close to its star that it orbits the planet once every 19 hours; and despite the scorching heat to be subjected to this proximity, the LTT 9779b still has a significant atmosphere.
Infrared observations collected by the now-retired Spitzer Space Telescope include the planet̵
In the first article, a team led by astronomer Ian Crossfield of the University of Kansas described the temperature profile of LTT 9779b.
In the second article, a team led by astronomer Diana Dragomir of the University of New Mexico describes the atmosphere of the exoplanet.
“For the first time, we measured the light coming from this planet, which should not exist,” Crossfield said.
“This planet is so intensely irradiated by its star that its temperature is over 3,000 degrees Fahrenheit. [1,650 degrees Celsius] and its atmosphere could evaporate completely. Still, our Spitzer observations show us its atmosphere through the infrared light the planet emits. “
He and his team are studying the phase curve of the exoplanet in infrared light. Here’s what it means: Because heat energy is emitted as infrared radiation, light at this wavelength can tell us the temperature of space objects many light-years away.
The system is oriented in such a way that the planet passes between us and the star, giving us clear wide-ranging views of both the night and day sides of the planet. Thus, to calculate the temperature of the exoplanet, astronomers can use the changing light of the entire system as the LTT 9779b orbit.
Interestingly, the hottest time of day for the LTT 9779b is almost thunderous at noon when the sun is directly overhead. On Earth, the hottest time of the day is actually a few hours in the afternoon, as heat enters the Earth’s atmosphere faster than it is radiated back into space.
In turn, this allows some educated assumptions about the atmosphere of LTT 9779b.
“The planet is much cooler than we expected, suggesting that it reflects away from the incident starlight that hits it, probably due to daytime clouds,” said astronomer Nicholas Cowan of the Institute for Exoplanetary Research (iREx) and McGill University in Canada.
“The planet also doesn’t transport much heat to its nocturnal side, but we think we understand that: Absorbed starlight is probably absorbed high into the atmosphere, from where energy is quickly radiated back into space.”
To further explore the atmosphere of LTT 9779b, Dragomir and her colleagues focused on secondary eclipses as the planet passed behind the star. This results in less dimming of the system’s light than when the planet passes in front of the star – known as transit, but this lower eclipse can help us understand the thermal structure of the exoplanet’s atmosphere.
“Hot Neptunes are rare and one in such an extreme environment as this is difficult to explain because its mass is not large enough to stay in the atmosphere for very long,” Dragomir said.
“So how did it do? LTT 9779b made us scratch our heads, but the fact that it has an atmosphere gives us a rare way to explore this type of planet, so we decided to explore it with another telescope.”
The researchers combined data from Spitzer’s secondary eclipse with data from NASA’s TESS space telescope to hunt exoplanets. This allowed them to obtain an emission spectrum from the atmosphere of LTT 9779b; i.e. the wavelengths of light absorbed and amplified by elements in it. They found that some wavelengths were absorbed by molecules – probably carbon monoxide.
This is not unexpected for such a hot planet. Carbon monoxide has been found in hot Jupiters, gas giants that also orbit their stars in a burning immediate vicinity. But gas giants are more massive than hot Neptunes and use their much higher gravity to preserve their atmosphere. It was thought that planets the size of Neptune did not have to be massive enough to do so.
Finding carbon monoxide in the atmosphere of hot Neptune can help us understand how this planet formed and why it still has its own atmosphere.
So while we know more about the LTT 9779b than we knew, there is still work to be done. Future observations could help us answer these and other questions, such as what else the atmosphere is made of and whether the exoplanet has started much larger and is currently in a process of rapid contraction.
Research like this will give us an excellent set of tools and experience for exploring the atmospheres of potentially habitable worlds.
“If anyone believes what astronomers say about finding signs of life or oxygen in other worlds, we’re going to have to show that we can actually do it first with easy things,” Crossfield 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 we can fix everything.”
Both articles are published in The Astrophysical Journal Letters, here and here.