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The discovery of the extraordinary exoplanet LTT 9779b was first announced a month ago. Just 260 light-years away, the planet was immediately deemed an excellent candidate for the follow-up study of its curious atmosphere. But it turns out we didn’t even have to wait too long to find out more.
LTT 9779b is a bit larger than Neptune, orbiting a Sun-like star – quite normal so far. But two things are really peculiar. It is so close to its star that the planet orbits once every 19 hours; and, despite the scorching heat it must be subjected to in that proximity, LTT 9779b still has a substantial atmosphere.
Infrared observations collected by the now retired Spitzer Space Telescope included the planet’s host star, and astronomers have now analyzed that data, publishing their results in a couple of studies.
In the first article, a team led by astronomer Ian Crossfield of the University of Kansas described the temperature profile of the LTT 9779b.
In the second article, a team led by astronomer Diana Dragomir of the University of New Mexico characterized the exoplanet’s atmosphere.
“For the first time, we measured the light from this planet that shouldn’t exist,” said Crossfield.
“This planet is so intensely radiated by its star that its temperature exceeds 3,000 degrees Fahrenheit [1,650 degrees Celsius] and its atmosphere may have evaporated completely. However, our observations of Spitzer show us his atmosphere through the infrared light emitted by the planet. “
A phase curve of an exoplanet. (ESA)
He and his team studied the exoplanet’s phase curve in infrared light. Here’s what it means: Since thermal energy is emitted as infrared radiation, light in this wavelength can tell us the temperature of cosmic objects many light years away.
The system is oriented so that the planet passes between us and the star, giving us clear lateral views of both the night and day side. Therefore, to calculate the temperature of the exoplanet, astronomers can use the changing light of the entire system as LTT 9779b orbits.
Interestingly, the hottest time of day for LTT 9779b is nearly noon, when the sun is directly overhead. On Earth, the hottest time of day is actually a few hours after noon, because heat enters the Earth’s atmosphere faster than it is radiated into space.
In turn, this allows for some plausible assumptions about LTT 9779b’s atmosphere.
“The planet is much colder than we expected, which suggests it is reflecting much of the incident starlight hitting it, presumably due to daytime clouds,” said astronomer Nicolas Cowan of the Institute for Research on Exoplanets. (iREx) and McGill University in Canada.
“Also, the planet doesn’t carry much heat to its night side, but we think we understand that: the starlight that is absorbed is likely absorbed high up in the atmosphere, from where energy is rapidly radiated into space.”
To further probe LTT 9779b’s atmosphere, Dragomir and his colleagues focused on secondary eclipses, when the planet passes behind the star. This results in a weaker dimming of the system’s light than when the planet passes in front of the star – known as a transit – but that weaker dimming can help us understand the thermal structure of an exoplanet’s atmosphere.
“Hot Neptunes are rare and one in such an extreme environment as this is difficult to explain because its mass isn’t large enough to hold an atmosphere for very long,” Dragomir said.
“So how did it go? LTT 9779b made us scratch our heads, but the fact that it has an atmosphere gives us a rare way to investigate this type of planet, so we decided to probe it with another telescope.”
The researchers combined data from Spitzer’s secondary eclipse with data from NASA’s TESS space telescope for hunting exoplanets. This allowed them to obtain an emission spectrum from the atmosphere of the LTT 9779b; that is, the wavelengths of the light absorbed and amplified by the elements it contains. They found that some wavelengths were absorbed by molecules, possibly carbon monoxide.
This is not unexpected for such a hot planet. Carbon monoxide has been detected in hot Jupiter, gas giants that orbit their stars even in close proximity. But gas giants are more massive than hot Neptunes and use their much higher gravity to maintain their atmosphere. It was thought that Neptune-sized planets didn’t have to be massive enough to do that.
Finding carbon monoxide in the atmosphere of a hot Neptune could 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 know, there is still some work to be done. Future observations could help us answer these and other questions, such as what else the atmosphere is made of, and the exoplanet started much larger and is currently undergoing rapid contraction.
Research such as this will provide us with an excellent toolkit and experience to probe the atmospheres of potentially habitable worlds as well.
“If anyone is to believe what astronomers say about looking for signs of life or oxygen on other worlds, we will first have to show that we can do it on the easy stuff,” Crossfield said.
“In this sense these larger and hottest planets like LTT 9779b act as training wheels and show that we really know what we are doing and that we can do everything right.”
The two documents were published in The Astrophysical Journal Letters, here and here.
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