Astronomers Analyze Atmosphere of Ultrahot Neptune

Astronomers Analyze Atmosphere of Ultrahot Neptune

Using data collected by the InfraRed Array Camera aboard NASA’s Spitzer Space Telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have studied the atmosphere of the recently-discovered ultrahot-Neptune exoplanet LTT 9779b.

An artist’s impression of the LTT 9779 system. Image credit: Ethen Schmidt, Kansas University.

LTT 9779b is a hot gas giant about 4.6 times larger than Earth and 29 times as massive.

The planet’s mean density is similar to that of Neptune and its atmosphere makes up around 9% of the total planetary mass.

It orbits LTT 9779, a two-billion-year-old Sun-like star located 260 light-years away in the constellation of Sculptor.

LTT 9779b has an orbital period of 19 hours and is located in the so-called ‘Neptunian desert’, a region devoid of planets when we look at the population of planetary masses and sizes.

“This ultrahot Neptune is a medium-sized exoplanet that orbits very close to its star, but its low density indicates that it still has an atmosphere weighing almost 10% of the planet’s mass,” said Dr. Diana Dragomir, an astronomer at the University of New Mexico and lead author of a paper published in the Astrophysical Journal Letters.

“Hot Neptunes are rare, and one in such an extreme environment as this one is difficult to explain because its mass isn’t large enough to hold on to an atmosphere for very long.”

“So how did it manage? LTT 9779b had us scratching 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.”

To investigate its atmospheric composition and shed further light on LTT 9779b’s origin, Dr. Dragomir and colleagues obtained secondary eclipse observations with Spitzer’s Infrared Array Camera.

The observations confirmed an atmospheric presence and enabled a measurement of the planet’s very high temperature — 2,032 degrees Celsius (3,690 degrees Fahrenheit).

“For the first time, we measured the light coming from this planet that shouldn’t exist,” said Dr. Ian Crossfield, an astronomer in the Department of Physics and Astronomy at the University of Kansas.

“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.”

After combining the Spitzer observations with a measurement of the secondary eclipse in the TESS bandpass, the scientists studied the resulting emission spectrum and identified evidence of molecular absorption in the atmosphere of LTT 9779b, which they believe is likely due to carbon monoxide.

This molecule is not unexpected in the atmospheres of hot large planets, but to find it in an ultrahot Neptune may provide clues on the origin of this planet and how it managed to hold onto its atmosphere.

“If there’s a lot of atmosphere surrounding the planet, as is the case for LTT 9779b, then you can study it more easily,” Dr. Dragomir said.

“A smaller atmosphere would be much harder to observe.”

In a companion study, also published in the Astrophysical Journal Letters, the team found signs that point to the atmosphere of LTT 9779b having a higher level of heavy elements than expected.

This is additionally intriguing because the two similarly-sized planets in our Solar System, Neptune and Uranus, are primarily composed of light elements like hydrogen and helium.

“We measure how much infrared light was being emitted by the planet as it rotates 360 degrees on its axis,” said Dr. Crossfield, lead author of the second paper.

“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.”

“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 Dr. Nicolas Cowan, an astronomer in the Institute for Research on Exoplanets and McGill University.

“The planet also doesn’t transport much heat to its nightside, but we think the starlight is likely absorbed high in the atmosphere, from whence the energy is quickly radiated back to space.”

“LTT 9779 is one of those super-exciting targets, a very rare gemstone for our understanding of hot Neptunes,” said Professor Björn Benneke, an astronomer at the Institute for Research on Exoplanets and the Université de Montréal.

“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.”

“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 NASA’s James Webb Space Telescope.”


Diana Dragomir et al. 2020. Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9779b. ApJL 903, L6; doi: 10.3847/2041-8213/abbc70

Ian J.M. Crossfield et al. 2020. Phase Curves of Hot Neptune LTT 9779b Suggest a High-metallicity Atmosphere. ApJL 903, L7; doi: 10.3847/2041-8213/abbc71

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