A Look at the Atmosphere of a Nearby Hot Neptune

Editor’s note: AAS Nova is on vacation until 2 November. Normal posting will resume at that time; in the meantime, we’ll be taking this opportunity to look at a few interesting AAS journal articles that have recently been in the news or drawn attention.

Among the types of known exoplanets we’ve spotted in our galaxy, there lie some gaps. One of them is the hot Neptune desert — a dearth of Neptune-sized planets that orbit close to their host stars. We speculate that these planets may be in short supply because they’re too small to retain their atmospheres that close to the heat of their hosts, but this desert still raises a lot of questions.

We now have an excellent opportunity to learn more, with the Transiting Exoplanet Survey Satellite (TESS) discovery of one such “planet that shouldn’t exist”: LTT 9779b. This hot Neptune is blasted by the radiation of its host as it tears around its star in less than 24 hours. And yet, somehow, it still has an atmosphere!

In a pair of papers recently published in ApJL, scientists Diana Dragomir (University of New Mexico) and Ian Crossfield (University of Kansas) lead explorations of this unexpected planet using TESS and the Spitzer infrared space telescope. They examine the atmosphere of LTT 9779b in two ways: by using phase-curve analysis, which charts the change in the planet’s brightness as it orbits around its host, and by measuring the planet’s emission using its secondary transit, which is when it passes behind its star (see the video below).

To learn more about the authors’ discoveries, check out the following articles and press releases.

Original articles:
“Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9779b,” Diana Dragomir et al 2020 ApJL 903 L6. doi:10.3847/2041-8213/abbc70
“Phase Curves of Hot Neptune LTT 9779b Suggest a High-metallicity Atmosphere,” Ian J. M. Crossfield et al 2020 ApJL 903 L7. doi:10.3847/2041-8213/abbc71
Press releases:
University of New Mexico: Data reveals evidence of molecular absorption in the atmosphere of a hot Neptune
University of Kansas: New study details atmosphere on ‘hot Neptune’ 260 light years away that ‘shouldn’t exist’