Peering Into the Atmosphere of the Hottest Planet Known


As the ultra-hot Jupiter KELT-9b blazes across the face of its host star, we have an excellent opportunity to examine its scalding atmosphere. A new study now reports on what we’ve found.

A Passing Glance

In our efforts to learn more about worlds beyond our solar system, atmospheres provide a critical key. Characterizing the atmospheres of exoplanets can provide us with insight into the planets’ compositions and climates, their evolution, and even — with some potential caveats — their habitability.

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

In particular, transiting exoplanets provide us with a unique opportunity. As a planet passes in front of its host star, we briefly observe the star’s light filtering through the planet’s atmosphere. By exploring the spectrum of that light, not only can we identify the presence of specific atoms and molecules in the planet’s atmosphere, but we can also learn more about where they are and what the atmospheric properties are at those locations.

In a new study led by Jake Turner (Cornell University), a team of scientists digs deep into such a transmission spectrum for the exoplanet KELT-9b.

Not Exactly Temperate

KELT-9b is an extreme world. Clocking in with a dayside temperature of more than 4,500 K (~7,600 °F), it is the hottest planet known — hotter than many stars! This ultra-hot Jupiter orbits at a mere 0.035 AU from its scalding A- or B-type host star, whizzing around its host in just 1.5 days.

The intense radiation bombarding KELT-9b almost certainly takes a toll: this energetic light should dissociate molecules into their component atoms and ionize metals in the hot atmosphere, and it may inflate the envelope of hydrogen gas around the planet to the point where the hot gas escapes.

atmospheric absorption lines

Observed and modeled Hα (top) and Ca II (bottom three) spectral lines in the atmosphere of the ultra-hot Jupiter KELT-9b. [Adapted from Turner et al. 2020]

Turner and collaborators explore the extreme conditions in KELT-9b’s atmosphere with high-resolution transmission spectra taken with the CARMENES instrument on the Calar Alto 3.5-m telescope in Spain.

Detecting Atmospheric Thermometers

The authors find absorption lines indicating the presence of ionized calcium, Ca II, in KELT-9b’s atmospheric spectra; this is just the second time that Ca II has been observed in a hot Jupiter’s atmosphere. They also find prominent Hα absorption — evidence that confirms the existence of an extended envelope of hydrogen surrounding the irradiated planet.

By modeling the spectra they obtain for KELT-9b, Turner and collaborators are able to identify the pressures, altitudes, and temperatures at which these spectral lines form in the atmosphere. They find that the Ca II lines probe the atmosphere at an altitude of about 1.32–1.40 times the planet’s radius. The Hα line provides information from higher up, at 1.44 planetary radii.

Together, these absorption lines act as atmospheric thermometers, providing a picture of KELT-9b’s atmospheric temperature profile and yielding insight into the energy that enters and leaves the planet’s atmosphere.

These results demonstrate the power of this technique, revealing the remarkable wealth of information we can glean from some distant starlight filtered through the atmosphere of an extreme world.


“Detection of Ionized Calcium in the Atmosphere of the Ultra-hot Jupiter KELT-9b,” Jake D. Turner et al 2020 ApJL 888 L13. doi:10.3847/2041-8213/ab60a9