A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host

Giant tidal tails of helium escaping the hot Jupiter HAT-P-32 b

Capturing planets in the act of losing their atmospheres provides rare opportunities to probe their evolution history. This analysis has been enabled by observations of the helium triplet at 10,833 angstrom, but past studies have focused on the narrow time window right around the planet's optical transit. We monitored the hot Jupiter HAT-P-32 b using high-resolution spectroscopy from the Hobby-Eberly Telescope covering the planet's full orbit. We detected helium escaping HAT-P-32 b at a 14σ significance,with extended leading and trailing tails spanning a projected length over 53 times the planet's radius. These tails are among the largest known structures associated with an exoplanet. We interpret our observations using three-dimensional hydrodynamic simulations, which predict Roche Lobe overflow with extended tails along the planet's orbital path.

Revealing the atmospheres of highly irradiated exoplanets: from ultra-hot Jupiters to rocky worlds

Spectroscopy of transiting exoplanets has revealed a wealth of information about their atmospheric compositions and thermal structures. In particular, studies of highly irradiated exoplanets at temperatures much higher than those found in our solar system have provided detailed information on planetary chemistry and physics because of the high level of precision which can be obtained from such observations. Here we use a variety of techniques to study the atmospheres of highly irradiated transiting exoplanets and address three large, open questions in exoplanet atmosphere spectroscopy. First, we use secondary eclipse and phase curve observations to investigate the thermal structures and heat redistribution of ultra-hot Jupiters, the hottest known exoplanets. We demonstrate how these planets form an unique class of objects influenced by high-temperature chemical effects such as molecular dissociation and H^- opacity. Second, we use observations of helium in the upper atmosphere of the exo-Neptune HAT-P-11b to probe atmospheric escape processes. Third, we develop tools to interpret JWST observations of highly irradiated exoplanets, including a data analysis pipeline to perform eclipse mapping of hot Jupiters and a method to infer albedos of and detect atmospheres on hot, terrestrial planets. Finally, we discuss remaining open questions in the field of highly irradiated exoplanets and opportunities to advance our understanding of these unique bodies in the coming years.

Nightside condensation of iron in an ultrahot giant exoplanet

Ultra-hot giant exoplanets receive thousands of times Earth’s insolation^1,2. Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry^3–5. Daysides are predicted to be cloud-free, dominated by atomic species⁶ and substantially hotter than nightsides^5,7,8. Atoms are expected to recombine into molecules over the nightside⁹, resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed^10–14, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night (“evening”) and night-to-day (“morning”) terminators could, however, be revealed as an asymmetric absorption signature during transit^4,7,15. Here, we report the detection of an asymmetric atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and temporally resolve this signature thanks to the combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by −11±0.7 km s^-1 on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside¹⁶. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. Iron must thus condense during its journey across the nightside.

Ground-based detection of an extended helium atmosphere in the Saturn-mass exoplanet WASP-69b

Hot gas giant exoplanets can lose part of their atmosphere due to strong stellar irradiation, and these losses can affect their physical and chemical evolution. Studies of atmospheric escape from exoplanets have mostly relied on space-based observations of the hydrogen Lyman-α line in the far ultraviolet region, which is strongly affected by interstellar absorption. Using ground-based high-resolution spectroscopy, we detected excess absorption in the helium triplet at 1083 nanometers during the transit of the Saturn-mass exoplanet WASP-69b, at a signal-to-noise ratio of 18. We measured line blueshifts of several kilometers per second and posttransit absorption, which we interpret as the escape of part of the atmosphere trailing behind the planet in comet-like form.

Atomic iron and titanium in the atmosphere of the exoplanet KELT-9b

To constrain the formation history of an exoplanet, we need to know its chemical composition^1-3. With an equilibrium temperature of about 4,050 kelvin⁴, the exoplanet KELT-9b (also known as HD 195689b) is an archetype of the class of ultrahot Jupiters that straddle the transition between stars and gas-giant exoplanets and are therefore useful for studying atmospheric chemistry. At these high temperatures, iron and several other transition metals are not sequestered in molecules or cloud particles and exist solely in their atomic forms⁵. However, despite being the most abundant transition metal in nature, iron has not hitherto been detected directly in an exoplanet because it is highly refractory. The high temperatures of KELT-9b imply that its atmosphere is a tightly constrained chemical system that is expected to be nearly in chemical equilibrium⁵ and cloud-free^6,7, and it has been predicted that spectral lines of iron should be detectable in the visible range of wavelengths⁵. Here we report observations of neutral and singly ionized atomic iron (Fe and Fe⁺) and singly ionized atomic titanium (Ti⁺) in the atmosphere of KELT-9b. We identify these species using cross-correlation analysis⁸ of high-resolution spectra obtained as the exoplanet passed in front of its host star. Similar detections of metals in other ultrahot Jupiters will provide constraints for planetary formation theories.