Exploring the Escaping Atmosphere of HAT-P-11b


The atmospheres of planets close to their host stars live a tenuous existence. New observations from the Hubble Space Telescope show signs of a Neptune-like exoplanet’s atmosphere being eroded away.

Evaporation at Work

Small planets observed to orbit closely around their host star fall into two main populations:

  1. those with radii smaller than 1.5 Earth radii, thought to be primarily rocky cores with little or no remaining atmosphere, and
  2. those with radii larger than 2 Earth radii, thought to retain some of their hydrogen and helium atmospheres.

What causes the difference between these two populations? We think that all close-in exoplanets are sculpted by the energetic radiation of their host stars. This radiation can erode away the primordial atmospheres — and for the smallest planets, this will leave only their rocky cores behind.

As we work to understand the detailed physics of this photoevaporation, it would be helpful to be able to directly watch a planet’s atmosphere escaping in this way. In a new study, scientist Megan Mansfield (University of Chicago) and collaborators present just the thing: observations of the escaping atmosphere of the exoplanet HAT-P-11b.

Observations of a Hot Neptune


Artist’s illustration of WASP-107b, the first planet for which Hubble discovered helium escaping from its atmosphere. [ESA/Hubble, NASA, M. Kornmesser]

HAT-P-11b is a Neptune-sized exoplanet that orbits very close to its host star in a system that’s located approximately 120 light-years from Earth. Using Hubble, Mansfield and collaborators discovered the subtle signature of helium escaping from the atmosphere of HAT-P-11b — making this the second planet for which this signature has been discovered by Hubble (the first was WASP 107-b) and one of only a handful of planets for which we’ve seen signs of atmospheric escape.

By comparing these observations to models, Mansfield and collaborators estimate that HAT-P-11b is losing mass at a rate of roughly 109–1011 g/s. This rate, while high, is still low enough that the planet has only lost a few percent of its mass over its history, leaving its bulk composition largely unaffected. This is consistent with what we would expect for a planet of its size: since it’s larger than 2 Earth radii, it should retain some of its hydrogen and helium atmosphere.

Narrowband spectrum of HAT-P-11b

Narrowband spectrum of HAT-P-11b (blue and gray points) compared to three 1D models of hydrodynamic escape (red, green, and orange lines). [Mansfield et al. 2018]

A New Approach

Why are these escaping-helium detections important? Observations like this one represent a new method for exploring exoplanet atmospheres! The helium signature detected from HAT-P-11b had long been theorized as a way to study escaping atmospheres, but until Hubble’s recent observations of helium in the atmosphere of WASP 107-b, the potential of this approach remained untapped.

Now two planets have been observed with this particular signal — and the signal from HAT-P-11b has been additionally confirmed with CARMENES instrument in Spain, marking the first time the same signature of photoevaporation has been detected by both ground- and space-based facilities.

Future observations like these — from both existing instruments and upcoming observatories like the James Webb Space Telescope — will hopefully continue to shed light on how atmospheres evaporate from small, close-in exoplanets.


“Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b,” Megan Mansfield et al 2018 ApJL 868 L34. doi:10.3847/2041-8213/aaf166