Astronomers use secondary eclipses to study exoplanet atmospheres
The Spitzer Space Telescope first detected molecules in the atmosphere of an exoplanet in 2007 during a secondary eclipse. (For more on exoplanet atmospheres, see the article by Heather Knutson, Physics Today, July 2013, page 64 .) A secondary eclipse, as seen from Earth, occurs when a planet passes behind its host star (see the diagram in the top panel below). During the secondary eclipse, thermal emission emitted from the planet does not contribute to the total spectrum of light seen from both the host star and the exoplanet (see the spectrum in the bottom panel below). The light detected during the eclipse can be compared with the total light detected right before and after the eclipse to reveal the contribution from the planet’s surface and atmosphere. Astronomers can’t detect a secondary eclipse for every exoplanet; the smaller and cooler the planet, the harder the eclipse is to identify.
The diagram in the top panel shows a secondary eclipse, with its corresponding light curve shown in the bottom panel. In the blue parts of the spectrum, light from the star is reflected off the planet and seen by observers on Earth. In the green parts, all reflected light is blocked by the star, and thus there is no planetary flux. A Markov Chain Monte Carlo fit to the time-series data marginalized over multiple astrophysical and systematic parameters (red line) is shown to guide the eye.
top figure by Jennifer Sieben; bottom figure adapted from T. P. Greene et al., Nature, 2023, doi:10.1038/s41586-023-05951-7
The secondary-eclipse method has been used previously to examine hot, Jupiter-type planets, but only with the precision afforded by the James Webb Space Telescope (JWST) have astronomers been able to use it to study the atmospheres of small, rocky planets as cool as those in our solar system. In November and December of last year, the JWST turned its mirrors to the TRAPPIST-1 planetary system.
Seven rocky, Earth-sized planets orbit the nearby dwarf star TRAPPIST-1. Each planet is tidally locked—one side always faces the star—which, in addition to x-ray and visible flares given off by the star, can lead to atmospheres being stripped away when the planets are still young. The system has been previously observed by both the Spitzer and Hubble Space Telescopes, although no atmospheric properties were found.
Thomas Greene from NASA’s Ames Research Center and his collaborators have used five secondary eclipses of the innermost planet TRAPPIST-1 b to determine the daytime temperature of the planet and constrain its atmospheric properties. They converted the measured eclipse depth to a dayside flux of 2.2 μJy, which corresponds to a blackbody brightness temperature of 503 K.
That temperature is at least 100 K warmer than predicted by most atmospheric models, which assume that heat would be moved around even a tidally locked planet. Neither do the measured secondary-eclipse depths match the atmospheric model with uniform heat distribution and reradiation. Instead, the observations are consistent with models of a thin atmosphere with no heat distribution.
The data also are consistent with models that show a slight amount of redistribution and ones that incorporate tidal effects that also heat the planet. The most likely explanation for Greene and collaborators’ observations is that TRAPPIST-1 b absorbs nearly all incident stellar flux and does not have a thick atmosphere. It will take more observations of future secondary eclipses to determine whether the planet has a thin atmosphere or none at all. Yet the confidence in only five eclipses so far demonstrates the ability of the JWST to resolve a very low contrast secondary eclipse with enough certainty for astronomers to conclude that there is no atmosphere, even around a small planet. (T. P. Greene et al., Nature, 2023, doi:10.1038/s41586-023-05951-7 .)