We may be measuring planetary temperatures with less than optimum tools. Calling it a “new phenomenon,” Cornell University’s Nikole Lewis described the background of a just published paper looking into hot Jupiter temperatures. Lewis had been increasingly puzzled by earlier work on the matter, which produced temperatures colder than scientists expected. The deputy director of the Carl Sagan Institute, Lewis joined colleagues Ryan MacDonald and Jayesh Goyal in looking for the reason, reporting their results in Astrophysical Journal Letters.
What emerged was the need to fine-tune our analysis of exoplanet atmospheres, as delivered by the technique called transmission spectroscopy, in which the light of a parent star is filtered through a planetary atmosphere during a transit. Have a look, for example, at an illustration of the hot-Jupiter WASP-43b as it transits its star. Scientists have been able to construct temperature maps for the planet as well as probing its atmosphere to understand the molecular chemistry within. But the data on atmospheric composition have to be interpreted correctly.
Image: In this artist’s illustration the Jupiter-sized planet WASP-43b orbits its parent star with a year lasting just 19 hours. The planet is tidally locked, meaning it keeps one hemisphere facing the star, just as the Moon keeps one face toward Earth. The color scale on the planet represents the temperature across its atmosphere. This is based on data from a 2014 study (not the Lewis paper) that mapped the temperature of WASP-43b in more detail than had been done for any other exoplanet at that time. Credit: NASA, ESA, and Z. Levay (STScI).
Hot Jupiters orbit close enough to their star to become tidally locked, while the intense gravitational forces at work can cause the planet to bulge, making it egg-shaped. What we would expect is a wide range of temperatures, varying by thousands of degrees, between the blistering ‘day’ side of the star and the frigid side turned away from the star. Averaging temperatures like these can be a problem, according to the Cornell researchers, and the range of temperatures likewise can promote entirely different chemistry between the two sides.
“When you treat a planet in only one dimension, you see a planet’s properties – such as temperature – incorrectly,” Lewis said. “You end up with biases. We knew the 1,000-degree differences were not correct, but we didn’t have a better tool. Now, we do.”
Let’s dig into this. We have over 40 examples of hot Jupiters with transmission spectra, meaning that the science of exoplanet atmospheres is becoming established — eventually, we’ll drill down to smaller rocky worlds to learn about possible biosignatures, but for now, we’re detecting various atoms and molecules in the atmospheres of close-in gas giants. Observations of high enough precision can use transmission spectroscopy to learn about temperatures at the terminator, the zone where day meets night and the temperature contrast can be huge.
Both optical and near-infrared data are needed to draw accurate conclusions. The anomaly that the authors are addressing is that almost all of the retrieved temperatures for hot Jupiters are cooler than planetary equilibrium temperatures derived for the planet. In fact, the retrieved temperatures for most hot Jupiters are between 200 and 600 K cooler than the equilibrium temperature ought to be. The equilibrium temperature is the point at which the planet emits as much thermal energy as it receives from the star. It is commonly denoted as Teq.
The authors propose that the colder temperatures being found via transmission spectra are the result of the use of 1-dimensional models. The atmosphere at the terminator is more complex than the 1D model can show. They call for more complex 3D general circulation models (GCM) to replace them. Sets of differential equations are put to work in a GCM, with the planet divided into a 3-dimensional grid and analyzed in terms of winds, heat transfer, radiation and other factors, taking into account their interactions with other parts of the grid. The difference is striking, as the image below, drawn from the paper, demonstrates.
Image: This is Figure 1 from the paper. Caption: Schematic explanation of the cold retrieved temperatures of exoplanet terminators. Left: a transiting exoplanet with a morning-evening temperature difference (observer’s perspective). Differing temperature and abundance profiles encode into the planet’s transmission spectrum. Right: the observed spectrum is analysed by retrieval techniques assuming a uniform terminator. The retrieved 1D temperature profile required to fit the observations is biased to colder temperatures. Credit: MacDonald et al.
Thus the earlier models bias the results toward colder temperatures. And this can be significant in our evaluation of planetary atmospheres, as the paper notes:
Those [chemical] species exhibiting compositional differences have retrieved 1D abundances biased lower than the true terminator-averaged values. Even species uniform around the terminator (here, H2O) are biased, though to higher abundances. Compositional biases become more severe as the retrieved P-T profile deviates further from the true terminator temperature. In the most extreme case, the retrieved H2O abundance is biased by over an order of magnitude, such that one would incorrectly believe a solar-metallicity atmosphere was 15 × super-solar at > 3σ confidence.
We need to resolve this matter, then, to conduct more accurate atmospheric work. The paper continues:
The retrieved cold temperatures of exoplanet terminators in the literature can be explained by inhomogenous morning-evening terminator compositions. The inferred temperatures arise from retrievals assuming uniform terminator properties. We have demonstrated analytically that the transit depth of a planet with different morning and evening terminator compositions, when equated to a 1D transit depth, results in a substantially colder temperature than the true average terminator temperature. This also holds for state-of-the-art retrieval codes, with the added complication that retrieved chemical abundances can also be significantly biased.
Using the older models has meant that the temperatures of hot Jupiters thus far measured may be biased by hundreds of degrees below their true value. The figure reaches 1000 K in the case of ultra-hot Jupiters. An implication here is that the chemistry derived from the older models is less reliable. The authors call for the use of more sophisticated retrieval tools, acknowledging the increased computer overhead involved with 3D approaches but arguing that such models will produce more accurate data on atmospheric temperatures and composition.
The paper is MacDonald et al., “Why Is it So Cold in Here? Explaining the Cold Temperatures Retrieved from Transmission Spectra of Exoplanet Atmospheres,” Astrophysical Journal Letters Vol. 893, No. 2 (23 April 2020). Abstract / preprint.