Taylor J. Bell

Image Credit: Danik Renaud

In the quest to understand the atmospheres of exoplanets, unexpected discoveries often challenge us and open new avenues of research. This was certainly the case when we observed the phase curve of WASP-12b at 4.5 microns. Our team was puzzled by the unusual shape of WASP-12b's 4.5-micron phase curve taken in 2010 and even more surprised to find similar patterns in a repeated phase curve taken in 2013. This blog post chronicles our journey from confusion to insight, detailing the process of testing various hypotheses and ultimately proposing an explanation involving mass loss and carbon monoxide (CO).

The Initial Confusion

In 2010, the Spitzer Space Telescope captured thermal phase curves of WASP-12b, an ultra-hot Jupiter orbiting just one stellar diameter above the surface of its star. The observations at 4.5 microns revealed a peculiar phase curve shape unlike any that Spitzer had seen before; while typical Spitzer phase curves have one peak near the time of eclipse, WASP-12b showed two peaks with both offset by ~90° from the time of eclipse. Such a feature is sometimes seen at optical wavelengths due to tidal distortion of the host star and/or the planet, but the amplitude of this two-peaked signal component in WASP-12b's 4.5-micron phase curve was enormous. Initially, we considered the possibility that the planet's shape might be significantly elongated due to tidal forces (given how extremely close the planet is to its host star), but this hypothesis required that the planet have an axis ratio of 1.8; this would mean that the planet would need to be roughly the shape of a rugby ball (see Figure 1)! What was even more confusing is that a similar two-peaked signal was not seen in a phase curve observation taken using Spitzer's 3.6-micron filter. If the planet were highly distorted, we wouldn't expect the "ellipsoidal variations" caused by tidal distortion to be so strongly wavelength-dependent. One possibility was that either the 3.6-micron or the 4.5-micron phase curves were corrupted by unusually bad instrumental noise which was either introducing the double-peaked signature in the 4.5-micron data or hid it in the 3.6-micron data.

Figure 1: Potential Hypotheses For the Unusual Spitzer Phase Curve

Image Credit: Taylor J. Bell

To better understand these highly unusual data, we conducted another pair of phase curve observations using the 3.6-micron and 4.5-micron filters in 2013 (see Figure 2). Remarkably, the 4.5-micron phase curve exhibited the same strange sinusoidal pattern, confirming that the phenomenon was real and not the cause of unusually strong instrumental noise. This consistency between data taken multiple years apart suggested a stable and significant astrophysical feature on or around WASP-12b, prompting us to investigate further.

Figure 2: Measured Spitzer Phase Curves

Image Credit: Taylor J. Bell

We explored several potential explanations. One group of hypotheses were stellar variability, star spots, or plages, but the consistency in the alignment of the double-peaked signal with the planet's orbit across both datasets made this highly improbable. Star spots or other irregularities would also not produce such a regular pattern specifically at 4.5 microns while not producing any such signal at 3.6 microns. Another consideration was that the shape of the transit seen at 4.5 microns did not seem unusual, and no transits or eclipses of other unknown planets or moons in the system were observed either.

The Gas Stream Hypothesis

As depicted in Figure 3, infrared light emitted from a dense stream of gas stripped from the planet provided a compelling fit to our observations. WASP-12b is already known to be rapidly losing mass, as evidenced by the detection of a large exosphere and significant mass transfer to its host star documented in previous studies. The planet is also known to be spiraling in towards its star based on previous studies, with the planet likely to be ripped apart by its star within the next ~3 million years. The elongated patch of hot gas resulting from mass loss processes would need to be pointed directly at or away from the host star to explain the timing of the two-peaked component of the Spitzer phase curve. When the planet is halfway between transit and eclipse, the gas stream would appear bright since it was fully stretched out in front of us; meanwhile, the stream would appear faint to us around transit and eclipse since we'd be looking along the gas stream and most of it would be hidden. If the gas stream were flowing directly away from the star, this might be contributing to the planet's decaying orbit, as the transfer of material from WASP-12b can alter the gravitational interactions and orbital dynamics. However, this still didn't explain why we didn't see a double-peaked signal in the 3.6-micron phase curve...

Figure 3: Perhaps We Are Seeing a Stream of Gas Stripped From WASP-12b?

Image Credit: Danik Renaud

The breakthrough came when we considered the potential role of carbon monoxide (CO) in the atmosphere of WASP-12b. CO is a fairly common molecule and is one of the hardest molecules to break apart because the carbon atom is triple-bonded to the oxygen atom, which is one the strongest types of molecular bonds. The relative abundance and strength of CO could help transport the molecule high into the planet's atmosphere and drag it out along with the hydrogen gas being stripped from the planet. If this gas stream were rich in CO, it could emit much more strongly at 4.5 microns than at 3.6 microns (see Figure 4), accounting for the observed differences between the two wavelengths.

Figure 4: Carbon Monoxide Emits At 4.5 Microns But Not At 3.6 Microns!

Image Credit: Taylor J. Bell

Conclusion: A New Understanding

Our study of WASP-12b's phase curves highlights the complexities and surprises inherent in exoplanetary science. The persistent and puzzling phase variations seen at 4.5 microns, now attributed to infrared emission from a dense gas stream rich in CO, demonstrate how repeated observations and hypothesis testing can lead to new insights. Future observations, particularly with new spectroscopic instruments like those on JWST, will further refine our understanding of this highly unusual planet and may uncover even more fascinating aspects of exoplanetary atmospheres.

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