The glow of an exoplanet may be caused by starlight reflecting off liquid iron

Do rainbows exist on distant worlds? Many phenomena that occur on Earth – such as rain, hurricanes and aurora borealis – also occur on other planets in our solar system if conditions are right. Now we have evidence from outside our solar system that one particularly exotic exoplanet may display something close to a rainbow.

It appears in the sky in the form of a halo of colors, a phenomenon called “glory” that occurs when light hits clouds made of homogeneous matter in the form of spherical droplets. This may be the explanation for the mystery related to observing the exoplanet WASP-76B. The planet, a scorching gas giant exposed to rains of molten iron, was also observed to have more light on its eastern end (a line used to separate the day side from the night side) than on its western end. Why was there more light on one side of the planet?

After spotting it using the CHEOPS space telescope, and then combining that with previous observations from Hubble, Spitzer and TESS, a team of researchers from the European Space Agency and the University of Bern in Switzerland now believe the most likely cause of the extra light is glory. .

Seeing the light

Over three years, CHEOPS made 23 observations of WASP-76B in both visible and infrared light. These included Phase curvestransit, and Secondary eclipse. Phase curves are continuous observations that track the complete revolution of a planet and show changes in its phase or the portion of its illuminated side facing the telescope. The telescope may see more or less from this side as the planet orbits its star. Phase curves can determine the change in overall brightness of a planet and star as the planet rotates.

A secondary eclipse occurs when a planet passes behind its host star and eclipses it. The light seen during this eclipse can later be compared with the total light before and after the occultation to give us a sense of the light reflected off the planet. Hot Jupiters like WASP-76B are usually observed through secondary eclipses.

Phase curve observations can continue while the planet occults its star. While observing the phase curve of WASP-76B, CHEOPS saw an excess of pre-ecliptic light on its night side. This was also seen in the TESS phase curve and secondary eclipse observations made previously.

The end of the rainbow?

One of the advantages of WASP-76b is that it is a very hot Jupiter, so at least its dayside does not have the clouds and haze that often obscure the atmospheres of hot and cold Jupiters. This makes detecting atmospheric emissions much easier. We have already noticed an asymmetry in iron content between the end of the day side and the night side, which was discovered in Previous study, making the planet particularly interesting. There was not as much gaseous iron in the upper atmosphere of the day limb compared to the night limb. This is probably due to iron raining down on the dayside of WASP-76b, which then condenses to form iron clouds on the nightside.

Hubble’s observations indicated that a thermal inversion — when the air near the planet’s surface begins to cool — was occurring on the night side. Cooling on this side would cause iron that had previously condensed into clouds, rained down on the dayside, and then evaporated from the intense heat to condense again. Droplets of liquid iron can form clouds.

These clouds are crucial because light from the host star, reflecting off the droplets in those clouds, can create the glory effect.

“Explaining the observation with a glory effect would require spherical droplets of aerosol and highly reflective, spherical-shaped clouds in the planet’s eastern hemisphere,” the researchers said in a paper recently published in the journal Astronomy and Astrophysics.

Extraterrestrial glories have been seen before. They are also known to form in clouds Venus. Just like WASP-76b, more pre-eclipse light has been observed on Venus, so while the glory is quite clear for the exoplanet, future observations using a more powerful telescope could help determine how similar the phenomenon on WASP-76 is to that existing on our planet. Venus. If they match, this would be the first glory ever observed on an exoplanet.

If future research finds a concrete way to find out if this really works, these phenomena could tell us more about the atmospheric composition of exoplanets, depending on the types of elements or molecules the light reflects off. They may even give up the presence of water, which could mean habitability. While the supposed glory of WASP-76b has not been conclusively proven, it is nothing but a rainbow in the darkness.

Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202348270