Indian astronomers from IIA Bengaluru find new method to study exoplanets' atmosphere

Indian astronomers, from the Indian Institute of Astrophysics (IIA) in Bengaluru, have found that observing the polarisation of light and studying polarisation signatures can help understand the atmosphere of exoplanets.

With this new method, the polarisation signatures or variations in scattering intensity of light, emerging from the exoplanets, can be observed using existing instruments and would help expand the study of planets beyond our solar system. This would add to the discoveries of 5,000 exoplanets that have been detected by astronomers till now. 

How does the method to study atmosphere of exoplanets work?

The method has been developed by Indian astronomers Sujan Sengupta and Aritra Chakrabarty, that has now been published as a study in The Astrophysical Journal. Two decades ago, IIA scientist Sengupta had suggested that the thermal radiation of hot young planets and the reflected light of planets orbiting other stars, known as exoplanets would be polarized and the measure of the polarization might unveil the chemical composition and other properties of the exoplanetary atmosphere. 

Chakrabarty, on the other hand, who is a postdoctoral researcher at IIA and a colleague of Sengupta, developed a detailed three-dimensional numerical method and simulated the polarization of exoplanets. The experts revealed that just like the planets of our solar system, exoplanets are slightly oblate due to their rapid spin rotation. Further, depending on its position around the star, only a part of the planetary disk gets illuminated by the starlight. This asymmetry of the light-emitting region gives rise to non-zero polarization. 

Both the astronomers developed a Python language-based numerical code that incorporates a state-of-the-art planetary atmosphere model. They employed all such asymmetries of an exoplanet orbiting the parent star at different inclination angles to calculate the amount of polarization at different latitudes and longitudes. They found that the polarization at different wavelengths is sufficiently high and hence can be detected even by a simple polarimeter if the starlight is blocked. According to the experts, this would help in studying the atmosphere of the exoplanets along with their chemical composition. Reflecting on this idea, Artitra Chakrabarty said-

Even if we cannot image the planet directly and the unpolarized starlight is allowed to mix up with the polarized reflected light of the planet, the amount should be a few ten parts of a million, but still can be detected by some of the existing high-end instruments such as HIPPI, POLISH, PlanetPol, etc. The research will help in designing instruments with appropriate sensitivity and guide the observers.

It is being said, that the polarimetric techniques in the near future will open up a new window for the study of the exoplanets and enable us to overcome many of the limitations of the traditional techniques.                               

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