A new technique to look for exomoons

An artist's impression of an earth-like exomoon orbiting a saturn-like exoplanet. Photo: NASA

An artist's impression of an earth-like exomoon orbiting a saturn-like exoplanet. Photo: NASA  

The recent discovery of Proxima b, an exoplanet in our neighbouring star system, Proxima Centauri in the Alpha Centauri group, has fuelled popular interest in studying new worlds. However, theoretical and observational study of exoplanets – planets orbiting stars other than the sun – is not new. Among the exoplanets discovered many cannot themselves support life. But it is expected that these could be surrounded by huge natural satellites having water and which are perhaps even habitable. This is one reason to look for such friendly “exomoons” as well as exoplanets.

Sujan Sengupta of Indian Institute of Astrophysics, Bengaluru, and Mark S. Marley of NASA Ames Research Centre, California, USA, have come up with a novel technique to detect the presence of exomoons using the variation in the polarization of light coming from the exoplanet. The research has been published in The Astrophysical Journal.

Detecting an exomoon is not an easy task. It is – relatively – easy to detect the exoplanets as they transit across the face of their star, by detecting the related variation in the intensity of the star’s light. With exomoons also, a similar method can be developed. This is because the scientists could study the dimming of that light as the exomoon transits the star. However, this would be a very faint signal.

A totally different approach, which depends on measuring the polarization of the light from the exoplanet, has been developed by Sengupta and Marley. As they propose in an earlier paper, light from these exoplanets is likely to be linearly polarized. However, when integrated over a perfectly circular disc, this would cancel. There are two effects which would result in a non-zero value for the observed polarization – one, the oblateness resulting from the planet having a fast spinning motion and, two, if a exomoon should partially block the light of the exoplanet when transiting, thereby introducing an asymmetry. If the planet is spinning very fast, the first effect would be dominant, but even then the authors show that it is possible to measure the slight correction due to the effect of the exomoons’ transit.

As Dr. Sengupta puts it, “When a dark moon appears between the planet and the observer, it shadows a tiny part of the planetary surface. So, a part of the planetary surface is blocked to the observer. Therefore, the polarization that arises due to scattering in that blocked out region is not added up to the net polarization. Hence, the final result is not exactly zero which should have been if there were no obstruction.”

This method offers a new way of detecting the exomoon – a fluctuation in the polarization of the observed light from the exoplanet. With this result, it becomes interesting for earth based telescopes and even space based ones, like the James Webb Space Telescope, to evolve instruments having one more capacity to help in the hunt for new planets.

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