Seeing worlds around brown dwarfs

Artist's rendering of the first planet outside of our solar system to be imaged orbiting a brown dwarf. Photo: ESO

Artist's rendering of the first planet outside of our solar system to be imaged orbiting a brown dwarf. Photo: ESO  

Stars come in different types. Our sun is a “main sequence” star – which, simply put, means it gets its energy by burning hydrogen through thermonuclear reactions. In order to do this, a star has to have a mass above 0.25 times the mass of the sun. However, nearly 15 per cent of stellar objects in our neighbourhood are the much-lighter “brown dwarfs,” and there is reason to believe these, too, could host habitable planets.

In this context, Sujan Sengupta, a faculty member of Indian Institute of Astrophysics, Bengaluru, has proposed analysing the polarisation of light from brown dwarfs to find out whether they are hosting transiting worlds. This work has been published in The Astronomical Journal.

Using existing detailed atmospheric models, the author calculates the scattering polarisation and infers that an earth-sized planet, when transiting across the brown dwarf, will show significant linear polarisation of the observed light when entering and leaving the transit. This change in polarisation value, as the planet transits its brown dwarf, can be measured using a polarimeter aimed in that direction and used to detect the exoplanet.

Failed stars

Brown dwarfs, which are starlike objects that have just failed to become main sequence stars, occupy the wide band of masses (between 13 and 75 Jupiter masses) between the lightest of stars and the heaviest of gas giant planets. Though they are not massive enough to burn the hydrogen they contain, during the first few million years of their existence, they give off energy by burning deuterium (a heavier isotope of hydrogen) and can be mistaken for low mass stars. These are candidates that can host habitable planets.

More common than brown dwarfs, among the stellar population in our galaxy, are M dwarfs, or red dwarfs, which are among the lighter stars in the main sequence. However, as Dr Sengupta argues in the paper, unlike sun-type stars, these red dwarfs are very active and also have intense magnetic fields which can cause them to give off flares, x-rays and ultraviolet rays. Because of this, even if they are shown to host planets in the habitable zone, the conditions on these planets may not be favourable for life.

So, if looking for habitable planets is the point of focus, we need to look at brown dwarfs, too, carefully.

While it is relatively easier to detect giant exoplanets around brown dwarfs, smaller, for instance, earth-sized, planets pose a problem for detection, due to the dim light given off by brown dwarfs. Both the transit method and direct imaging methods are challenged because of this, and the polarisation method can step in to fill the gap.

The implied existence of huge number of exoplanets around brown dwarfs thus stands to be confirmed by this powerful technique.

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Printable version | Aug 6, 2020 5:29:04 PM |

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