Characterization of a water world in a multi-exoplanetary system

A team of astronomers from 11 countries, led by researchers at the Instituto de Astrofísica e Ciências do Espaço (IA), were able to determine the precise mass of two small exoplanets orbiting the variable star HD 106315.

These two planets were previously detected by the Kepler satellite (NASA) via the transit method, which allowed researchers to determine the diameter of the planets. HD 106315b has a period of 9,5 days and a diameter of 2,44 times the diameter of the Earth, while HD 106315c has an orbital period of about 21 days, and a diameter 4,35 times the diameter of the Earth.

But to characterize a planet (for example, to determine if it is gaseous or rocky, or whether they have an atmosphere), researchers also need to know the mass of the planet, measured by the radial velocities, so that together with size, they can infer its density.

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Credit: ESA/ATG medialab

Artist's impression of a planet transiting a star.

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Credit: Barros et. al, 2017

Mass-radius relationship for different compositions of small planets: The blue line represents a composition of 50% mantle/50% water; the dark green line 100% mantle; the light green line an Earth-like 32,5% core/67,5% mantle; the brown line a Mercury-like 68% core/32% mantle. The planets HD 106315 b and c are on the far right of the graph, with planet "b" in the blue line. The Solar System planets, Venus, Earth, Uranus and Neptune are indicated as reference, and show that while HD 106315 c is quite similar to Neptune, HD 106315 b is a water world, very different from Solar System planets.

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A team of astronomers from 11 countries, led by researchers at the Instituto de Astrofísica e Ciências do Espaço (IA), were able to determine the precise mass of two small exoplanets orbiting the variable star HD 106315.