Neither rock nor water worlds: Astronomers discover two “in-between” mini-Neptunes

An international group of astronomers reported another discovery made with the help of the TESS spacecraft. It managed to detect two mini-Neptunes. 

Mini-Neptune in an artist’s impression. Source: NASA/Ames/JPL-Caltech

The discovery was made during observations of TOI-6054. It is an F-type subgiant star located at a distance of about 259 light-years from Earth. It’s about 66 per cent larger, but only slightly more massive than the Sun. TESS was able to detect two transit signals in the light curve of this star. Subsequent measurements of the radial velocity of TOI-6054, made with the NEID spectrograph mounted on the WIYN 3.5-metre telescope, confirmed their planetary nature.

The closest exoplanet to the star has been designated TOI-6054 b. It has a radius of about 2.65 Earth radii, and its mass is about 12.4 times that of our planet. This gives an average density of 3.65 g/cm3, which is less than the average density of Mars but greater than the average density of the Moon. The exoplanet orbits at a distance of 0.077 a. e. (11.5 million kilometres) from the star. Its orbital period is 7.5 days and its equilibrium temperature is estimated at 1090 °C.

The second exoplanet, named TOI-6054 c, has a radius of 2.81 Earth radii and has a mass of about 9.2 Earth masses. Its average density is calculated to be approximately 2.27 g/cm3. The exoplanet has an orbital period of 12.56 days and is about 0.11 a. e. (16, 5 million kilometres) away from its parent star. The equilibrium temperature of TOI-6054 c is estimated to be 870 °C.

Based on the parameters obtained, astronomers classified TOI-6054 b and TOI-6054 c as mini-Neptunes, noting that they are too large to have a purely rocky composition. Both exoplanets are likely neither rocky worlds with a hydrogen-helium shell nor watery worlds without hydrogen or helium, but occupy an intermediate position between the two categories. 

Summarising the results, the researchers emphasise that the worlds of the TOI-6054 system should be an excellent target for the James Webb Space Telescope. Its power should be enough to reveal the main features of their atmospheres.

Earlier we reported on how astronomers managed to measure the wind speed in the atmosphere of a glowing gas giant.

According to Phys.org

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