Water-rich mini-Neptunes probably formed far out in the icy regions of their planetary system before migrating inwards, while combinations of rock and gas would tell us that these planets stayed in the same place as they formed. Different theories predict different outer layers: Do they have deep oceans of liquid water, a puffy hydrogen and helium atmosphere or an atmosphere of pure water vapour?ĭiscovering the composition of mini-Neptunes is important to understand the formation history of this type of planet. Astronomers predict that they have an iron-rocky core with thick outer layers of lighter material. One of the outstanding questions about mini-Neptunes is what they are made of.
Their discovery is essential because it brings our sample of known exoplanets closer to the longer orbits that we find in our own Solar System. The four newly discovered planets have orbits between 21 and 53 days around four different stars. During this hit-or-miss procedure Cheops was able to measure a transit for each of the exoplanets, confirming their existence, discovering their true orbital periods and taking the next step in their characterisation. Scientists calculated the most likely orbital periods and pointed Cheops at the same stars at the time they expected the planets to transit. During its extended mission, TESS revisited these stars and the same transit was seen again, implying the existence of planets. A hint to a transit – the dimming of light as a planet passes in front of its star from our viewpoint – was spotted for each star. However, this spacecraft only looked for 27 days at each star. The first sign of the existence of these four new exoplanets was found by the NASA TESS mission. While hot Jupiters orbit their star in a matter of hours to days and typically have surface temperatures of more than 1000 ☌, warm mini-Neptunes take longer to orbit their host stars and have cooler surface temperatures of only around 300 ☌. They are smaller, cooler, and more difficult to find than the so-called hot Jupiter exoplanets which have been found in abundance.
Mini-Neptunes are among the most common types of exoplanets known, and astronomers are starting to find more and more orbiting bright stars. These so-called mini-Neptunes are unlike any planet in our Solar System and provide a ‘missing link’ between Earth-like and Neptune-like planets that is not yet understood. These exoplanets have sizes between Earth and Neptune and orbit their stars closer than Mercury our Sun. ESA’s exoplanet mission Cheops confirmed the existence of four warm exoplanets orbiting four stars in our Milky Way.
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