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. Mini-Neptunes are among the most common types of exoplanets known, and astronomers are starting to find more and more orbiting bright stars.
Mini-Neptunes are mysterious objects. They are smaller, cooler, and more difficult to find than the so-called hot Jupiter exoplanets which have been found in abundance. While hot Jupiters orbit their star in a matter of hours to days and typically have surface temperatures of more than 1000 C, warm mini-Neptunes take longer to orbit their host stars and have cooler surface temperatures of only around 300 C.
The first sign of the existence of these four new exoplanets was found by the NASA TESS mission. However, this spacecraft only looked for 27 days at each star. 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. During its extended mission, TESS revisited these stars and the same transit was seen again, implying the existence of planets.
Scientists calculated the most likely orbital periods and pointed Cheops at the same stars at the time they expected the planets to transit. 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.
The four newly discovered planets have orbits between 21 and 53 days around four different stars. 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.
One of the outstanding questions about mini-Neptunes is what they are made of. Astronomers predict that they have an iron-rocky core with thick outer layers of lighter material. 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?
Discovering the composition of mini-Neptunes is important to understand the formation history of this type of planet. 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.
The new Cheops measurements helped determine the radius of the four exoplanets, while their mass could be determined using observations from ground-based telescopes. Combining the mass and radius of a planet gives an estimate of its overall density.
The density can only give a first estimate of the mass of the iron-rocky core. While this new information about the density is an important step forward in understanding mini-Neptunes, it does not contain enough information to offer a conclusion for the outer layers.
The four newly confirmed exoplanets orbit bright stars, which make them the perfect candidates for a follow-up visit by the NASA/ESA/CSA James Webb Space Telescope or ESA's future Ariel mission. These spectroscopic missions could discover what their atmospheres contain and provide a definitive answer to the composition of their outer layers.
A full characterisation is needed to understand how these bodies formed. Knowing the composition of these planets will tell us by what mechanism they formed in early planetary systems. This in turn helps us better understand the origins and evolution of our own Solar System.
The results were published in four papers:
'Refined parameters of the HD 22946 planetary system and the true orbital period of the planet d' by Z. Garai et al. is published in Astronomy and Astrophysics.
'Two Warm Neptunes transiting HIP 9618 revealed by TESS and Cheops' by H. P. Osborn et al. is published in the Monthly Notices of the Royal Astronomical Society.
'TESS and CHEOPS Discover Two Warm Sub-Neptunes Transiting the Bright K-dwarf HD15906' by A. Tuson et al. is published in the Monthly Notices of the Royal Astronomical Society.
'TOI-5678 b: a 48-day transiting Neptune-mass planet characterized with CHEOPS and HARPS' by S. Ulmer-Moll et al. is published in Astronomy and Astrophysics.
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