ESA’s Cheops exoplanet mission has confirmed that there are four warm exoplanets orbiting four stars in our Milky Way. These exoplanets are between Earth and Neptune in size and orbit their stars closer to our Sun than Mercury.
These mini-Neptunes are unlike any other planet in the Solar System and provide an as-yet-unexplained “missing link” between Earth-like and Neptune-like planets. Mini-Neptunes are one of the most common types of exoplanets known, and astronomers are beginning to find more and more orbiting bright stars.
Mini-Neptunes are mysterious objects. They are smaller, cooler, and harder to find than the hot Jupiter exoplanets that have been found in abundance. While hot Jupiters orbit their stars in hours to days and surface temperatures are typically above 1000 °C, warm mini-Neptunes take longer to orbit their stars and have cooler surface temperatures of around 300 °C.
The first indication of the existence of these four new exoplanets was found by NASA’s TESS mission. However, this spacecraft only looked at each star for 27 days. A transit indication for each star is a dimming of light as the planet passes in front of its star. During its extended mission, TESS revisited these stars and the same transit was observed again, indicating the presence of planets.
Scientists calculated the most likely orbital periods and aimed Cheops at one star while waiting for the planets to pass. During this hit-or-miss procedure, Cheops was able to measure transits for each of the exoplanets, confirm their existence, determine their true orbital periods, and take the next step in their characterization.
The four newly discovered planets orbit four different stars between 21 and 53 days. Their discovery is significant because it brings our known sample of exoplanets closer to the long orbits found in our own Solar System.
One of the big questions about Mini-Neptunes is what they’re made of. Astronomers predict that they have an iron-rock core with thick outer layers of lighter material. Different theories suggest different outer layers: do they have deep oceans of liquid water, an atmosphere of rough hydrogen and helium, or an atmosphere of pure water vapor?
Deciphering the composition of mini-Neptunes is important for understanding the history of the formation of this type of planet. Water-rich mini-Neptunes may have formed in the icy regions of the planetary system before migrating inward, and the combination of rock and gas suggests that these planets remained where they formed.
The new Cheops measurements helped determine the radii of four exoplanets, and their masses can be determined by observations from ground-based telescopes. Combining a planet’s mass and radius gives an estimate of its total density.
Density can only provide a first estimate of the mass of the iron-stone core. While this new density information is an important step in understanding mini-Neptune, there is not enough information to draw conclusions for its outer layers.
Four newly confirmed exoplanets orbit bright stars, making them ideal candidates for a visit by the NASA/ESA/CSA James Webb Space Telescope or ESA’s future Ariel mission. These spectroscopic missions can determine what is in their atmospheres and provide precise answers to the composition of their outer layers.
A full description is needed to understand how these bodies came to be. Knowing the composition of these planets reveals the mechanism by which they formed in early planetary systems. This, in turn, will help us better understand the origin and evolution of the Solar System.
The results were published in four papers:
“Refined parameters of the planetary system HD 22946 and the true orbital period of planet d” Z. Garai et al. Published in Astronomy & Astrophysics. (open access)
‘HIP 9618 transit biennial Neptune from TESS & Cheops by HP Osborn et al. Published in Monthly Notices of the Royal Astronomical Society. (open access)
A. Tuson et al. “TESS and CHEOPS Reveal Two Warm Sub-Neptune Passes by Bright K-Dwarf HD15906”. Published in Monthly Notices of the Royal Astronomical Society. (open access)
“TOI-5678 b: A 48-day transit Neptune-mass planet characterized by CHEOPS and HARPS” S. Ulmer-Moll et al. Published in Astronomy & Astrophysics. (open access)
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