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Department of Physics

The Cavendish Laboratory
Illustration of an exoplanet and its host star. Credit NASA, ESA, CSA, Dani Player (STScI)

ESA’s exoplanet mission Cheops confirmed the existence of four warm exoplanets orbiting four stars in our Milky Way. These exoplanets have sizes between Earth and Neptune and orbit their stars closer than Mercury orbits our Sun.

The Universe is far too big and there is far too much data sent by those powerful telescopes to search by eye, so I created a pipeline to automatically search through the TESS data for duotransits and after running it on tens of thousands of stars, I was so excited to discover five planets that were missed by previous searches! Amy Tuson

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.

One of these four new warm mini-Neptunes was discovered by Amy Tuson, a PhD student at the University of Cambridge, using a unique combination of space telescope observations.

The first sign of the existence of these four new exoplanets was found by the NASA TESS mission. However, this spacecraft only looked at each star for 27 days during its primary mission. A hint of 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 about two years after the first observation and the same transit was seen again, implying the existence of the planets. With only two transits separated by about two years, these planets were labelled ‘duotransits’.

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.

By combining TESS and Cheops data, along with some additional ground-based observations, Amy Tuson, who works with Didier Queloz at Cambridge’s Cavendish Laboratory, was able to confirm the presence of two planets transiting the bright star HD 15906.

“From the TESS data alone, the inner planet had four transits and a unique period of 10.9 days. The outer planet was a duotransit – it had 36 possible values for its period and so we needed Cheops to determine the correct one,” explained Amy. “The first two Cheops observations were flat lines, but on the third try we observed a transit and confirmed the period of the outer planet to be 21.6 days.”

The Universe is far too big and there is far too much data sent by those powerful telescopes to search by eye, so how do we find TESS duotransits to follow-up with Cheops in the first place?

A big part of Amy’s PhD has been developing a specialised pipeline, better suited to finding those planets that only have two transits and go mostly undetected by more conventional  pipelines.

“I created my pipeline to automatically search through the TESS data for duotransits,” explained Amy. “After running it on tens of thousands of stars, I was so excited to discover five planets that were missed by previous searches! This includes TOI 5678 b which is one of the other warm planets confirmed by ESA’s Cheops mission.”


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 big goals of exoplanet science right now are the discovery of habitable planets, finding an Earth twin and the search for life in the Universe”, said Didier Queloz, Professor of Astrophysics at the Cavendish Laboratory and Amy’s PhD supervisor.

“To get there, we need to start by expanding the sample of long period planets and improving our understanding of them. The new discoveries, as well as the unique observing technique we are using, are helping to create a foundation for this future work.”

The results were published in four papers:


What are mini Neptunes and how do you search for them? PhD student Amy Tuson explains.

Adapted from an ESA press release.


Illustration of an exoplanet and its host star. Credit: NASA, ESA, CSA, Dani Player (STScI)