CHEOPS is a joint mission by the European Space Agency (ESA) and Switzerland, under the leadership of the University of Bern in collaboration with the University of Geneva (UNIGE). After almost three months of extensive testing, with part of it in the midst of the lockdown to contain the coronavirus, on Wednesday, March 25, 2020, ESA declared the CHEOPS space telescope ready for science. With this achievement, ESA has handed over the responsibility to operate CHEOPS to the mission consortium, which consists of scientists and engineers from approximately 30 institutions in 11 European countries.

Successful completion of the CHEOPS test phase despite the coronavirus crisis

The successful completion of the test phase took place in very challenging times, with essentially all the mission team being required to work from home towards the end of the phase. “The completion of the test phase was only possible with the full commitment of all the participants, and because the mission has an operational control system that is largely automated, allowing commands to be sent and data to be received from home,” explains Willy Benz, Professor of Astrophysics at the University of Bern and Principal Investigator of the CHEOPS mission.

A team of scientists, engineers and technicians put CHEOPS through a period of extensive testing and calibration from the beginning of January until the end of March. “We were thrilled when we realized that all the systems worked as expected or even better than expected,” explains CHEOPS Instrument Scientist Andrea Fortier from the Univerisity of Bern, who led the commissioning team of the consortium.

Meeting high requirements on measuring accuracy

The team began by focusing on the evaluation of the photometric performance of the space telescope. CHEOPS has been conceptualized as a device of exceptional precision capable of detecting exoplanets the size of planet Earth. “The most critical test was in the precise measurement of the brightness of a star to a variance of 0.002% (20 parts-per-million),” explains Willy Benz. This precision is required so as to clearly recognize the dimming caused by the passage of an Earth-sized planet in front of a Sun-like star (an event known as a “transit”, which can last several hours). CHEOPS was also required to demonstrate its ability to maintain this degree of precision for up to two days.

CHEOPS surpasses the requirements

To verify this, the team focused on a star known as HD 88111. The star is located in the Hydra constellation, some 175 light years away from Earth, and it is not known to host planets. CHEOPS took an image of the star every 30 seconds for 47 consecutive hours (see Figure 1). Every image was carefully analyzed, initially using a specialized automatic software package, and subsequently by the team members, to determine in each image the brightness of the star as accurately as possible. The team had expected the brightness of the star to change during the period of observation due to a variety of effects, such as other stars in the field of view, the tiny jitter motion of the satellite, or the impact of cosmic ray hits on the detector.

The results of the 5,640 photos taken by CHEOPS over 47 hours are shown in Figure 2 as a “light curve”. The curve depicts the change over time in the brightness measurements from all the images, showing a root-mean-square scatter of 0.0015% (15 parts-per-million). “The light curve measured by CHEOPS was pleasingly flat. The space telescope easily surpasses the requirement for being able to measure brightness to a precision of 0.002% (20 parts-per-million),” explains Christopher Broeg, Mission Manager for the CHEOPS mission at the University of Bern.

An exoplanet that would float

The team observed other stars, including some known to host planets (these are called exoplanets). CHEOPS focused on the planetary system HD 93396 which is in the Sextans constellation, some 320 light years away from Earth. This system consists of a giant exoplanet called KELT-11b, which was discovered in 2016 to orbit this star in 4.7 days. The star is almost three times the size of the sun.

The team chose this particular system because the star is so big that the planet takes a long time to pass in front of it: in fact, almost eight hours. “This gave CHEOPS the opportunity to demonstrate its ability to capture long transit events otherwise difficult to observe from the ground, as the ‘astronomical’ part of the night for ground-based astronomy usually takes less than eight hours,” explains Didier Queloz, professor at the Astronomy Department of the Faculty of Science at the University of Geneva and spokesperson of the CHEOPS Science Team. The first transit light curve of CHEOPS is shown in Figure 3, where the dip due to the planet occurs approximately nine hours after the he beginning of the observation.

The transit of KELT-11b measured by CHEOPS enabled determining the size of the exoplanet. It has a diameter of 181,600 km, which CHEOPS is able to measure with an accuracy of 4'290 km. The diameter of the Earth, in comparison, is only approximately 12,700 km, while that of Jupiter – the biggest planet in our solar system – is 139,900 km. Exoplanet KELT-11b is therefore bigger than Jupiter, but its mass is five times lower, which means it has an extremely low density: “It would float on water in a big-enough swimming pool,” says David Ehrenreich, CHEOPS Mission Scientist from the University of Geneva. The limited density is attributed to the close proximity of the planet to its star. Figure 4 shows a drawing of the first transit planet system to be successfully observed by CHEOPS.

Benz explains that the measurements by CHEOPS are five times more accurate than those from Earth. “That gives us a foretaste for what we can achieve with CHEOPS over the months and years to come,” continues Benz.

Since its launch last year from the European Space Agency (ESA)’s spaceport in Kourou, French Guiana, on December 18 the CHEOPS telescope in Earth’s orbit has demonstrated its functionality and precision beyond expectations. That it would ever to get to that point, was never a certainty, as it was a pioneering project in different aspects.

First Swiss-led ESA space mission

From 26 project proposals, CHEOPS was selected as ESA’s so-called first small ‘S-class mission’ back in 2012. The conditions of such a mission were that it had to be realized within a few years and would not cost ESA more than 50 million euro. It is the first Swiss-led mission of ESA, and as such, Switzerland was the primary financial contributor after ESA. Furthermore, CHEOPS is the first ESA mission dedicated to characterizing known exoplanets.

Exoplanets, i.e. planets outside the Solar system, were first found in 1995 by two Swiss astronomers, Michel Mayor and Didier Queloz from the University of Geneva, who were last year awarded the Nobel Prize for this discovery. Since then, over 4000 such planets were found, and CHEOPS was proposed to determine the composition of the most interesting specimens. This ambitious goal required a dedicated collaboration of a consortium of more than a hundred scientists and engineers from eleven European countries, who were involved in constructing CHEOPS under the leadership of the University of Bern and ESA over five years.

“We think that we meet the requirements to deliver ultra-precise measurements to the scientists. Otherwise we would not be flying", said Christopher Broeg, project manager for the CHEOPS mission from the University of Bern, upon completion of construction. And meet them, they did. In close collaboration with the University of Geneva, where the Science Operations Center of CHEOPS is located, the instrument has already collected plenty of data. Despite the extraordinary circumstances the Covid-19 pandemic has created, CHEOPS scientists have recently carried out a detailed study of an extreme exoplanet based on some of this data. “And the next papers are already in preparation”, as Willy Benz, Professor of astrophysics at the University of Bern and head of the CHEOPS consortium, reports.

In the beginning of October 2020, the CHEOPS space telescope had to make an evasive maneuver due to a piece of space debris. "In the event of a collision, this could have meant the destruction of the CHEOPS space telescope", as Willy Benz points out.

Strong collaboration between universities and industrial partners

“This mission really stands out in Swiss space activity”, as Renato Krpoun, head of the Swiss Space Office, puts it, “and reflects on the strong collaboration with industrial partners”. Christian Leumann, rector of the University of Bern, points out that the CHEOPS mission "underlines our presence in space research and gives Switzerland international visibility." Yves Flückiger, rector of the University of Geneva, adds: “I am more than content with the close collaboration between our universities as well as with the industrial partners.” 

Exciting scientific year to come

The mission presents an excellent opportunity for scientists world-wide, explains Kate Isaak, ESA Project Scientist: “With twenty percent of the observing time available through the ESA-run Guest Observers Programme, scientists from around the world will be able to capitalize directly on the unique capabilities of CHEOPS.”

“Judging by the latest observation successes, 2021 will be a very exciting year for CHEOPS, scientifically speaking” as David Ehrenreich, Professor of Astronomy at the University of Geneva and Consortium Mission Scientist of CHEOPS, concludes.

Quelle: University of Bern

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Update: 27.01.2021

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CHEOPS finds unique planetary system

Musical notes that sound pleasant together can form a harmony. These notes are usually in a special relationship with each other: when expressed as frequencies, their ratios result in simple fractions, such as four-thirds or three-halves. Similarly, a planetary system can also form a kind of harmony when planets, whose orbital period ratios form simple fractions, regularly attract each other with their gravity. When one planet takes three days to orbit its star and its neighbor takes two days, for example. Using the CHEOPS space telescope, scientists, led by astrophysicist Adrien Leleu of the Center for Space and Habitability of the University of Bern, the University of Geneva and the National Center of Competence in Research PlanetS, found such relationships between five of six planets orbiting the star TOI-178, located over 200 light years away from Earth. The results were published in the journal Astronomy and Astrophysics.

A missing piece in an unexpected puzzle

“This result surprised us, as previous observations with the Transiting Exoplanet Survey Satellite (TESS) of NASA pointed toward a three planets system, with two planets orbiting very close together. We therefore observed the system with additional instruments, such as the ground based ESPRESSO spectrograph at the European Southern Observatory (ESO)’s Paranal Observatory in Chile, but the results were inconclusive.”, Leleu remembers. When he and his colleagues first proposed to investigate the system more closely, they were therefore not sure what they would find. The high precision and target-pointing agility of CHEOPS was required to bring clarity, but that turned out to be more difficult than expected. “After analyzing the data from eleven days of observing the system with CHEOPS, it seemed that there were more planets than we had initially thought”, Leleu says. The team identified a possible solution with five planets and decided to invest another day of precious observation time on the system to confirm. They found that there were indeed five planets present with orbital periods of around 2, 3, 6, 10 and 20 days respectively.

While a system with five planets would have been quite a remarkable finding in itself, Leleu and his colleagues noticed that there might be more to the story: the system appeared to be in harmony. “Our theory implied that there could be an additional planet in this harmony; however its orbital period needed to be very nearly 15 days.”, Leleu explains. To check if their theory was in fact true, the team scheduled yet another observation with CHEOPS, at the exact time that this missing planet would pass by – if it existed. But then, an accident threatened to cancel their plans.

Prediction confirmed despite near-collision

“Just before the time of the observation, a piece of space debris threatened to collide with the CHEOPS satellite”, as co-author and Professor of Astrophysics at the University of Bern, Yann Alibert, recalls. Therefore, the control center of the European Space Agency (ESA) initiated an evasive maneuver of the satellite and all observations were interrupted. “But to our great relief, this manoeuver was done very efficiently and the satellite could resume observations just in time to capture the mysterious planet passing by”, as Nathan Hara, co-author and astrophysicist from the University of Geneva reports. “A few days later, the data clearly indicated the presence of the additional planet and thus confirmed that there were indeed six planets in the TOI-178 system”, Hara explains.

A system that challenges current understanding

Thanks to the precision of CHEOPS’ measurements as well as previous data from the TESS mission, the ESO’s spectrograph ESPRESSO, and others, the scientists could not only measure the periods and sizes of the planets of 1.1 to 3 times the radius of the Earth, but also estimate their densities. With that came another surprise: compared to the harmonic, orderly way the planets orbit around their star, their densities appear to be a wild mixture.

“It is the first time we observe something like this”, as ESA Project Scientist Kate Isaak points out and adds that “in the few systems we know with such a harmony, the density of planets steadily decreases as we move away from the star. In the TOI-178 system, a dense, terrestrial planet like Earth appears to be right next to a very fluffy planet with half the density of Neptune followed by one very similar to Neptune”.

As Adrien Leleu concludes, “the system therefore turned out to be one that challenges our understanding of the formation and evolution of planetary systems”.

Quelle: University of Bern

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ESA’s exoplanet watcher Cheops reveals unique planetary system

ESA’s exoplanet mission Cheops has revealed a unique planetary system consisting of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. The sizes and masses of the planets, however, don’t follow such an orderly pattern. This finding challenges current theories of planet formation.

The discovery of increasing numbers of planetary systems, none like our own Solar System, continues to improve our understanding of how planets form and evolve. A striking example is the planetary system called TOI-178, some 200 light-years away in the constellation of Sculptor.

Astronomers already expected this star to host two or more exoplanets after observing it with NASA’s Transiting Exoplanet Survey Satellite (TESS). New, highly precise observations with Cheops, ESA’s Characterising Exoplanet Satellite that was launched in 2019, now show that TOI-178 harbours at least six planets and that this foreign solar system has a very unique layout. The team, led by Adrien Leleu of University of Geneva and the University of Bern in Switzerland, published their results today in Astronomy & Astrophysics.

One of the special characteristics of the TOI-178 system that the scientists were able to uncover with Cheops is that the planets – except the one closest to the star – follow a rhythmic dance as they move in their orbits. This phenomenon is called orbital resonance, and it means that there are patterns that repeat themselves as the planets go around the star, with some planets aligning every few orbits.

A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. For every orbit of Europa, Ganymede completes two orbits, and Io completes four (this is a 4:2:1 pattern).

In the TOI-178 system, the resonant motion is much more complex as it involves five planets, following a 18:9:6:4:3 pattern. While the second planet from the star (the first in the pattern) completes 18 orbits, the third planet from the star (second in the pattern) completes nine orbits, and so on.

Initially, the scientists only found four of the planets in resonance, but by following the pattern the scientists calculated that there must be another planet in the system (the fourth following the pattern, the fifth planet from the star).

“We predicted its trajectory very precisely by assuming that it was in resonance with the other planets,” Adrien explains. An additional observation with Cheops confirmed that the missing planet indeed existed in the predicted orbit.

After they had uncovered the rare orbital arrangements, the scientists were curious to see whether the planet densities (size and mass) also follow an orderly pattern. To investigate this, Adrien and his team combined data from Cheops with observations taken with ground-based telescopes at the European Southern Observatory’s (ESO) Paranal Observatory in Chile.

But while the planets in the TOI-178 system orbit their star in a very orderly manner, their densities do not follow any particular pattern. One of the exoplanets, a dense, terrestrial planet like Earth is right next to a similar-sized but very fluffy planet ­­– like a mini-Jupiter, and next to that is one very similar to Neptune.

“This is not what we expected, and is the first time that we observe such a setup in a planetary system,” says Adrien. “In the few systems we know where the planets orbit in this resonant rhythm, the densities of the planets gradually decrease as we move away from the star, and it is also what we expect from theory.”

Catastrophic events such as giant impacts could normally explain large variations in planet densities, but the TOI-178 system would not be so neatly in harmony if that had been the case.

“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” explains co-author Yann Alibert from the University of Bern.

Revealing the complex architecture of the TOI-178 system, which challenges current theories of planet formation, was made possible thanks to almost 12 days of observations with Cheops (11 days of continuous observations, plus two shorter observations).

“Solving this exciting puzzle required quite some effort to plan, in particular to schedule the 11-day continuous observation needed in order to catch the signatures of the different planets,” says ESA Cheops project scientist Kate Isaak. “This study highlights very nicely the follow-up potential of Cheops – not only to better characterise known planets, but to hunt down and confirm new ones.”

Adrien and his team want to continue to use Cheops to study the TOI system in even more detail.

“We might find more planets that could be in the habitable zone – where liquid water might be present on the surface of a planet – which begins outside of the orbits of the planets that we discovered to date,” says Adrien. “We also want to find out what happened to the innermost planet that is not in resonance with the others. We suspect that it broke out of resonance due to tidal forces.”

Astronomers will use Cheops to observe hundreds of known exoplanets orbiting bright stars.

“Cheops will not only deepen our understanding of the formation of exoplanets, but also that of our own planet and the Solar System,” adds Kate.

Notes for editors

‘Six transiting planets and a chain of Laplace resonances in TOI-178’ by A. Leleu et al. appears in Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202039767

More about Cheops

Cheops is an ESA mission developed in partnership with Switzerland, with a dedicated consortium led by the University of Bern, and with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden and the UK.

ESA is the Cheops mission architect, responsible for procurement and testing of the satellite, the launch and early operations phase, and in-orbit commissioning, as well as the Guest Observers’ Programme through which scientists world-wide can apply to observe with Cheops. The consortium of 11 ESA Member States led by Switzerland provided essential elements of the mission. The prime contractor for the design and construction of the spacecraft is Airbus Defence and Space in Madrid, Spain.

The Cheops mission consortium runs the Mission Operations Centre located at INTA, in Torrejón de Ardoz near Madrid, Spain, and the Science Operations Centre, located at the University of Geneva, Switzerland.

Quelle: ESA

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Update: 30.06.2021

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Unique exoplanet photobombs CHEOPS study of nearby star system

While studying two exoplanets in a bright nearby star system, the CHEOPS satellite has unexpectedly spotted the system’s third known planet crossing the face of the star. This transit reveals exciting details about a rare planet “with no known equivalent”, as the scientific team led by the Universities of Geneva and Bern, and members of the National Center of Competence in Research PlanetS, point out.

Photobombs – when something or someone unexpectedly enters a camera’s field of sight during the taking of a photograph – happen every day. Sometimes it is a friend, other times a stranger or perhaps a bird. Rarely, however, is it a whole planet. Yet, this is exactly what happened while CHEOPS, the Swiss-led space telescope, was taking pictures of a planetary system 50 light-years away.

A planet like no other

The planetary system is located in the constellation Lupus (latin for Wolf), around a star called Nu2 Lupi, visible to the naked eye (but not from Switzerland). In 2019, Swiss astronomers announced the detection of three exoplanets around this bright, Sun-like star. The three exoplanets have masses between those of Earth and Neptune (17 times the Earth) and take 12, 28 and 107 days to circle their parent star. “What makes these exoplanets really outstanding is that we can see them passing just in front of their star; they’re said to ‘transit’”, says Yann Alibert, professor of astrophysics at the University of Bern and co-author of the study which has been published in Nature Astronomy. “We knew that already for the two inner planets, which led us to point CHEOPS to the system in the first place. However, the third planet is quite far away from the star, no one was expected to see its transit!”, Alibert adds. In fact, the farther away the planet is from its star, the less likely it is to transit.

This proved a game changer, as it is the first time an exoplanet with a revolution period of over 100 days – which corresponds to a distance from the star somewhere between that of Mercury and Venus from the Sun – has been spotted transiting a star that is bright enough to be visible to the naked eye. 

“Due to its relatively long period, the amount of stellar radiation reaching the planet is mild in comparison to many other discovered exoplanets. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its origin”, says David Ehrenreich, professor at the University of Geneva and mission scientist of CHEOPS, who co-signed the study. But so far, the few such exoplanets astronomers had found orbited faint stars. In other words: little of their light reaches Earth and therefore makes them difficult to study. Not this time: “Since its bright host star is quite close to us, it is easier to study. This makes it a golden target for future study with no known equivalent”, Ehrenreich adds.

Further insights from other telescopes

The high-precision measurements of CHEOPS reveal the third planet, called nu2 Lupi d, to be about 2.5 times the size of Earth and almost 9 times its mass. By combining these measurements with archival data from other observatories and numerical models developed by the University of Bern, Laetitia Delrez, a visiting researcher at the University of Geneva and lead-author of the study, was able to accurately characterize the density and composition the planet and its neighbors. “The innermost planet is mainly rocky, while the outer two appear to be enshrouded in envelopes of hydrogen and helium gases beneath which they hold large amounts of water”, Delrez explains. Far more water, in fact, than the Earth has: a quarter of each planet’s mass is made up of water, compared to less than 0.1% for Earth. This water, however, is not liquid, instead taking the form of high-pressure ice or high-temperature steam, making the planets uninhabitable. But these insights could only be the beginning.

“Now that we discovered that all three planets transit and have precisely measured their properties, the next step is to study them with bigger and more powerful instruments than CHEOPS, like the Hubble Space Telescope or its successor, the James Webb Space Telescope. They could reveal further details, such as the composition of the atmosphere” says Ehrenreich. Given its overall properties and orbit, planet d is going to become the poster-child of exoplanets with a mild-temperature atmosphere around a star similar to the Sun.

Quelle: University of Bern