“The Alien Planet So Light It Would Float on Water”

 

Star HD 88111

 

On March 25, 2020, in the midst of the lockdown to contain the coronavirus, the European Space Agency declared the CHEOPS space telescope “mission-ready” for science, handing over the responsibility to operate CHEOPS to the mission consortium, which consists of scientists and engineers from approximately 30 institutions in 11 European countries. Measurements by CHEOPS are five times more accurate than those from Earth-based telescopes.

Launched in December 2019, Cheops, or the Characterizing Exoplanet Satellite, opened its eye to the Universe at the end of January and shortly after took its first, intentionally blurred images of stars. The deliberate defocusing is at the core of the mission’s observing strategy, which improves the measurement precision by spreading the light coming from distant stars over many pixels of its detector.

One of the stars the team observed was 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.

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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.

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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.

The image at the top of the page shows the star known as HD 88111, taken by ESA’s Cheops exoplanet watcher during its in-orbit commissioning in early 2020. The star is located in the Hydra constellation, some 175 light years away from Earth, and it is not known to host any orbiting planets. To demonstrate the stability of the satellite and instrument, Cheops took an image of this star every 30 seconds for 47 consecutive hours.

The images taken by Cheops are intentionally blurred: this deliberate defocusing is at the core of the mission’s observing strategy, which improves the measurement precision by spreading the light coming from distant stars over many pixels of its detector.

The Daily Galaxy, Sam Cabot, via ESA