Elusive planets play hide and seek with CHEOPS

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CHEOPS is a joint mission of the European Space Agency (ESA) and the University of Geneva, led by the University of Bern, Switzerland. Since its launch in December 2019, the extremely precise measurements of CHEOPS have contributed to several important discoveries in the field of exoplanets.

NCCR PlanetS members Dr. Solen Ulmer-Moll of the Universities of Bern and Geneva and Dr. Hugh Osborne of the University of Bern used the unique synergy of CHEOPS and NASA’s TESS satellite to detect elusive exoplanets. The planets, named TOI 5678 b and HIP 9618 c, respectively, are about the size of Neptune, or slightly smaller, with 4.9 and 3.4 Earth radii. Related articles have recently been published in the journals Astronomy & Astrophysics and Monthly Notices of the Royal Astronomical Society. Two other members of the international team, Amy Tuson of the University of Cambridge (UK) and Dr Zoltan Garay of the ELTE Gothard Astrophysical Observatory (Hungary), used the same method to identify two similar planets in other countries, publishing in the same journals. systems.

Synergy of two satellites

The CHEOPS satellite monitors the brightness of stars if an orbiting planet passes in front of its star from our point of view, and captures its slight dimming. By searching for these dim events, called “transits,” scientists have been able to discover many of the thousands of exoplanets orbiting stars other than our Sun.

“NASA’s TESS satellite is excellent at detecting exoplanet transits, even for the most difficult minor planets. However, it changes its field of view every 27 days to rapidly scan much of the sky, which prevents it from finding planets with long orbital periods,” explains Hugh Osborne. However, the TESS satellite was able to observe single transits around the stars TOI 5678 and HIP 9618. Returning to the same field of view two years later, he was again able to observe similar transits around the same stars. Despite these observations, it was not possible to make an unequivocal conclusion about the presence of planets around these stars, because the information was incomplete.

Solen Ulmer-Moll: “That’s where CHEOPS comes into play: by focusing on one star at a time, CHEOPS is the perfect next mission to continue observing these stars to find the missing pieces of information.”

A long game of “hide and seek”.

Suspecting the existence of exoplanets, the CHEOPS team devised a method to avoid blindly spending expensive observing time in hopes of detecting additional transits. They adopted a targeted approach based on very few definitions of transits observed by TESS. Based on this, Osborne developed software that suggests and prioritizes candidate periods for each planet. “Then we play a game of hide-and-seek with the planets using the CHEOPS satellite,” says Osborne.

“We aim CHEOPS at a specific time, and depending on whether we observe a transit or not, we can rule out some possibilities and try again at another time until we have a unique solution for the orbital period.” It took five and four attempts, respectively, for scientists to definitively confirm the existence of the two exoplanets, finding that TOI 5678 b has a period of 48 days and HIP 9618 c has a period of 52.5 days.

Great goals for JWST

For scientists, the story does not end there. With their newfound bounding periods, they could refer to ground-based observations using another method called radial velocity, which allowed the team to determine masses of 20 and 7.5 Earth masses for TOI 5678 b and HIP 9618 c, respectively. With both the size and mass of a planet, its density is known, and scientists can get an idea of ​​what it’s made of. “But the density is not high enough for mini-Neptunes, and there are still several hypotheses about the composition of the planets: they could be rocky planets with a lot of gas, or planets with water-rich and very vaporous atmospheres. “, – explains Ulmer-Moll. “The four newly discovered exoplanets orbit bright stars, making them a prime target of interest for the James Webb Space Telescope’s JWST mission, which will help unravel the mysteries of their composition,” continued Ulmer-Moll.

Most of the exoplanet atmospheres observed so far have come from Hot Jupiters, which are very large and hot exoplanets orbiting close to their parent star. “The temperature of the four new planets we discovered is ‘only’ 217 to 277ºC. This temperature allows clouds and molecules to survive, otherwise they would be destroyed by the intense heat of Hot Jupiters. They could be detected by JWST,” Osborne explains. Smaller in size and with longer orbital periods than hot Jupiter, the newly discovered four planets are the first step toward tracking the transits of Earth-like planets.

Publication Details:

HIP 9618 transit biennial Neptune from TESS & Cheops by HP Osborn et al. Published in Monthly Notices of the Royal Astronomical Society.


TOI-5678 b: 48-day transit Neptune-mass planet, S. Ulmer-Moll et al. Published in Astronomy & Astrophysics.


Refined parameters of the planetary system HD 22946 and the true orbital period of planet d Z. Garai et al. Published in Astronomy & Astrophysics.


TESS and CHEOPS A. Tuson et al. Published in Monthly Notices of the Royal Astronomical Society.


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