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The Characterizing Exoplanet Satellite undergoing tests prior to launch. The European satellite could pinpoint the sizes of worlds between Neptune and Earth in size.

European Space Agency/G. Porter

European exoplanet mission will scrutinize known worlds

Astronomers have discovered more than 4000 other worlds so far, but know little about them. That will change after the successful launch today of the European Space Agency’s (ESA’s) first exoplanet mission, an orbiting telescope called the Characterizing Exoplanet Satellite (CHEOPS). Rather than hunt for new worlds, CHEOPS will examine known exoplanets to improve estimates of their sizes.

“It’s not a discovery machine but a follow-up machine,” says Principal Investigator Willy Benz of the University of Bern. “We’ll pick the most interesting targets and make more accurate measurements.”

CHEOPS was launched this morning by a Soyuz-Fregat rocket from Europe’s spaceport in French Guiana into a 700-kilometer-high polar orbit. The satellite will hug the line between night and day so it can always look out over the night side of Earth and avoid picking up stray light reflected off the surface.

Earlier missions, including NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS), discovered exoplanets by watching a patch of sky for days or weeks and looking for the dip in the brightness of a star when a planet passes in front of its face, or transits. From the depth of the dip astronomers can gauge how much of the star’s light is blocked and hence the size of the planet. CHEOPS will also use the transit technique, but it will focus on one star at a time and only those that are bright, close to Earth, and already known to have at least one exoplanet.

CHEOPS has a 35-centimeter telescope, three times the width of those on TESS, which launched in 2018, so it can collect more light from faint targets. Kepler, which operated from 2009 to 2018, had a much larger 95-centimeter telescope but was designed for surveying many stars at once. By optimizing its optics to focus on a single target and reducing noise, CHEOPS should be able to match Kepler’s precision on bright stars, Benz says.

One of CHEOPS’s main aims is to squeeze down the error bars on planetary sizes. It will focus specifically on planets between Neptune and Earth in size—a common planetary type that is unknown in our solar system. Somewhere in that range, there’s a transition between gaseous and rocky planets that astronomers are eager to probe. “You can focus on interesting systems that you really want to get precise information on,” says Scott Gaudi of Ohio State University in Columbus. And that will help astronomers pick out the best candidates for scrutiny by upcoming observatories such as NASA’s James Webb Space Telescope and ESA’s Ariel, which will probe the constituents of their atmospheres.

Although CHEOPS is not intended to be a discovery machine, with its extra sensitivity and long observations it may end up spotting fainter transits by planetary siblings orbiting alongside its targets. CHEOPS may find that systems containing a Neptune-size planet also tend to have smaller ones, like Earth, Mars, and Venus, says Carole Haswell of the Open University in Milton Keynes, U.K. “It will fill in our picture of planetary systems,” she says.

There’s also the possibility of detecting moons around exoplanets. There have been hints of a possible exomoon over the past couple of years, but these have been disputed because of the difficulty of interpreting noisy data. CHEOPS’s unique selling point, Haswell says, is less noisy data, which “should help us resolve some of these debates.”